On Fri, Jul 17, 2026 at 12:28:45PM -0400, Michael S. Tsirkin wrote: > On Fri, Jul 17, 2026 at 04:31:03PM +0200, Greg Kroah-Hartman wrote: > > On Fri, Jul 17, 2026 at 09:08:23AM -0400, Michael S. Tsirkin wrote: > > > On Fri, Jul 17, 2026 at 02:07:50PM +0200, Greg Kroah-Hartman wrote: > > > > On Fri, Jul 17, 2026 at 06:52:46AM -0400, Michael S. Tsirkin wrote: > > > > > On Fri, Jul 17, 2026 at 12:46:52PM +0200, Greg Kroah-Hartman wrote: > > > > > > On Fri, Jul 17, 2026 at 06:23:57AM -0400, Michael S. Tsirkin wrote: > > > > > > > On Fri, Jul 17, 2026 at 12:15:09PM +0200, Greg Kroah-Hartman > > > > > > > wrote: > > > > > > > > On Fri, Jul 17, 2026 at 06:10:41AM -0400, Michael S. Tsirkin > > > > > > > > wrote: > > > > > > > > > On Fri, Jul 17, 2026 at 11:14:23AM +0200, Greg Kroah-Hartman > > > > > > > > > wrote: > > > > > > > > > > On Fri, Jul 17, 2026 at 04:59:32AM -0400, Michael S. > > > > > > > > > > Tsirkin wrote: > > > > > > > > > > > On Fri, Jul 17, 2026 at 10:39:40AM +0200, David > > > > > > > > > > > Hildenbrand (Arm) wrote: > > > > > > > > > > > > On 7/17/26 07:48, Michael S. Tsirkin wrote: > > > > > > > > > > > > > On Thu, Jul 16, 2026 at 05:59:05PM +0200, David > > > > > > > > > > > > > Hildenbrand (Arm) wrote: > > > > > > > > > > > > >>> Or do we just always trust virtio mem devices > > > > > > > > > > > > >>> explicitly? > > > > > > > > > > > > >> > > > > > > > > > > > > >> It's hard for me to understand where we draw the > > > > > > > > > > > > >> line, really. > > > > > > > > > > > > >> > > > > > > > > > > > > >> But maybe MST can clarify what we care about in > > > > > > > > > > > > >> virtio world where the > > > > > > > > > > > > >> hypervisor is fully in charge of the device, > > > > > > > > > > > > > > > > > > > > > > > > > > Generally: > > > > > > > > > > > > > - The guest is expected to whitelist drivers (most > > > > > > > > > > > > > drivers have not > > > > > > > > > > > > > been audited). > > > > > > > > > > > > > > > > > > > > > > > > But even if you audited your driver, who makes sure > > > > > > > > > > > > that we consider all ways > > > > > > > > > > > > where the device could mess with us? > > > > > > > > > > > > > > > > > > > > > > A lot of this is up to a correct setup. For example, make > > > > > > > > > > > sure all > > > > > > > > > > > filesystems are encrypted and refuse to mount unencrypted > > > > > > > > > > > ones. > > > > > > > > > > > > > > > > > > > > > > > Something feels off here. > > > > > > > > > > > > > > > > > > > > > > > > Handling selected out-of-spec scenarios like this feels > > > > > > > > > > > > like a band-aid. Happy > > > > > > > > > > > > to be corrected. > > > > > > > > > > > > > > > > > > > > > > Well Documentation/security/snp-tdx-threat-model.rst puts > > > > > > > > > > > it like this: > > > > > > > > > > > It is important to note > > > > > > > > > > > that this doesn’t imply that the host or VMM are > > > > > > > > > > > intentionally > > > > > > > > > > > malicious, but that there exists a security value in > > > > > > > > > > > having a small CoCo > > > > > > > > > > > VM TCB. > > > > > > > > > > > > > > > > > > > > > > and > > > > > > > > > > > > > > > > > > > > > > While traditionally the host has unlimited access to > > > > > > > > > > > guest data and can > > > > > > > > > > > leverage this access to attack the guest, the CoCo > > > > > > > > > > > systems mitigate such > > > > > > > > > > > attacks by adding security features like guest data > > > > > > > > > > > confidentiality and > > > > > > > > > > > integrity protection. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > now, when we are talking about "mitigation" it is indeed > > > > > > > > > > > becoming a bit > > > > > > > > > > > murky. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > For me, a rule of thumb I came up with is that if the > > > > > > > > > > > validation happens > > > > > > > > > > > to also be helful for users e.g. to work around buggy > > > > > > > > > > > devices, > > > > > > > > > > > or maybe because we feel failing gracefully is nice > > > > > > > > > > > because this > > > > > > > > > > > will allow to later make use of this config and old > > > > > > > > > > > drivers will > > > > > > > > > > > fail but at least not panic, then it is good to include. > > > > > > > > > > > > > > > > > > > > Why not do what USB does? Don't trust the device until > > > > > > > > > > AFTER probe() > > > > > > > > > > succeeds? All of the needed checking should happen before > > > > > > > > > > then, as that > > > > > > > > > > is a "slow path" so lots of validation and the like can > > > > > > > > > > happen at that > > > > > > > > > > point. > > > > > > > > > > > > > > > > > > > > After that, during the normal data paths, after the driver > > > > > > > > > > is bound, > > > > > > > > > > trust it all you want as attempting to validate every > > > > > > > > > > single packet is > > > > > > > > > > just going to be impossible. > > > > > > > > > > > > > > > > > > > > thanks, > > > > > > > > > > > > > > > > > > > > greg k-h > > > > > > > > > > > > > > > > > > People do expect that data path validation at this point. > > > > > > > > > > > > > > > > Ok, so you want this patch :) > > > > > > > > > > > > > > > > And more, as you need to treat everything from the host as > > > > > > > > "untrusted", > > > > > > > > and it must be "verified". > > > > > > > > > > > > > > Well. First it's not me) Second it's only specific configurations > > > > > > > - > > > > > > > for example there's no short term plan to validate filesystem > > > > > > > code, people > > > > > > > are expected to rely on encryption. The reasons have more to do > > > > > > > with the available manpower than anything else. > > > > > > > > > > > > Sure, but again, for subsystems, you have to define your threat > > > > > > model as > > > > > > the LLMs are churning against the code base and coming up with lots > > > > > > of > > > > > > crazy ideas if a device should or should not be trusted and > > > > > > spitting out > > > > > > patches and reports like the ones that are in the first few patches > > > > > > of > > > > > > this series. > > > > > > > > > > > > So please, pick a model, let's document it, and go with that. I am > > > > > > getting directly conflicting responses here. > > > > > > > > > > > > thanks, > > > > > > > > > > > > greg k-h > > > > > > > > > > Supposed to be this one: > > > > > Documentation/security/snp-tdx-threat-model.rst > > > > > > > > > > what is missing? > > > > > > > > A policy decision that needs to be made. All that document does is > > > > describe a bunch of different "threats" yet does not decide what to do > > > > about them at all from what I can tell. > > > > > > That would be this section I think: > > > > > > The **Linux kernel CoCo VM security objectives** can be summarized as > > > follows: > > > > > > it does, indeed, not go into detail about how to interact, safely, > > > with untrusted entities. Does it really need to be spelled out? > > > > Seems like it as I didn't figure it out at all :) > > > > > > And that's just for one subset of the CoC world, right? Is that > > > > something that all virtio drivers need/want to care about? > > > > > > What is missing, and what you seem to be asking for, is an opinionated > > > stance on which drivers we care about in this world? > > > True. > > > > Yes. > > > > > coco guys tried to annotate drivers at some point to do exactly that. > > > this was rejected upstream from the position that this is not > > > different from handling buggy hardware, and just to fix all drivers. > > > so it's up to users, and I guess for virtio the answer is yes > > > with some exceptions because we don't have a better answer right now. > > > > Ok, so back to the original question here: > > > > > > So I don't see a real answer to the "does Linux trust the host to give > > > > you good data or not" question in that file, am I missing it? > > > > > > > > thanks, > > > > > > > > greg k-h > > > > > > This? Note the last sentence. > > > > > > The **Linux CoCo VM attack surface** is any interface exposed from a CoCo > > > guest Linux kernel towards an untrusted host that is not covered by the > > > CoCo technology SW/HW protection. This includes any possible > > > side-channels, as well as transient execution side channels. Examples of > > > explicit (not side-channel) interfaces include accesses to port I/O, MMIO > > > and DMA interfaces, access to PCI configuration space, VMM-specific > > > hypercalls (towards Host-side VMM), access to shared memory pages, > > > interrupts allowed to be injected into the guest kernel by the host, as > > > well as CoCo technology-specific hypercalls, if present. Additionally, the > > > host in a CoCo system typically controls the process of creating a CoCo > > > guest: it has a method to load into a guest the firmware and bootloader > > > images, the kernel image together with the kernel command line. All of > > > this > > > data should also be considered untrusted until its integrity and > > > authenticity is established via attestation. > > > > Great, so you are saying that we need to fix any bug found where a host > > could be sending "bad" data over the virtio path before, and after, the > > driver is bound to the device. That's a solid answer, and let's let the > > LLMs run with that! > > > > Which also implies that the first 3 patches here are acceptable, right? > > :) > > > > thanks, > > > > greg k-h > > The 1st patch has nothing to do with coco. Bad values it checks do not > lead to any information leak. I'm fine with it as such > but I > would like the commit log to make that clearer.
Or to be more precise, I'd like the motivation to be documented, given it is not coco, and then I can judge the merits. > > -- > MST
