Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
On Mon, Jun 21, 2021 at 12:16:55PM +0200, Christian König wrote:
> Am 18.06.21 um 20:45 schrieb Daniel Vetter:
> > On Fri, Jun 18, 2021 at 8:02 PM Christian König
> > wrote:
> > > Am 18.06.21 um 19:20 schrieb Daniel Vetter:
> > > [SNIP]
> > > The whole thing was introduced with this commit here:
> > >
> > > commit f2c24b83ae90292d315aa7ac029c6ce7929e01aa
> > > Author: Maarten Lankhorst
> > > Date: Wed Apr 2 17:14:48 2014 +0200
> > >
> > > drm/ttm: flip the switch, and convert to dma_fence
> > >
> > > Signed-off-by: Maarten Lankhorst
> > >
> > >int ttm_bo_move_accel_cleanup(struct ttm_buffer_object *bo,
> > >
> > > - bo->sync_obj = driver->sync_obj_ref(sync_obj);
> > > + reservation_object_add_excl_fence(bo->resv, fence);
> > > if (evict) {
> > >
> > > Maarten replaced the bo->sync_obj reference with the dma_resv exclusive
> > > fence.
> > >
> > > This means that we need to apply the sync_obj semantic to all drivers
> > > using a DMA-buf with its dma_resv object, otherwise you break imports
> > > from TTM drivers.
> > >
> > > Since then and up till now the exclusive fence must be waited on and
> > > never replaced with anything which signals before the old fence.
> > >
> > > Maarten and I think Thomas did that and I was always assuming that you
> > > know about this design decision.
> > Surprisingly I do actually know this.
> >
> > Still the commit you cite did _not_ change any of the rules around
> > dma_buf: Importers have _no_ obligation to obey the exclusive fence,
> > because the buffer is pinned. None of the work that Maarten has done
> > has fundamentally changed this contract in any way.
>
> Well I now agree that the rules around dma_resv are different than I
> thought, but this change should have raised more eyebrows.
>
> The problem is this completely broke interop with all drivers using TTM and
> I think might even explain some bug reports.
>
> I re-introduced the moving fence by adding bo->moving a few years after the
> initial introduction of dma_resv, but that was just to work around
> performance problems introduced by using the exclusive fence for both use
> cases.
Ok that part is indeed not something I've known.
> > If amdgpu (or any other ttm based driver) hands back and sgt without
> > waiting for ttm_bo->moving or the exclusive fence first, then that's a
> > bug we need to fix in these drivers. But if ttm based drivers did get
> > this wrong, then they got this wrong both before and after the switch
> > over to using dma_resv - this bug would go back all the way to Dave's
> > introduction of drm_prime.c and support for that.
>
> I'm not 100% sure, but I think before the switch to the dma_resv object
> drivers just waited for the BOs to become idle and that should have
> prevented this.
>
> Anyway let's stop discussing history and move forward. Sending patches for
> all affected TTM driver with CC: stable tags in a minute.
>
>
> > The only thing which importers have to do is not wreak the DAG nature
> > of the dma_resv fences and drop dependencies. Currently there's a
> > handful of drivers which break this (introduced over the last few
> > years), and I have it somewhere on my todo list to audit them all.
>
> Please give that some priority.
>
> Ignoring the moving fence is a information leak, but messing up the DAG
> gives you access to freed up memory.
Yeah will try to. I've also been hung up a bit on how to fix that, but I
think just closing the DAG-breakage is simplest. Any userspace which then
complains about the additional sync that causes would then be motivated to
look into the import ioctl Jason has. And I think the impact in practice
should be minimal, aside from some corner cases.
> > The goal with extracting dma_resv from ttm was to make implicit sync
> > working and get rid of some terrible stalls on the userspace side.
> > Eventually it was also the goal to make truly dynamic buffer
> > reservation possible, but that took another 6 or so years to realize
> > with your work. And we had to make dynamic dma-buf very much opt-in,
> > because auditing all the users is very hard work and no one
> > volunteered. And for dynamic dma-buf the rule is that the exclusive
> > fence must _never_ be ignored, and the two drivers supporting it (mlx5
> > and amdgpu) obey that.
> >
> > So yeah for ttm drivers dma_resv is primarily for memory management,
> > with a side effect of also supporting implicit sync.
> >
> > For everyone else (and this includes a pile of render drivers, all the
> > atomic kms drivers, v4l and I have no idea what else on top) dma_resv
> > was only ever about implicit sync, and it can be ignored. And it (the
> > implicit sync side) has to be ignored to be able to support vulkan
> > winsys buffers correctly without stalling where we shouldn't. Also we
> > have to ignore it on atomic kms side too (and depending upon whether
> > writeback is supported atomic kms is perfectly capable of reading out
> >
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
Am 18.06.21 um 20:45 schrieb Daniel Vetter:
On Fri, Jun 18, 2021 at 8:02 PM Christian König
wrote:
Am 18.06.21 um 19:20 schrieb Daniel Vetter:
[SNIP]
The whole thing was introduced with this commit here:
commit f2c24b83ae90292d315aa7ac029c6ce7929e01aa
Author: Maarten Lankhorst
Date: Wed Apr 2 17:14:48 2014 +0200
drm/ttm: flip the switch, and convert to dma_fence
Signed-off-by: Maarten Lankhorst
int ttm_bo_move_accel_cleanup(struct ttm_buffer_object *bo,
- bo->sync_obj = driver->sync_obj_ref(sync_obj);
+ reservation_object_add_excl_fence(bo->resv, fence);
if (evict) {
Maarten replaced the bo->sync_obj reference with the dma_resv exclusive
fence.
This means that we need to apply the sync_obj semantic to all drivers
using a DMA-buf with its dma_resv object, otherwise you break imports
from TTM drivers.
Since then and up till now the exclusive fence must be waited on and
never replaced with anything which signals before the old fence.
Maarten and I think Thomas did that and I was always assuming that you
know about this design decision.
Surprisingly I do actually know this.
Still the commit you cite did _not_ change any of the rules around
dma_buf: Importers have _no_ obligation to obey the exclusive fence,
because the buffer is pinned. None of the work that Maarten has done
has fundamentally changed this contract in any way.
Well I now agree that the rules around dma_resv are different than I
thought, but this change should have raised more eyebrows.
The problem is this completely broke interop with all drivers using TTM
and I think might even explain some bug reports.
I re-introduced the moving fence by adding bo->moving a few years after
the initial introduction of dma_resv, but that was just to work around
performance problems introduced by using the exclusive fence for both
use cases.
If amdgpu (or any other ttm based driver) hands back and sgt without
waiting for ttm_bo->moving or the exclusive fence first, then that's a
bug we need to fix in these drivers. But if ttm based drivers did get
this wrong, then they got this wrong both before and after the switch
over to using dma_resv - this bug would go back all the way to Dave's
introduction of drm_prime.c and support for that.
I'm not 100% sure, but I think before the switch to the dma_resv object
drivers just waited for the BOs to become idle and that should have
prevented this.
Anyway let's stop discussing history and move forward. Sending patches
for all affected TTM driver with CC: stable tags in a minute.
The only thing which importers have to do is not wreak the DAG nature
of the dma_resv fences and drop dependencies. Currently there's a
handful of drivers which break this (introduced over the last few
years), and I have it somewhere on my todo list to audit them all.
Please give that some priority.
Ignoring the moving fence is a information leak, but messing up the DAG
gives you access to freed up memory.
The goal with extracting dma_resv from ttm was to make implicit sync
working and get rid of some terrible stalls on the userspace side.
Eventually it was also the goal to make truly dynamic buffer
reservation possible, but that took another 6 or so years to realize
with your work. And we had to make dynamic dma-buf very much opt-in,
because auditing all the users is very hard work and no one
volunteered. And for dynamic dma-buf the rule is that the exclusive
fence must _never_ be ignored, and the two drivers supporting it (mlx5
and amdgpu) obey that.
So yeah for ttm drivers dma_resv is primarily for memory management,
with a side effect of also supporting implicit sync.
For everyone else (and this includes a pile of render drivers, all the
atomic kms drivers, v4l and I have no idea what else on top) dma_resv
was only ever about implicit sync, and it can be ignored. And it (the
implicit sync side) has to be ignored to be able to support vulkan
winsys buffers correctly without stalling where we shouldn't. Also we
have to ignore it on atomic kms side too (and depending upon whether
writeback is supported atomic kms is perfectly capable of reading out
any buffer passed to it).
Oh! That might actually explain some issues, but that just completely
breaks when TTM based drivers use atomic.
In other words for the first use is actually rather likely for TTM based
drivers to need to move the buffer around so that scanout is possible.
And that in turn means you need to wait for this move to finish even if
you have an explicit fence to wait for. IIRC amdgpu rolled its own
implementation of this and radeon doesn't have atomic, but nouveau is
most like broken.
So we do need a better solution for this sooner or later.
It's absolutely not that this is my invention, I'm just telling you how
it ever was.
Anyway this means we have a seriously misunderstanding and yes now some
of our discussions about dynamic P2P suddenly make much more sense.
Yeah I think
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
Hi Daniel, thanks for jumping in here. And yes, you are absolutely right we need to get this fixed and not yell at each other that we have a different understanding of things. Your proposal sounds sane to me, but I wouldn't call it slots. Rather something like "use cases" since we can have multiple fences for each category I think. And I see at four here: 1. Internal kernel memory management. Everybody needs to wait for this, it's equal to bo->moving. 2. Writers for implicit sync, implicit sync readers should wait for them. 3. Readers for implicit sync, implicit sync writers should wait for them. 4. Things like TLB flushes and page table updates, no implicit sync but memory management must take them into account before moving/freeing backing store. Happy weekend and hopefully not so much heat guys. Cheers, Christian. Am 18.06.21 um 20:20 schrieb Daniel Stone: Sorry for the mobile reply, but V4L2 is absolutely not write-only; there has never been an intersection of V4L2 supporting dmabuf and not supporting reads. I see your point about the heritage of dma_resv but it’s a red herring. It doesn’t matter who’s right, or who was first, or where the code was extracted from. It’s well defined that amdgpu defines resv to be one thing, that every other non-TTM user defines it to be something very different, and that the other TTM users define it to be something in the middle. We’ll never get to anything workable if we keep arguing who’s right. Everyone is wrong, because dma_resv doesn’t globally mean anything. It seems clear that there are three classes of synchronisation barrier (not using the ‘f’ word here), in descending exclusion order: - memory management barriers (amdgpu exclusive fence / ttm_bo->moving) - implicit synchronisation write barriers (everyone else’s exclusive fences, amdgpu’s shared fences) - implicit synchronisation read barriers (everyone else’s shared fences, also amdgpu’s shared fences sometimes) I don’t see a world in which these three uses can be reduced to two slots. What also isn’t clear to me though, is how the memory-management barriers can exclude all other access in the original proposal with purely userspace CS. Retaining the three separate modes also seems like a hard requirement to not completely break userspace, but then I don’t see how three separate slots would work if they need to be temporally ordered. amdgpu fixed this by redefining the meaning of the two slots, others fixed this by not doing one of the three modes. So how do we square the circle without encoding a DAG into the kernel? Do the two slots need to become a single list which is ordered by time + ‘weight’ and flattened whenever modified? Something else? Have a great weekend. -d On 18 Jun 2021, at 5:43 pm, Christian König wrote: Am 18.06.21 um 17:17 schrieb Daniel Vetter: [SNIP] Ignoring _all_ fences is officially ok for pinned dma-buf. This is what v4l does. Aside from it's definitely not just i915 that does this even on the drm side, we have a few more drivers nowadays. No it seriously isn't. If drivers are doing this they are more than broken. See the comment in dma-resv.h * Based on bo.c which bears the following copyright notice, * but is dual licensed: The handling in ttm_bo.c is and always was that the exclusive fence is used for buffer moves. As I said multiple times now the *MAIN* purpose of the dma_resv object is memory management and *NOT* synchronization. Those restrictions come from the original design of TTM where the dma_resv object originated from. The resulting consequences are that: a) If you access the buffer without waiting for the exclusive fence you run into a potential information leak. We kind of let that slip for V4L since they only access the buffers for writes, so you can't do any harm there. b) If you overwrite the exclusive fence with a new one without waiting for the old one to signal you open up the possibility for userspace to access freed up memory. This is a complete show stopper since it means that taking over the system is just a typing exercise. What you have done by allowing this in is ripping open a major security hole for any DMA-buf import in i915 from all TTM based driver. This needs to be fixed ASAP, either by waiting in i915 and all other drivers doing this for the exclusive fence while importing a DMA-buf or by marking i915 and all other drivers as broken. Sorry, but if you allowed that in you seriously have no idea what you are talking about here and where all of this originated from. Regards, Christian. ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
Am 18.06.21 um 19:20 schrieb Daniel Vetter:
On Fri, Jun 18, 2021 at 6:43 PM Christian König
wrote:
Am 18.06.21 um 17:17 schrieb Daniel Vetter:
[SNIP]
Ignoring _all_ fences is officially ok for pinned dma-buf. This is
what v4l does. Aside from it's definitely not just i915 that does this
even on the drm side, we have a few more drivers nowadays.
No it seriously isn't. If drivers are doing this they are more than broken.
See the comment in dma-resv.h
* Based on bo.c which bears the following copyright notice,
* but is dual licensed:
The handling in ttm_bo.c is and always was that the exclusive fence is
used for buffer moves.
As I said multiple times now the *MAIN* purpose of the dma_resv object
is memory management and *NOT* synchronization.
Those restrictions come from the original design of TTM where the
dma_resv object originated from.
The resulting consequences are that:
a) If you access the buffer without waiting for the exclusive fence you
run into a potential information leak.
We kind of let that slip for V4L since they only access the buffers
for writes, so you can't do any harm there.
b) If you overwrite the exclusive fence with a new one without waiting
for the old one to signal you open up the possibility for userspace to
access freed up memory.
This is a complete show stopper since it means that taking over the
system is just a typing exercise.
What you have done by allowing this in is ripping open a major security
hole for any DMA-buf import in i915 from all TTM based driver.
This needs to be fixed ASAP, either by waiting in i915 and all other
drivers doing this for the exclusive fence while importing a DMA-buf or
by marking i915 and all other drivers as broken.
Sorry, but if you allowed that in you seriously have no idea what you
are talking about here and where all of this originated from.
Dude, get a grip, seriously. dma-buf landed in 2011
commit d15bd7ee445d0702ad801fdaece348fdb79e6581
Author: Sumit Semwal
Date: Mon Dec 26 14:53:15 2011 +0530
dma-buf: Introduce dma buffer sharing mechanism
and drm prime landed in the same year
commit 3248877ea1796915419fba7c89315fdbf00cb56a
(airlied/drm-prime-dmabuf-initial)
Author: Dave Airlie
Date: Fri Nov 25 15:21:02 2011 +
drm: base prime/dma-buf support (v5)
dma-resv was extracted much later
commit 786d7257e537da0674c02e16e3b30a44665d1cee
Author: Maarten Lankhorst
Date: Thu Jun 27 13:48:16 2013 +0200
reservation: cross-device reservation support, v4
Maarten's patch only extracted the dma_resv stuff so it's there,
optionally. There was never any effort to roll this out to all the
existing drivers, of which there were plenty.
It is, and has been since 10 years, totally fine to access dma-buf
without looking at any fences at all. From your pov of a ttm driver
dma-resv is mainly used for memory management and not sync, but I
think that's also due to some reinterpretation of the actual sync
rules on your side. For everyone else the dma_resv attached to a
dma-buf has been about implicit sync only, nothing else.
No, that was way before my time.
The whole thing was introduced with this commit here:
commit f2c24b83ae90292d315aa7ac029c6ce7929e01aa
Author: Maarten Lankhorst
Date: Wed Apr 2 17:14:48 2014 +0200
drm/ttm: flip the switch, and convert to dma_fence
Signed-off-by: Maarten Lankhorst
int ttm_bo_move_accel_cleanup(struct ttm_buffer_object *bo,
- bo->sync_obj = driver->sync_obj_ref(sync_obj);
+ reservation_object_add_excl_fence(bo->resv, fence);
if (evict) {
Maarten replaced the bo->sync_obj reference with the dma_resv exclusive
fence.
This means that we need to apply the sync_obj semantic to all drivers
using a DMA-buf with its dma_resv object, otherwise you break imports
from TTM drivers.
Since then and up till now the exclusive fence must be waited on and
never replaced with anything which signals before the old fence.
Maarten and I think Thomas did that and I was always assuming that you
know about this design decision.
It's absolutely not that this is my invention, I'm just telling you how
it ever was.
Anyway this means we have a seriously misunderstanding and yes now some
of our discussions about dynamic P2P suddenly make much more sense.
Regards,
Christian.
_only_ when you have a dynamic importer/exporter can you assume that
the dma_resv fences must actually be obeyed. That's one of the reasons
why we had to make this a completely new mode (the other one was
locking, but they really tie together).
Wrt your problems:
a) needs to be fixed in drivers exporting buffers and failing to make
sure the memory is there by the time dma_buf_map_attachment returns.
b) needs to be fixed in the importers, and there's quite a few of
those. There's more than i915 here, which is why I think we should
have the dma_resv_add_shared_exclusive helper extracted from amdgpu.
Avoids hand-rolling this about 5 times (6 if we include
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
Am 18.06.21 um 17:17 schrieb Daniel Vetter: [SNIP] Ignoring _all_ fences is officially ok for pinned dma-buf. This is what v4l does. Aside from it's definitely not just i915 that does this even on the drm side, we have a few more drivers nowadays. No it seriously isn't. If drivers are doing this they are more than broken. See the comment in dma-resv.h * Based on bo.c which bears the following copyright notice, * but is dual licensed: The handling in ttm_bo.c is and always was that the exclusive fence is used for buffer moves. As I said multiple times now the *MAIN* purpose of the dma_resv object is memory management and *NOT* synchronization. Those restrictions come from the original design of TTM where the dma_resv object originated from. The resulting consequences are that: a) If you access the buffer without waiting for the exclusive fence you run into a potential information leak. We kind of let that slip for V4L since they only access the buffers for writes, so you can't do any harm there. b) If you overwrite the exclusive fence with a new one without waiting for the old one to signal you open up the possibility for userspace to access freed up memory. This is a complete show stopper since it means that taking over the system is just a typing exercise. What you have done by allowing this in is ripping open a major security hole for any DMA-buf import in i915 from all TTM based driver. This needs to be fixed ASAP, either by waiting in i915 and all other drivers doing this for the exclusive fence while importing a DMA-buf or by marking i915 and all other drivers as broken. Sorry, but if you allowed that in you seriously have no idea what you are talking about here and where all of this originated from. Regards, Christian. ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
On Fri, Jun 18, 2021 at 8:02 PM Christian König
wrote:
>
> Am 18.06.21 um 19:20 schrieb Daniel Vetter:
> > On Fri, Jun 18, 2021 at 6:43 PM Christian König
> > wrote:
> >> Am 18.06.21 um 17:17 schrieb Daniel Vetter:
> >>> [SNIP]
> >>> Ignoring _all_ fences is officially ok for pinned dma-buf. This is
> >>> what v4l does. Aside from it's definitely not just i915 that does this
> >>> even on the drm side, we have a few more drivers nowadays.
> >> No it seriously isn't. If drivers are doing this they are more than broken.
> >>
> >> See the comment in dma-resv.h
> >>
> >>* Based on bo.c which bears the following copyright notice,
> >>* but is dual licensed:
> >>
> >>
> >>
> >> The handling in ttm_bo.c is and always was that the exclusive fence is
> >> used for buffer moves.
> >>
> >> As I said multiple times now the *MAIN* purpose of the dma_resv object
> >> is memory management and *NOT* synchronization.
> >>
> >> Those restrictions come from the original design of TTM where the
> >> dma_resv object originated from.
> >>
> >> The resulting consequences are that:
> >>
> >> a) If you access the buffer without waiting for the exclusive fence you
> >> run into a potential information leak.
> >> We kind of let that slip for V4L since they only access the buffers
> >> for writes, so you can't do any harm there.
> >>
> >> b) If you overwrite the exclusive fence with a new one without waiting
> >> for the old one to signal you open up the possibility for userspace to
> >> access freed up memory.
> >> This is a complete show stopper since it means that taking over the
> >> system is just a typing exercise.
> >>
> >>
> >> What you have done by allowing this in is ripping open a major security
> >> hole for any DMA-buf import in i915 from all TTM based driver.
> >>
> >> This needs to be fixed ASAP, either by waiting in i915 and all other
> >> drivers doing this for the exclusive fence while importing a DMA-buf or
> >> by marking i915 and all other drivers as broken.
> >>
> >> Sorry, but if you allowed that in you seriously have no idea what you
> >> are talking about here and where all of this originated from.
> > Dude, get a grip, seriously. dma-buf landed in 2011
> >
> > commit d15bd7ee445d0702ad801fdaece348fdb79e6581
> > Author: Sumit Semwal
> > Date: Mon Dec 26 14:53:15 2011 +0530
> >
> > dma-buf: Introduce dma buffer sharing mechanism
> >
> > and drm prime landed in the same year
> >
> > commit 3248877ea1796915419fba7c89315fdbf00cb56a
> > (airlied/drm-prime-dmabuf-initial)
> > Author: Dave Airlie
> > Date: Fri Nov 25 15:21:02 2011 +
> >
> > drm: base prime/dma-buf support (v5)
> >
> > dma-resv was extracted much later
> >
> > commit 786d7257e537da0674c02e16e3b30a44665d1cee
> > Author: Maarten Lankhorst
> > Date: Thu Jun 27 13:48:16 2013 +0200
> >
> > reservation: cross-device reservation support, v4
> >
> > Maarten's patch only extracted the dma_resv stuff so it's there,
> > optionally. There was never any effort to roll this out to all the
> > existing drivers, of which there were plenty.
> >
> > It is, and has been since 10 years, totally fine to access dma-buf
> > without looking at any fences at all. From your pov of a ttm driver
> > dma-resv is mainly used for memory management and not sync, but I
> > think that's also due to some reinterpretation of the actual sync
> > rules on your side. For everyone else the dma_resv attached to a
> > dma-buf has been about implicit sync only, nothing else.
>
> No, that was way before my time.
>
> The whole thing was introduced with this commit here:
>
> commit f2c24b83ae90292d315aa7ac029c6ce7929e01aa
> Author: Maarten Lankhorst
> Date: Wed Apr 2 17:14:48 2014 +0200
>
> drm/ttm: flip the switch, and convert to dma_fence
>
> Signed-off-by: Maarten Lankhorst
>
> int ttm_bo_move_accel_cleanup(struct ttm_buffer_object *bo,
>
> - bo->sync_obj = driver->sync_obj_ref(sync_obj);
> + reservation_object_add_excl_fence(bo->resv, fence);
> if (evict) {
>
> Maarten replaced the bo->sync_obj reference with the dma_resv exclusive
> fence.
>
> This means that we need to apply the sync_obj semantic to all drivers
> using a DMA-buf with its dma_resv object, otherwise you break imports
> from TTM drivers.
>
> Since then and up till now the exclusive fence must be waited on and
> never replaced with anything which signals before the old fence.
>
> Maarten and I think Thomas did that and I was always assuming that you
> know about this design decision.
Surprisingly I do actually know this.
Still the commit you cite did _not_ change any of the rules around
dma_buf: Importers have _no_ obligation to obey the exclusive fence,
because the buffer is pinned. None of the work that Maarten has done
has fundamentally changed this contract in any way.
If amdgpu (or any other ttm based driver) hands back and sgt without
waiting for ttm_bo->moving or the exclusive fence first, then that's a
bug we need to fix
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
Am 18.06.21 um 16:31 schrieb Daniel Vetter: [SNIP] And that drivers choose to ignore the exclusive fence is an absolutely no-go from a memory management and security point of view. Exclusive access means exclusive access. Ignoring that won't work. Yeah, this is why I've been going all over the place about lifting ttm_bo->moving to dma_resv. And also that I flat out don't trust your audit, if you havent found these drivers then very clearly you didn't audit much at all :-) I just didn't though that anybody could be so stupid to allow such a thing in. The only thing which saved us so far is the fact that drivers doing this are not that complex. BTW: How does it even work? I mean then you would run into the same problem as amdgpu with its page table update fences, e.g. that your shared fences might signal before the exclusive one. So we don't ignore any fences when we rip out the backing storage. And yes there's currently a bug in all these drivers that if you set both the "ignore implicit fences" and the "set the exclusive fence" flag, then we just break this. Which is why I think we want to have a dma_fence_add_shared_exclusive() helper extracted from your amdgpu code, which we can then use everywhere to plug this. Daniel are you realizing what you are talking about here? Does that also apply for imported DMA-bufs? If yes than that is a security hole you can push an elephant through. Can you point me to the code using that? For dma-buf this isn't actually a problem, because dma-buf are pinned. You can't move them while other drivers are using them, hence there's not actually a ttm_bo->moving fence we can ignore. p2p dma-buf aka dynamic dma-buf is a different beast, and i915 (and fwiw these other drivers) need to change before they can do dynamic dma-buf. Otherwise we have an information leak worth a CVE and that is certainly not something we want. Because yes otherwise we get a CVE. But right now I don't think we have one. Yeah, agree. But this is just because of coincident and not because of good engineering :) Well the good news is that I think we're now talking slightly less past each another than the past few weeks :-) We do have a quite big confusion on what exactly the signaling ordering is supposed to be between exclusive and the collective set of shared fences, and there's some unifying that needs to happen here. But I think what Jason implements here in the import ioctl is the most defensive version possible, so really can't break any driver. It really works like you have an ad-hoc gpu engine that does nothing itself, but waits for the current exclusive fence and then sets the exclusive fence with its "CS" completion fence. That's imo perfectly legit use-case. The use case is certainly legit, but I'm not sure if merging this at the moment is a good idea. Your note that drivers are already ignoring the exclusive fence in the dma_resv object was eye opening to me. And I now have the very strong feeling that the synchronization and the design of the dma_resv object is even more messy then I thought it is. To summarize we can be really lucky that it didn't blow up into our faces already. I don't think there was that much luck involved (ok I did find a possible bug in i915 already around cpu cache flushing) - for SoC the exclusive slot in dma_resv really is only used for implicit sync and nothing else. The fun only starts when you throw in pipelined backing storage movement. I guess this also explains why you just seemed to ignore me when I was asking for a memory management exclusive fence for the p2p stuff, or some other way to specifically handling movements (like ttm_bo->moving or whatever it is). From my pov we clearly needed that to make p2p dma-buf work well enough, mixing up the memory management exclusive slot with the implicit sync exclusive slot never looked like a bright idea to me. I think at least we now have some understanding here. Well to be honest what you have just told me means that i915 is seriously broken. Ignoring the exclusive fence on an imported DMA-buf is an absolutely *NO-GO* even without P2P. What you have stitched together here allows anybody to basically read any memory on the system with both i915 and nouveau, radeon or amdgpu. We need to fix that ASAP! Regards, Christian. Same for the export one. Waiting for a previous snapshot of implicit fences is imo perfectly ok use-case and useful for compositors - client might soon start more rendering, and on some drivers that always results in the exclusive slot being set, so if you dont take a snapshot you oversync real bad for your atomic flip. The export use case is unproblematic as far as I can see. Those changes are years in the past. If we have a real problem here (not sure on that yet), then we'll have to figure out how to fix it without nuking uAPI. Well, that was the basic idea of attaching flags to the fences in the dma_resv object. In other words you clearly
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
Sorry for the mobile reply, but V4L2 is absolutely not write-only; there has never been an intersection of V4L2 supporting dmabuf and not supporting reads. I see your point about the heritage of dma_resv but it’s a red herring. It doesn’t matter who’s right, or who was first, or where the code was extracted from. It’s well defined that amdgpu defines resv to be one thing, that every other non-TTM user defines it to be something very different, and that the other TTM users define it to be something in the middle. We’ll never get to anything workable if we keep arguing who’s right. Everyone is wrong, because dma_resv doesn’t globally mean anything. It seems clear that there are three classes of synchronisation barrier (not using the ‘f’ word here), in descending exclusion order: - memory management barriers (amdgpu exclusive fence / ttm_bo->moving) - implicit synchronisation write barriers (everyone else’s exclusive fences, amdgpu’s shared fences) - implicit synchronisation read barriers (everyone else’s shared fences, also amdgpu’s shared fences sometimes) I don’t see a world in which these three uses can be reduced to two slots. What also isn’t clear to me though, is how the memory-management barriers can exclude all other access in the original proposal with purely userspace CS. Retaining the three separate modes also seems like a hard requirement to not completely break userspace, but then I don’t see how three separate slots would work if they need to be temporally ordered. amdgpu fixed this by redefining the meaning of the two slots, others fixed this by not doing one of the three modes. So how do we square the circle without encoding a DAG into the kernel? Do the two slots need to become a single list which is ordered by time + ‘weight’ and flattened whenever modified? Something else? Have a great weekend. -d > On 18 Jun 2021, at 5:43 pm, Christian König wrote: > > Am 18.06.21 um 17:17 schrieb Daniel Vetter: >> [SNIP] >> Ignoring _all_ fences is officially ok for pinned dma-buf. This is >> what v4l does. Aside from it's definitely not just i915 that does this >> even on the drm side, we have a few more drivers nowadays. > > No it seriously isn't. If drivers are doing this they are more than broken. > > See the comment in dma-resv.h > > * Based on bo.c which bears the following copyright notice, > * but is dual licensed: > > > > The handling in ttm_bo.c is and always was that the exclusive fence is used > for buffer moves. > > As I said multiple times now the *MAIN* purpose of the dma_resv object is > memory management and *NOT* synchronization. > > Those restrictions come from the original design of TTM where the dma_resv > object originated from. > > The resulting consequences are that: > > a) If you access the buffer without waiting for the exclusive fence you run > into a potential information leak. > We kind of let that slip for V4L since they only access the buffers for > writes, so you can't do any harm there. > > b) If you overwrite the exclusive fence with a new one without waiting for > the old one to signal you open up the possibility for userspace to access > freed up memory. > This is a complete show stopper since it means that taking over the > system is just a typing exercise. > > > What you have done by allowing this in is ripping open a major security hole > for any DMA-buf import in i915 from all TTM based driver. > > This needs to be fixed ASAP, either by waiting in i915 and all other drivers > doing this for the exclusive fence while importing a DMA-buf or by marking > i915 and all other drivers as broken. > > Sorry, but if you allowed that in you seriously have no idea what you are > talking about here and where all of this originated from. > > Regards, > Christian. ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
On Fri, Jun 18, 2021 at 6:43 PM Christian König wrote: > > Am 18.06.21 um 17:17 schrieb Daniel Vetter: > > [SNIP] > > Ignoring _all_ fences is officially ok for pinned dma-buf. This is > > what v4l does. Aside from it's definitely not just i915 that does this > > even on the drm side, we have a few more drivers nowadays. > > No it seriously isn't. If drivers are doing this they are more than broken. > > See the comment in dma-resv.h > > * Based on bo.c which bears the following copyright notice, > * but is dual licensed: > > > > The handling in ttm_bo.c is and always was that the exclusive fence is > used for buffer moves. > > As I said multiple times now the *MAIN* purpose of the dma_resv object > is memory management and *NOT* synchronization. > > Those restrictions come from the original design of TTM where the > dma_resv object originated from. > > The resulting consequences are that: > > a) If you access the buffer without waiting for the exclusive fence you > run into a potential information leak. > We kind of let that slip for V4L since they only access the buffers > for writes, so you can't do any harm there. > > b) If you overwrite the exclusive fence with a new one without waiting > for the old one to signal you open up the possibility for userspace to > access freed up memory. > This is a complete show stopper since it means that taking over the > system is just a typing exercise. > > > What you have done by allowing this in is ripping open a major security > hole for any DMA-buf import in i915 from all TTM based driver. > > This needs to be fixed ASAP, either by waiting in i915 and all other > drivers doing this for the exclusive fence while importing a DMA-buf or > by marking i915 and all other drivers as broken. > > Sorry, but if you allowed that in you seriously have no idea what you > are talking about here and where all of this originated from. Dude, get a grip, seriously. dma-buf landed in 2011 commit d15bd7ee445d0702ad801fdaece348fdb79e6581 Author: Sumit Semwal Date: Mon Dec 26 14:53:15 2011 +0530 dma-buf: Introduce dma buffer sharing mechanism and drm prime landed in the same year commit 3248877ea1796915419fba7c89315fdbf00cb56a (airlied/drm-prime-dmabuf-initial) Author: Dave Airlie Date: Fri Nov 25 15:21:02 2011 + drm: base prime/dma-buf support (v5) dma-resv was extracted much later commit 786d7257e537da0674c02e16e3b30a44665d1cee Author: Maarten Lankhorst Date: Thu Jun 27 13:48:16 2013 +0200 reservation: cross-device reservation support, v4 Maarten's patch only extracted the dma_resv stuff so it's there, optionally. There was never any effort to roll this out to all the existing drivers, of which there were plenty. It is, and has been since 10 years, totally fine to access dma-buf without looking at any fences at all. From your pov of a ttm driver dma-resv is mainly used for memory management and not sync, but I think that's also due to some reinterpretation of the actual sync rules on your side. For everyone else the dma_resv attached to a dma-buf has been about implicit sync only, nothing else. _only_ when you have a dynamic importer/exporter can you assume that the dma_resv fences must actually be obeyed. That's one of the reasons why we had to make this a completely new mode (the other one was locking, but they really tie together). Wrt your problems: a) needs to be fixed in drivers exporting buffers and failing to make sure the memory is there by the time dma_buf_map_attachment returns. b) needs to be fixed in the importers, and there's quite a few of those. There's more than i915 here, which is why I think we should have the dma_resv_add_shared_exclusive helper extracted from amdgpu. Avoids hand-rolling this about 5 times (6 if we include the import ioctl from Jason). Also I've like been trying to explain this ever since the entire dynamic dma-buf thing started. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
On Fri, Jun 18, 2021 at 4:42 PM Christian König wrote: > > Am 18.06.21 um 16:31 schrieb Daniel Vetter: > > [SNIP] > >> And that drivers choose to ignore the exclusive fence is an absolutely > >> no-go from a memory management and security point of view. Exclusive > >> access means exclusive access. Ignoring that won't work. > > Yeah, this is why I've been going all over the place about lifting > > ttm_bo->moving to dma_resv. And also that I flat out don't trust your > > audit, if you havent found these drivers then very clearly you didn't > > audit much at all :-) > > I just didn't though that anybody could be so stupid to allow such a > thing in. > > >> The only thing which saved us so far is the fact that drivers doing this > >> are not that complex. > >> > >> BTW: How does it even work? I mean then you would run into the same > >> problem as amdgpu with its page table update fences, e.g. that your > >> shared fences might signal before the exclusive one. > > So we don't ignore any fences when we rip out the backing storage. > > > > And yes there's currently a bug in all these drivers that if you set > > both the "ignore implicit fences" and the "set the exclusive fence" > > flag, then we just break this. Which is why I think we want to have a > > dma_fence_add_shared_exclusive() helper extracted from your amdgpu > > code, which we can then use everywhere to plug this. > > Daniel are you realizing what you are talking about here? Does that also > apply for imported DMA-bufs? > > If yes than that is a security hole you can push an elephant through. > > Can you point me to the code using that? > > >>> For dma-buf this isn't actually a problem, because dma-buf are pinned. You > >>> can't move them while other drivers are using them, hence there's not > >>> actually a ttm_bo->moving fence we can ignore. > >>> > >>> p2p dma-buf aka dynamic dma-buf is a different beast, and i915 (and fwiw > >>> these other drivers) need to change before they can do dynamic dma-buf. > >>> > Otherwise we have an information leak worth a CVE and that is certainly > not > something we want. > >>> Because yes otherwise we get a CVE. But right now I don't think we have > >>> one. > >> Yeah, agree. But this is just because of coincident and not because of > >> good engineering :) > > Well the good news is that I think we're now talking slightly less > > past each another than the past few weeks :-) > > > >>> We do have a quite big confusion on what exactly the signaling ordering is > >>> supposed to be between exclusive and the collective set of shared fences, > >>> and there's some unifying that needs to happen here. But I think what > >>> Jason implements here in the import ioctl is the most defensive version > >>> possible, so really can't break any driver. It really works like you have > >>> an ad-hoc gpu engine that does nothing itself, but waits for the current > >>> exclusive fence and then sets the exclusive fence with its "CS" completion > >>> fence. > >>> > >>> That's imo perfectly legit use-case. > >> The use case is certainly legit, but I'm not sure if merging this at the > >> moment is a good idea. > >> > >> Your note that drivers are already ignoring the exclusive fence in the > >> dma_resv object was eye opening to me. And I now have the very strong > >> feeling that the synchronization and the design of the dma_resv object > >> is even more messy then I thought it is. > >> > >> To summarize we can be really lucky that it didn't blow up into our > >> faces already. > > I don't think there was that much luck involved (ok I did find a > > possible bug in i915 already around cpu cache flushing) - for SoC the > > exclusive slot in dma_resv really is only used for implicit sync and > > nothing else. The fun only starts when you throw in pipelined backing > > storage movement. > > > > I guess this also explains why you just seemed to ignore me when I was > > asking for a memory management exclusive fence for the p2p stuff, or > > some other way to specifically handling movements (like ttm_bo->moving > > or whatever it is). From my pov we clearly needed that to make p2p > > dma-buf work well enough, mixing up the memory management exclusive > > slot with the implicit sync exclusive slot never looked like a bright > > idea to me. > > > > I think at least we now have some understanding here. > > Well to be honest what you have just told me means that i915 is > seriously broken. > > Ignoring the exclusive fence on an imported DMA-buf is an absolutely > *NO-GO* even without P2P. > > What you have stitched together here allows anybody to basically read > any memory on the system with both i915 and nouveau, radeon or amdgpu. > > We need to fix that ASAP! Ignoring _all_ fences is officially ok for pinned dma-buf. This is what v4l does. Aside from it's definitely not just i915 that does this even on the drm side, we have a few more drivers nowadays. The rules are that after you've called dma_buf_map_attachment the
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
On Fri, Jun 18, 2021 at 11:15 AM Christian König wrote: > > Am 17.06.21 um 21:58 schrieb Daniel Vetter: > > On Thu, Jun 17, 2021 at 09:37:36AM +0200, Christian König wrote: > >> [SNIP] > >>> But, to the broader point, maybe? I'm a little fuzzy on exactly where > >>> i915 inserts and/or depends on fences. > >>> > When you combine that with complex drivers which use TTM and buffer > moves underneath you can construct an information leak using this and > give userspace access to memory which is allocated to the driver, but > not yet initialized. > > This way you can leak things like page tables, passwords, kernel data > etc... in large amounts to userspace and is an absolutely no-go for > security. > >>> Ugh... Unfortunately, I'm really out of my depth on the implications > >>> going on here but I think I see your point. > >>> > That's why I'm said we need to get this fixed before we upstream this > patch set here and especially the driver change which is using that. > >>> Well, i915 has had uAPI for a while to ignore fences. > >> Yeah, exactly that's illegal. > > You're a few years too late with closing that barn door. The following > > drives have this concept > > - i915 > > - msm > > - etnaviv > > > > Because you can't write a competent vulkan driver without this. > > WHAT? ^^ > > > This was discussed at absolute epic length in various xdcs iirc. We did > > ignore a > > bit the vram/ttm/bo-moving problem because all the people present were > > hacking on integrated gpu (see list above), but that just means we need to > > treat the ttm_bo->moving fence properly. > > I should have visited more XDCs in the past, the problem is much larger > than this. > > But I now start to understand what you are doing with that design and > why it looks so messy to me, amdgpu is just currently the only driver > which does Vulkan and complex memory management at the same time. > > >> At least the kernel internal fences like moving or clearing a buffer object > >> needs to be taken into account before a driver is allowed to access a > >> buffer. > > Yes i915 needs to make sure it never ignores ttm_bo->moving. > > No, that is only the tip of the iceberg. See TTM for example also puts > fences which drivers needs to wait for into the shared slots. Same thing > for use cases like clear on release etc > > From my point of view the main purpose of the dma_resv object is to > serve memory management, synchronization for command submission is just > a secondary use case. > > And that drivers choose to ignore the exclusive fence is an absolutely > no-go from a memory management and security point of view. Exclusive > access means exclusive access. Ignoring that won't work. Yeah, this is why I've been going all over the place about lifting ttm_bo->moving to dma_resv. And also that I flat out don't trust your audit, if you havent found these drivers then very clearly you didn't audit much at all :-) > The only thing which saved us so far is the fact that drivers doing this > are not that complex. > > BTW: How does it even work? I mean then you would run into the same > problem as amdgpu with its page table update fences, e.g. that your > shared fences might signal before the exclusive one. So we don't ignore any fences when we rip out the backing storage. And yes there's currently a bug in all these drivers that if you set both the "ignore implicit fences" and the "set the exclusive fence" flag, then we just break this. Which is why I think we want to have a dma_fence_add_shared_exclusive() helper extracted from your amdgpu code, which we can then use everywhere to plug this. > > For dma-buf this isn't actually a problem, because dma-buf are pinned. You > > can't move them while other drivers are using them, hence there's not > > actually a ttm_bo->moving fence we can ignore. > > > > p2p dma-buf aka dynamic dma-buf is a different beast, and i915 (and fwiw > > these other drivers) need to change before they can do dynamic dma-buf. > > > >> Otherwise we have an information leak worth a CVE and that is certainly not > >> something we want. > > Because yes otherwise we get a CVE. But right now I don't think we have > > one. > > Yeah, agree. But this is just because of coincident and not because of > good engineering :) Well the good news is that I think we're now talking slightly less past each another than the past few weeks :-) > > We do have a quite big confusion on what exactly the signaling ordering is > > supposed to be between exclusive and the collective set of shared fences, > > and there's some unifying that needs to happen here. But I think what > > Jason implements here in the import ioctl is the most defensive version > > possible, so really can't break any driver. It really works like you have > > an ad-hoc gpu engine that does nothing itself, but waits for the current > > exclusive fence and then sets the exclusive fence with its "CS" completion > > fence. >
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
On Fri, Jun 18, 2021 at 4:15 AM Christian König wrote: > > Am 17.06.21 um 21:58 schrieb Daniel Vetter: > > On Thu, Jun 17, 2021 at 09:37:36AM +0200, Christian König wrote: > >> [SNIP] > >>> But, to the broader point, maybe? I'm a little fuzzy on exactly where > >>> i915 inserts and/or depends on fences. > >>> > When you combine that with complex drivers which use TTM and buffer > moves underneath you can construct an information leak using this and > give userspace access to memory which is allocated to the driver, but > not yet initialized. > > This way you can leak things like page tables, passwords, kernel data > etc... in large amounts to userspace and is an absolutely no-go for > security. > >>> Ugh... Unfortunately, I'm really out of my depth on the implications > >>> going on here but I think I see your point. > >>> > That's why I'm said we need to get this fixed before we upstream this > patch set here and especially the driver change which is using that. > >>> Well, i915 has had uAPI for a while to ignore fences. > >> Yeah, exactly that's illegal. > > You're a few years too late with closing that barn door. The following > > drives have this concept > > - i915 > > - msm > > - etnaviv > > > > Because you can't write a competent vulkan driver without this. > > WHAT? ^^ I think it's fair to say that you can't write a competent Vulkan driver with implicit sync getting in the way. Since AMD removes all the implicit sync internally, this solves most of the problems there. RADV does suffer some heartache around WSI which is related but I'd hardly say that makes it incompetent. > > This was discussed at absolute epic length in various xdcs iirc. We did > > ignore a > > bit the vram/ttm/bo-moving problem because all the people present were > > hacking on integrated gpu (see list above), but that just means we need to > > treat the ttm_bo->moving fence properly. > > I should have visited more XDCs in the past, the problem is much larger > than this. > > But I now start to understand what you are doing with that design and > why it looks so messy to me, amdgpu is just currently the only driver > which does Vulkan and complex memory management at the same time. I'm reading "complex memory management" here and elsewhere as "has VRAM". All memory management is complex; shuffling to/from VRAM just adds more layers. > >> At least the kernel internal fences like moving or clearing a buffer object > >> needs to be taken into account before a driver is allowed to access a > >> buffer. > > Yes i915 needs to make sure it never ignores ttm_bo->moving. > > No, that is only the tip of the iceberg. See TTM for example also puts > fences which drivers needs to wait for into the shared slots. Same thing > for use cases like clear on release etc > > From my point of view the main purpose of the dma_resv object is to > serve memory management, synchronization for command submission is just > a secondary use case. > > And that drivers choose to ignore the exclusive fence is an absolutely > no-go from a memory management and security point of view. Exclusive > access means exclusive access. Ignoring that won't work. > > The only thing which saved us so far is the fact that drivers doing this > are not that complex. I think there's something important in Daniel's list above with drivers that have a "no implicit sync uAPI": None of them are TTM based. We (i915) have been doing our own thing for memory management for a while and it may not follow your TTM mental model. Sure, there's a dma_resv in our BOs and we can import/export dma-buf but that doesn't mean that, internally, we think of it the same way. I say this in very generic terms because there are a whole lot of details that I don't know. What I do know is that, whatever we're doing, it's been pretty robust for many years. That said, we are moving to TTM so, if I'm right that this is a GEM <-> TTM conflict, we've got some thinking to do. > BTW: How does it even work? I mean then you would run into the same > problem as amdgpu with its page table update fences, e.g. that your > shared fences might signal before the exclusive one. > > > For dma-buf this isn't actually a problem, because dma-buf are pinned. You > > can't move them while other drivers are using them, hence there's not > > actually a ttm_bo->moving fence we can ignore. > > > > p2p dma-buf aka dynamic dma-buf is a different beast, and i915 (and fwiw > > these other drivers) need to change before they can do dynamic dma-buf. > > > >> Otherwise we have an information leak worth a CVE and that is certainly not > >> something we want. > > Because yes otherwise we get a CVE. But right now I don't think we have > > one. > > Yeah, agree. But this is just because of coincident and not because of > good engineering :) > > > We do have a quite big confusion on what exactly the signaling ordering is > > supposed to be between exclusive and the
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
Am 17.06.21 um 21:58 schrieb Daniel Vetter: On Thu, Jun 17, 2021 at 09:37:36AM +0200, Christian König wrote: [SNIP] But, to the broader point, maybe? I'm a little fuzzy on exactly where i915 inserts and/or depends on fences. When you combine that with complex drivers which use TTM and buffer moves underneath you can construct an information leak using this and give userspace access to memory which is allocated to the driver, but not yet initialized. This way you can leak things like page tables, passwords, kernel data etc... in large amounts to userspace and is an absolutely no-go for security. Ugh... Unfortunately, I'm really out of my depth on the implications going on here but I think I see your point. That's why I'm said we need to get this fixed before we upstream this patch set here and especially the driver change which is using that. Well, i915 has had uAPI for a while to ignore fences. Yeah, exactly that's illegal. You're a few years too late with closing that barn door. The following drives have this concept - i915 - msm - etnaviv Because you can't write a competent vulkan driver without this. WHAT? ^^ This was discussed at absolute epic length in various xdcs iirc. We did ignore a bit the vram/ttm/bo-moving problem because all the people present were hacking on integrated gpu (see list above), but that just means we need to treat the ttm_bo->moving fence properly. I should have visited more XDCs in the past, the problem is much larger than this. But I now start to understand what you are doing with that design and why it looks so messy to me, amdgpu is just currently the only driver which does Vulkan and complex memory management at the same time. At least the kernel internal fences like moving or clearing a buffer object needs to be taken into account before a driver is allowed to access a buffer. Yes i915 needs to make sure it never ignores ttm_bo->moving. No, that is only the tip of the iceberg. See TTM for example also puts fences which drivers needs to wait for into the shared slots. Same thing for use cases like clear on release etc From my point of view the main purpose of the dma_resv object is to serve memory management, synchronization for command submission is just a secondary use case. And that drivers choose to ignore the exclusive fence is an absolutely no-go from a memory management and security point of view. Exclusive access means exclusive access. Ignoring that won't work. The only thing which saved us so far is the fact that drivers doing this are not that complex. BTW: How does it even work? I mean then you would run into the same problem as amdgpu with its page table update fences, e.g. that your shared fences might signal before the exclusive one. For dma-buf this isn't actually a problem, because dma-buf are pinned. You can't move them while other drivers are using them, hence there's not actually a ttm_bo->moving fence we can ignore. p2p dma-buf aka dynamic dma-buf is a different beast, and i915 (and fwiw these other drivers) need to change before they can do dynamic dma-buf. Otherwise we have an information leak worth a CVE and that is certainly not something we want. Because yes otherwise we get a CVE. But right now I don't think we have one. Yeah, agree. But this is just because of coincident and not because of good engineering :) We do have a quite big confusion on what exactly the signaling ordering is supposed to be between exclusive and the collective set of shared fences, and there's some unifying that needs to happen here. But I think what Jason implements here in the import ioctl is the most defensive version possible, so really can't break any driver. It really works like you have an ad-hoc gpu engine that does nothing itself, but waits for the current exclusive fence and then sets the exclusive fence with its "CS" completion fence. That's imo perfectly legit use-case. The use case is certainly legit, but I'm not sure if merging this at the moment is a good idea. Your note that drivers are already ignoring the exclusive fence in the dma_resv object was eye opening to me. And I now have the very strong feeling that the synchronization and the design of the dma_resv object is even more messy then I thought it is. To summarize we can be really lucky that it didn't blow up into our faces already. Same for the export one. Waiting for a previous snapshot of implicit fences is imo perfectly ok use-case and useful for compositors - client might soon start more rendering, and on some drivers that always results in the exclusive slot being set, so if you dont take a snapshot you oversync real bad for your atomic flip. The export use case is unproblematic as far as I can see. Those changes are years in the past. If we have a real problem here (not sure on that yet), then we'll have to figure out how to fix it without nuking uAPI. Well, that was the basic idea of attaching flags to
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
On Thu, Jun 17, 2021 at 09:37:36AM +0200, Christian König wrote: > Am 16.06.21 um 20:30 schrieb Jason Ekstrand: > > On Tue, Jun 15, 2021 at 3:41 AM Christian König > > wrote: > > > Hi Jason & Daniel, > > > > > > maybe I should explain once more where the problem with this approach is > > > and why I think we need to get that fixed before we can do something > > > like this here. > > > > > > To summarize what this patch here does is that it copies the exclusive > > > fence and/or the shared fences into a sync_file. This alone is totally > > > unproblematic. > > > > > > The problem is what this implies. When you need to copy the exclusive > > > fence to a sync_file then this means that the driver is at some point > > > ignoring the exclusive fence on a buffer object. > > Not necessarily. Part of the point of this is to allow for CPU waits > > on a past point in buffers timeline. Today, we have poll() and > > GEM_WAIT both of which wait for the buffer to be idle from whatever > > GPU work is currently happening. We want to wait on something in the > > past and ignore anything happening now. > > Good point, yes that is indeed a valid use case. > > > But, to the broader point, maybe? I'm a little fuzzy on exactly where > > i915 inserts and/or depends on fences. > > > > > When you combine that with complex drivers which use TTM and buffer > > > moves underneath you can construct an information leak using this and > > > give userspace access to memory which is allocated to the driver, but > > > not yet initialized. > > > > > > This way you can leak things like page tables, passwords, kernel data > > > etc... in large amounts to userspace and is an absolutely no-go for > > > security. > > Ugh... Unfortunately, I'm really out of my depth on the implications > > going on here but I think I see your point. > > > > > That's why I'm said we need to get this fixed before we upstream this > > > patch set here and especially the driver change which is using that. > > Well, i915 has had uAPI for a while to ignore fences. > > Yeah, exactly that's illegal. You're a few years too late with closing that barn door. The following drives have this concept - i915 - msm - etnaviv Because you can't write a competent vulkan driver without this. This was discussed at absolute epic length in various xdcs iirc. We did ignore a bit the vram/ttm/bo-moving problem because all the people present were hacking on integrated gpu (see list above), but that just means we need to treat the ttm_bo->moving fence properly. > At least the kernel internal fences like moving or clearing a buffer object > needs to be taken into account before a driver is allowed to access a > buffer. Yes i915 needs to make sure it never ignores ttm_bo->moving. For dma-buf this isn't actually a problem, because dma-buf are pinned. You can't move them while other drivers are using them, hence there's not actually a ttm_bo->moving fence we can ignore. p2p dma-buf aka dynamic dma-buf is a different beast, and i915 (and fwiw these other drivers) need to change before they can do dynamic dma-buf. > Otherwise we have an information leak worth a CVE and that is certainly not > something we want. Because yes otherwise we get a CVE. But right now I don't think we have one. We do have a quite big confusion on what exactly the signaling ordering is supposed to be between exclusive and the collective set of shared fences, and there's some unifying that needs to happen here. But I think what Jason implements here in the import ioctl is the most defensive version possible, so really can't break any driver. It really works like you have an ad-hoc gpu engine that does nothing itself, but waits for the current exclusive fence and then sets the exclusive fence with its "CS" completion fence. That's imo perfectly legit use-case. Same for the export one. Waiting for a previous snapshot of implicit fences is imo perfectly ok use-case and useful for compositors - client might soon start more rendering, and on some drivers that always results in the exclusive slot being set, so if you dont take a snapshot you oversync real bad for your atomic flip. > > Those changes are years in the past. If we have a real problem here (not > > sure on > > that yet), then we'll have to figure out how to fix it without nuking > > uAPI. > > Well, that was the basic idea of attaching flags to the fences in the > dma_resv object. > > In other words you clearly denote when you have to wait for a fence before > accessing a buffer or you cause a security issue. Replied somewhere else, and I do kinda like the flag idea. But the problem is we first need a ton more encapsulation and review of drivers before we can change the internals. One thing at a time. And yes for amdgpu this gets triple-hard because you both have the ttm_bo->moving fence _and_ the current uapi of using fence ownership _and_ you need to figure out how to support vulkan properly with true opt-in fencing. I'm pretty
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
Am 16.06.21 um 20:30 schrieb Jason Ekstrand: On Tue, Jun 15, 2021 at 3:41 AM Christian König wrote: Hi Jason & Daniel, maybe I should explain once more where the problem with this approach is and why I think we need to get that fixed before we can do something like this here. To summarize what this patch here does is that it copies the exclusive fence and/or the shared fences into a sync_file. This alone is totally unproblematic. The problem is what this implies. When you need to copy the exclusive fence to a sync_file then this means that the driver is at some point ignoring the exclusive fence on a buffer object. Not necessarily. Part of the point of this is to allow for CPU waits on a past point in buffers timeline. Today, we have poll() and GEM_WAIT both of which wait for the buffer to be idle from whatever GPU work is currently happening. We want to wait on something in the past and ignore anything happening now. Good point, yes that is indeed a valid use case. But, to the broader point, maybe? I'm a little fuzzy on exactly where i915 inserts and/or depends on fences. When you combine that with complex drivers which use TTM and buffer moves underneath you can construct an information leak using this and give userspace access to memory which is allocated to the driver, but not yet initialized. This way you can leak things like page tables, passwords, kernel data etc... in large amounts to userspace and is an absolutely no-go for security. Ugh... Unfortunately, I'm really out of my depth on the implications going on here but I think I see your point. That's why I'm said we need to get this fixed before we upstream this patch set here and especially the driver change which is using that. Well, i915 has had uAPI for a while to ignore fences. Yeah, exactly that's illegal. At least the kernel internal fences like moving or clearing a buffer object needs to be taken into account before a driver is allowed to access a buffer. Otherwise we have an information leak worth a CVE and that is certainly not something we want. Those changes are years in the past. If we have a real problem here (not sure on that yet), then we'll have to figure out how to fix it without nuking uAPI. Well, that was the basic idea of attaching flags to the fences in the dma_resv object. In other words you clearly denote when you have to wait for a fence before accessing a buffer or you cause a security issue. Christian. --Jason Regards, Christian. Am 10.06.21 um 23:09 schrieb Jason Ekstrand: Modern userspace APIs like Vulkan are built on an explicit synchronization model. This doesn't always play nicely with the implicit synchronization used in the kernel and assumed by X11 and Wayland. The client -> compositor half of the synchronization isn't too bad, at least on intel, because we can control whether or not i915 synchronizes on the buffer and whether or not it's considered written. The harder part is the compositor -> client synchronization when we get the buffer back from the compositor. We're required to be able to provide the client with a VkSemaphore and VkFence representing the point in time where the window system (compositor and/or display) finished using the buffer. With current APIs, it's very hard to do this in such a way that we don't get confused by the Vulkan driver's access of the buffer. In particular, once we tell the kernel that we're rendering to the buffer again, any CPU waits on the buffer or GPU dependencies will wait on some of the client rendering and not just the compositor. This new IOCTL solves this problem by allowing us to get a snapshot of the implicit synchronization state of a given dma-buf in the form of a sync file. It's effectively the same as a poll() or I915_GEM_WAIT only, instead of CPU waiting directly, it encapsulates the wait operation, at the current moment in time, in a sync_file so we can check/wait on it later. As long as the Vulkan driver does the sync_file export from the dma-buf before we re-introduce it for rendering, it will only contain fences from the compositor or display. This allows to accurately turn it into a VkFence or VkSemaphore without any over- synchronization. This patch series actually contains two new ioctls. There is the export one mentioned above as well as an RFC for an import ioctl which provides the other half. The intention is to land the export ioctl since it seems like there's no real disagreement on that one. The import ioctl, however, has a lot of debate around it so it's intended to be RFC-only for now. Mesa MR:
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
On Tue, Jun 15, 2021 at 3:41 AM Christian König wrote: > > Hi Jason & Daniel, > > maybe I should explain once more where the problem with this approach is > and why I think we need to get that fixed before we can do something > like this here. > > To summarize what this patch here does is that it copies the exclusive > fence and/or the shared fences into a sync_file. This alone is totally > unproblematic. > > The problem is what this implies. When you need to copy the exclusive > fence to a sync_file then this means that the driver is at some point > ignoring the exclusive fence on a buffer object. Not necessarily. Part of the point of this is to allow for CPU waits on a past point in buffers timeline. Today, we have poll() and GEM_WAIT both of which wait for the buffer to be idle from whatever GPU work is currently happening. We want to wait on something in the past and ignore anything happening now. But, to the broader point, maybe? I'm a little fuzzy on exactly where i915 inserts and/or depends on fences. > When you combine that with complex drivers which use TTM and buffer > moves underneath you can construct an information leak using this and > give userspace access to memory which is allocated to the driver, but > not yet initialized. > > This way you can leak things like page tables, passwords, kernel data > etc... in large amounts to userspace and is an absolutely no-go for > security. Ugh... Unfortunately, I'm really out of my depth on the implications going on here but I think I see your point. > That's why I'm said we need to get this fixed before we upstream this > patch set here and especially the driver change which is using that. Well, i915 has had uAPI for a while to ignore fences. Those changes are years in the past. If we have a real problem here (not sure on that yet), then we'll have to figure out how to fix it without nuking uAPI. --Jason > Regards, > Christian. > > Am 10.06.21 um 23:09 schrieb Jason Ekstrand: > > Modern userspace APIs like Vulkan are built on an explicit > > synchronization model. This doesn't always play nicely with the > > implicit synchronization used in the kernel and assumed by X11 and > > Wayland. The client -> compositor half of the synchronization isn't too > > bad, at least on intel, because we can control whether or not i915 > > synchronizes on the buffer and whether or not it's considered written. > > > > The harder part is the compositor -> client synchronization when we get > > the buffer back from the compositor. We're required to be able to > > provide the client with a VkSemaphore and VkFence representing the point > > in time where the window system (compositor and/or display) finished > > using the buffer. With current APIs, it's very hard to do this in such > > a way that we don't get confused by the Vulkan driver's access of the > > buffer. In particular, once we tell the kernel that we're rendering to > > the buffer again, any CPU waits on the buffer or GPU dependencies will > > wait on some of the client rendering and not just the compositor. > > > > This new IOCTL solves this problem by allowing us to get a snapshot of > > the implicit synchronization state of a given dma-buf in the form of a > > sync file. It's effectively the same as a poll() or I915_GEM_WAIT only, > > instead of CPU waiting directly, it encapsulates the wait operation, at > > the current moment in time, in a sync_file so we can check/wait on it > > later. As long as the Vulkan driver does the sync_file export from the > > dma-buf before we re-introduce it for rendering, it will only contain > > fences from the compositor or display. This allows to accurately turn > > it into a VkFence or VkSemaphore without any over- synchronization. > > > > This patch series actually contains two new ioctls. There is the export > > one mentioned above as well as an RFC for an import ioctl which provides > > the other half. The intention is to land the export ioctl since it seems > > like there's no real disagreement on that one. The import ioctl, however, > > has a lot of debate around it so it's intended to be RFC-only for now. > > > > Mesa MR: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037 > > IGT tests: https://patchwork.freedesktop.org/series/90490/ > > > > v10 (Jason Ekstrand, Daniel Vetter): > > - Add reviews/acks > > - Add a patch to rename _rcu to _unlocked > > - Split things better so import is clearly RFC status > > > > v11 (Daniel Vetter): > > - Add more CCs to try and get maintainers > > - Add a patch to document DMA_BUF_IOCTL_SYNC > > - Generally better docs > > - Use separate structs for import/export (easier to document) > > - Fix an issue in the import patch > > > > v12 (Daniel Vetter): > > - Better docs for DMA_BUF_IOCTL_SYNC > > > > v12 (Christian König): > > - Drop the rename patch in favor of Christian's series > > - Add a comment to the commit message for the dma-buf sync_file export > > ioctl saying
Re: [Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
Hi Jason & Daniel, maybe I should explain once more where the problem with this approach is and why I think we need to get that fixed before we can do something like this here. To summarize what this patch here does is that it copies the exclusive fence and/or the shared fences into a sync_file. This alone is totally unproblematic. The problem is what this implies. When you need to copy the exclusive fence to a sync_file then this means that the driver is at some point ignoring the exclusive fence on a buffer object. When you combine that with complex drivers which use TTM and buffer moves underneath you can construct an information leak using this and give userspace access to memory which is allocated to the driver, but not yet initialized. This way you can leak things like page tables, passwords, kernel data etc... in large amounts to userspace and is an absolutely no-go for security. That's why I'm said we need to get this fixed before we upstream this patch set here and especially the driver change which is using that. Regards, Christian. Am 10.06.21 um 23:09 schrieb Jason Ekstrand: Modern userspace APIs like Vulkan are built on an explicit synchronization model. This doesn't always play nicely with the implicit synchronization used in the kernel and assumed by X11 and Wayland. The client -> compositor half of the synchronization isn't too bad, at least on intel, because we can control whether or not i915 synchronizes on the buffer and whether or not it's considered written. The harder part is the compositor -> client synchronization when we get the buffer back from the compositor. We're required to be able to provide the client with a VkSemaphore and VkFence representing the point in time where the window system (compositor and/or display) finished using the buffer. With current APIs, it's very hard to do this in such a way that we don't get confused by the Vulkan driver's access of the buffer. In particular, once we tell the kernel that we're rendering to the buffer again, any CPU waits on the buffer or GPU dependencies will wait on some of the client rendering and not just the compositor. This new IOCTL solves this problem by allowing us to get a snapshot of the implicit synchronization state of a given dma-buf in the form of a sync file. It's effectively the same as a poll() or I915_GEM_WAIT only, instead of CPU waiting directly, it encapsulates the wait operation, at the current moment in time, in a sync_file so we can check/wait on it later. As long as the Vulkan driver does the sync_file export from the dma-buf before we re-introduce it for rendering, it will only contain fences from the compositor or display. This allows to accurately turn it into a VkFence or VkSemaphore without any over- synchronization. This patch series actually contains two new ioctls. There is the export one mentioned above as well as an RFC for an import ioctl which provides the other half. The intention is to land the export ioctl since it seems like there's no real disagreement on that one. The import ioctl, however, has a lot of debate around it so it's intended to be RFC-only for now. Mesa MR: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037 IGT tests: https://patchwork.freedesktop.org/series/90490/ v10 (Jason Ekstrand, Daniel Vetter): - Add reviews/acks - Add a patch to rename _rcu to _unlocked - Split things better so import is clearly RFC status v11 (Daniel Vetter): - Add more CCs to try and get maintainers - Add a patch to document DMA_BUF_IOCTL_SYNC - Generally better docs - Use separate structs for import/export (easier to document) - Fix an issue in the import patch v12 (Daniel Vetter): - Better docs for DMA_BUF_IOCTL_SYNC v12 (Christian König): - Drop the rename patch in favor of Christian's series - Add a comment to the commit message for the dma-buf sync_file export ioctl saying why we made it an ioctl on dma-buf Cc: Christian König Cc: Michel Dänzer Cc: Dave Airlie Cc: Bas Nieuwenhuizen Cc: Daniel Stone Cc: mesa-dev@lists.freedesktop.org Cc: wayland-de...@lists.freedesktop.org Test-with: 20210524205225.872316-1-ja...@jlekstrand.net Christian König (1): dma-buf: Add dma_fence_array_for_each (v2) Jason Ekstrand (5): dma-buf: Add dma_resv_get_singleton (v6) dma-buf: Document DMA_BUF_IOCTL_SYNC (v2) dma-buf: Add an API for exporting sync files (v12) RFC: dma-buf: Add an extra fence to dma_resv_get_singleton_unlocked RFC: dma-buf: Add an API for importing sync files (v7) Documentation/driver-api/dma-buf.rst | 8 ++ drivers/dma-buf/dma-buf.c| 103 + drivers/dma-buf/dma-fence-array.c| 27 +++ drivers/dma-buf/dma-resv.c | 110 +++ include/linux/dma-fence-array.h | 17 + include/linux/dma-resv.h | 2 + include/uapi/linux/dma-buf.h | 103 - 7 files changed,
[Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)
Modern userspace APIs like Vulkan are built on an explicit synchronization model. This doesn't always play nicely with the implicit synchronization used in the kernel and assumed by X11 and Wayland. The client -> compositor half of the synchronization isn't too bad, at least on intel, because we can control whether or not i915 synchronizes on the buffer and whether or not it's considered written. The harder part is the compositor -> client synchronization when we get the buffer back from the compositor. We're required to be able to provide the client with a VkSemaphore and VkFence representing the point in time where the window system (compositor and/or display) finished using the buffer. With current APIs, it's very hard to do this in such a way that we don't get confused by the Vulkan driver's access of the buffer. In particular, once we tell the kernel that we're rendering to the buffer again, any CPU waits on the buffer or GPU dependencies will wait on some of the client rendering and not just the compositor. This new IOCTL solves this problem by allowing us to get a snapshot of the implicit synchronization state of a given dma-buf in the form of a sync file. It's effectively the same as a poll() or I915_GEM_WAIT only, instead of CPU waiting directly, it encapsulates the wait operation, at the current moment in time, in a sync_file so we can check/wait on it later. As long as the Vulkan driver does the sync_file export from the dma-buf before we re-introduce it for rendering, it will only contain fences from the compositor or display. This allows to accurately turn it into a VkFence or VkSemaphore without any over- synchronization. This patch series actually contains two new ioctls. There is the export one mentioned above as well as an RFC for an import ioctl which provides the other half. The intention is to land the export ioctl since it seems like there's no real disagreement on that one. The import ioctl, however, has a lot of debate around it so it's intended to be RFC-only for now. Mesa MR: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037 IGT tests: https://patchwork.freedesktop.org/series/90490/ v10 (Jason Ekstrand, Daniel Vetter): - Add reviews/acks - Add a patch to rename _rcu to _unlocked - Split things better so import is clearly RFC status v11 (Daniel Vetter): - Add more CCs to try and get maintainers - Add a patch to document DMA_BUF_IOCTL_SYNC - Generally better docs - Use separate structs for import/export (easier to document) - Fix an issue in the import patch v12 (Daniel Vetter): - Better docs for DMA_BUF_IOCTL_SYNC v12 (Christian König): - Drop the rename patch in favor of Christian's series - Add a comment to the commit message for the dma-buf sync_file export ioctl saying why we made it an ioctl on dma-buf Cc: Christian König Cc: Michel Dänzer Cc: Dave Airlie Cc: Bas Nieuwenhuizen Cc: Daniel Stone Cc: mesa-dev@lists.freedesktop.org Cc: wayland-de...@lists.freedesktop.org Test-with: 20210524205225.872316-1-ja...@jlekstrand.net Christian König (1): dma-buf: Add dma_fence_array_for_each (v2) Jason Ekstrand (5): dma-buf: Add dma_resv_get_singleton (v6) dma-buf: Document DMA_BUF_IOCTL_SYNC (v2) dma-buf: Add an API for exporting sync files (v12) RFC: dma-buf: Add an extra fence to dma_resv_get_singleton_unlocked RFC: dma-buf: Add an API for importing sync files (v7) Documentation/driver-api/dma-buf.rst | 8 ++ drivers/dma-buf/dma-buf.c| 103 + drivers/dma-buf/dma-fence-array.c| 27 +++ drivers/dma-buf/dma-resv.c | 110 +++ include/linux/dma-fence-array.h | 17 + include/linux/dma-resv.h | 2 + include/uapi/linux/dma-buf.h | 103 - 7 files changed, 369 insertions(+), 1 deletion(-) -- 2.31.1 ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
