Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()
On Thu, Jun 4, 2020 at 5:57 PM Kees Cook wrote:
>
> On Thu, Jun 04, 2020 at 10:42:45AM +0200, gli...@google.com wrote:
> > Under certain circumstances (we found this out running Docker on a
> > Clang-built kernel with CONFIG_INIT_STACK_ALL) ovl_copy_xattr() may
> > return uninitialized value of |error| from ovl_copy_xattr().
> > It is then returned by ovl_create() to lookup_open(), which casts it to
> > an invalid dentry pointer, that can be further read or written by the
> > lookup_open() callers.
> >
> > The uninitialized value is returned when all the xattr on the file
> > are ovl_is_private_xattr(), which is actually a successful case,
> > therefore we initialize |error| with 0.
> >
> > Signed-off-by: Alexander Potapenko
> > Cc: Kees Cook
> > Cc: Roy Yang
> > Cc: # 4.1
>
> Please include a Fixes (more below) and Link tags for details to help
> guide backporting, then you don't need to bother with with "# 4.1",
> the -stable tools will figure it out with a "Fixes" tag.
>
> Thanks for the v2!
>
> Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1050405
> Reviewed-by: Kees Cook
>
> > The bug seem to date back to at least v4.1 where the annotation has been
> > introduced (i.e. the compilers started noticing error could be used
> > before being initialized). I hovever didn't try to prove that the
> > problem is actually reproducible on such ancient kernels. We've seen it
> > on a real machine running v4.4 as well.
>
> It seems like it came from this, but that's v4.5:
>
> Fixes: e4ad29fa0d22 ("ovl: use a minimal buffer in ovl_copy_xattr")
I believe it's actually:
Fixes: 0956254a2d5b ("ovl: don't copy up opaqueness")
That patch added the ovl_is_private_xattr() check in ovl_copy_xattr(),
without which 'error' was always initilalized.
Thanks,
Miklos
Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()
On Thu, Jun 4, 2020 at 5:57 PM Kees Cook wrote:
>
> On Thu, Jun 04, 2020 at 10:42:45AM +0200, gli...@google.com wrote:
> > Under certain circumstances (we found this out running Docker on a
> > Clang-built kernel with CONFIG_INIT_STACK_ALL) ovl_copy_xattr() may
> > return uninitialized value of |error| from ovl_copy_xattr().
> > It is then returned by ovl_create() to lookup_open(), which casts it to
> > an invalid dentry pointer, that can be further read or written by the
> > lookup_open() callers.
> >
> > The uninitialized value is returned when all the xattr on the file
> > are ovl_is_private_xattr(), which is actually a successful case,
> > therefore we initialize |error| with 0.
> >
> > Signed-off-by: Alexander Potapenko
> > Cc: Kees Cook
> > Cc: Roy Yang
> > Cc: # 4.1
>
> Please include a Fixes (more below) and Link tags for details to help
> guide backporting, then you don't need to bother with with "# 4.1",
> the -stable tools will figure it out with a "Fixes" tag.
>
> Thanks for the v2!
>
> Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1050405
> Reviewed-by: Kees Cook
>
> > The bug seem to date back to at least v4.1 where the annotation has been
> > introduced (i.e. the compilers started noticing error could be used
> > before being initialized). I hovever didn't try to prove that the
> > problem is actually reproducible on such ancient kernels. We've seen it
> > on a real machine running v4.4 as well.
>
> It seems like it came from this, but that's v4.5:
>
> Fixes: e4ad29fa0d22 ("ovl: use a minimal buffer in ovl_copy_xattr")
https://bugs.chromium.org/p/chromium/issues/detail?id=1050405 mentions v4.4.212.
Your patch could've been slipped into that kernel as well.
> What did you find in v4.1? It looks like error isn't uninitialized in
> v4.1:
The annotation appeared first in
https://elixir.bootlin.com/linux/v4.1.18/source/fs/overlayfs/copy_up.c#L27,
does that count as 4.1 or 4.2?
> But v4.1.52 backported the above patch (e4ad29fa0d22), which is why I
> don't try to figure these things out manually. Once we find the commit,
> the tools will figure it out. I think you just need:
>
> Fixes: e4ad29fa0d22 ("ovl: use a minimal buffer in ovl_copy_xattr")
> Cc: sta...@vger.kernel.org
Sounds good!
Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()
On Thu, Jun 04, 2020 at 10:57:24AM +0200, Miklos Szeredi wrote:
> On Thu, Jun 4, 2020 at 10:43 AM wrote:
> >
> > Under certain circumstances (we found this out running Docker on a
> > Clang-built kernel with CONFIG_INIT_STACK_ALL) ovl_copy_xattr() may
> > return uninitialized value of |error| from ovl_copy_xattr().
> > It is then returned by ovl_create() to lookup_open(), which casts it to
> > an invalid dentry pointer, that can be further read or written by the
> > lookup_open() callers.
> >
> > The uninitialized value is returned when all the xattr on the file
> > are ovl_is_private_xattr(), which is actually a successful case,
> > therefore we initialize |error| with 0.
> >
> > Signed-off-by: Alexander Potapenko
> > Cc: Kees Cook
> > Cc: Roy Yang
> > Cc: # 4.1
> >
> > ---
> >
> > The bug seem to date back to at least v4.1 where the annotation has been
> > introduced (i.e. the compilers started noticing error could be used
> > before being initialized). I hovever didn't try to prove that the
> > problem is actually reproducible on such ancient kernels. We've seen it
> > on a real machine running v4.4 as well.
> >
> > v2:
> > -- Per Vivek Goyal's suggestion, changed |error| to be 0
>
> Thanks, applied patch posted here (with your signed-off as well, since
> the patch is the same...):
>
> https://lore.kernel.org/linux-unionfs/874ks212uj.fsf@m5Zedd9JOGzJrf0/
Can you please add:
Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1050405
Fixes: e4ad29fa0d22 ("ovl: use a minimal buffer in ovl_copy_xattr")
Reviewed-by: Kees Cook
(and adjust the CC field to drop the "# 4.1" so tools can figure it
out?)
Thanks!
--
Kees Cook
Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()
On Thu, Jun 04, 2020 at 10:42:45AM +0200, gli...@google.com wrote:
> Under certain circumstances (we found this out running Docker on a
> Clang-built kernel with CONFIG_INIT_STACK_ALL) ovl_copy_xattr() may
> return uninitialized value of |error| from ovl_copy_xattr().
> It is then returned by ovl_create() to lookup_open(), which casts it to
> an invalid dentry pointer, that can be further read or written by the
> lookup_open() callers.
>
> The uninitialized value is returned when all the xattr on the file
> are ovl_is_private_xattr(), which is actually a successful case,
> therefore we initialize |error| with 0.
>
> Signed-off-by: Alexander Potapenko
> Cc: Kees Cook
> Cc: Roy Yang
> Cc: # 4.1
Please include a Fixes (more below) and Link tags for details to help
guide backporting, then you don't need to bother with with "# 4.1",
the -stable tools will figure it out with a "Fixes" tag.
Thanks for the v2!
Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1050405
Reviewed-by: Kees Cook
> The bug seem to date back to at least v4.1 where the annotation has been
> introduced (i.e. the compilers started noticing error could be used
> before being initialized). I hovever didn't try to prove that the
> problem is actually reproducible on such ancient kernels. We've seen it
> on a real machine running v4.4 as well.
It seems like it came from this, but that's v4.5:
Fixes: e4ad29fa0d22 ("ovl: use a minimal buffer in ovl_copy_xattr")
What did you find in v4.1? It looks like error isn't uninitialized in
v4.1:
int ovl_copy_xattr(struct dentry *old, struct dentry *new)
{
ssize_t list_size, size;
char *buf, *name, *value;
int error;
if (!old->d_inode->i_op->getxattr ||
!new->d_inode->i_op->getxattr)
return 0;
list_size = vfs_listxattr(old, NULL, 0);
if (list_size <= 0) {
if (list_size == -EOPNOTSUPP)
return 0;
return list_size;
}
buf = kzalloc(list_size, GFP_KERNEL);
if (!buf)
return -ENOMEM;
error = -ENOMEM;
...
But v4.1.52 backported the above patch (e4ad29fa0d22), which is why I
don't try to figure these things out manually. Once we find the commit,
the tools will figure it out. I think you just need:
Fixes: e4ad29fa0d22 ("ovl: use a minimal buffer in ovl_copy_xattr")
Cc: sta...@vger.kernel.org
and things like v4.1.52 will get fixed (if anyone is actually doing
updates for v4.1.z any more...)
--
Kees Cook
Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()
On Thu, Jun 4, 2020 at 10:43 AM wrote: > > Under certain circumstances (we found this out running Docker on a > Clang-built kernel with CONFIG_INIT_STACK_ALL) ovl_copy_xattr() may > return uninitialized value of |error| from ovl_copy_xattr(). > It is then returned by ovl_create() to lookup_open(), which casts it to > an invalid dentry pointer, that can be further read or written by the > lookup_open() callers. > > The uninitialized value is returned when all the xattr on the file > are ovl_is_private_xattr(), which is actually a successful case, > therefore we initialize |error| with 0. > > Signed-off-by: Alexander Potapenko > Cc: Kees Cook > Cc: Roy Yang > Cc: # 4.1 > > --- > > The bug seem to date back to at least v4.1 where the annotation has been > introduced (i.e. the compilers started noticing error could be used > before being initialized). I hovever didn't try to prove that the > problem is actually reproducible on such ancient kernels. We've seen it > on a real machine running v4.4 as well. > > v2: > -- Per Vivek Goyal's suggestion, changed |error| to be 0 Thanks, applied patch posted here (with your signed-off as well, since the patch is the same...): https://lore.kernel.org/linux-unionfs/874ks212uj.fsf@m5Zedd9JOGzJrf0/ Miklos
Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()
On Wed, Jun 3, 2020 at 11:46 PM Vivek Goyal wrote:
>
> On Wed, Jun 03, 2020 at 07:47:14PM +0200, gli...@google.com wrote:
> > Under certain circumstances (we found this out running Docker on a
> > Clang-built kernel with CONFIG_INIT_STACK_ALL) ovl_copy_xattr() may
> > return uninitialized value of |error| from ovl_copy_xattr().
>
> If we are returning uninitialized value of error, doesn't that mean
> that somewhere in the function we are returning without setting error.
> And that probably means that's a bug and we should fix it?
Could be. My understanding of that code is quite limited, so I'm happy
to change the patch if necessary.
> I am wondering if this is triggered by loop finishing because all
> the xattr on the file are ovl_is_private_xattr().
Yes, that's the case. The loop makes one iteration, then
ovl_is_private_xattr() returns true, then the loop ends.
> In that case, we
> will come out of the loop without setting error. This is in fact
> success and we should return 0 instead of some random error?
Thanks for letting me know. I'll change that to 0 then.
> Thanks
> Vivek
>
>
> > It is then returned by ovl_create() to lookup_open(), which casts it to
> > an invalid dentry pointer, that can be further read or written by the
> > lookup_open() callers.
> >
> > Signed-off-by: Alexander Potapenko
> > Cc: Kees Cook
> > Cc: Roy Yang
> > Cc: # 4.1
> >
> > ---
> >
> > It's unclear to me whether error should be initially 0 or some error
> > code (both seem to work), but I thought returning an error makes sense,
> > as the situation wasn't anticipated by the code authors.
> >
> > The bug seem to date back to at least v4.1 where the annotation has been
> > introduced (i.e. the compilers started noticing error could be used
> > before being initialized). I hovever didn't try to prove that the
> > problem is actually reproducible on such ancient kernels. We've seen it
> > on a real machine running v4.4 as well.
> > ---
> > fs/overlayfs/copy_up.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> > index 9709cf22cab3..428d43e2d016 100644
> > --- a/fs/overlayfs/copy_up.c
> > +++ b/fs/overlayfs/copy_up.c
> > @@ -47,7 +47,7 @@ int ovl_copy_xattr(struct dentry *old, struct dentry *new)
> > {
> > ssize_t list_size, size, value_size = 0;
> > char *buf, *name, *value = NULL;
> > - int uninitialized_var(error);
> > + int error = -EINVAL;
> > size_t slen;
> >
> > if (!(old->d_inode->i_opflags & IOP_XATTR) ||
> > --
> > 2.27.0.rc2.251.g90737beb825-goog
> >
>
--
Alexander Potapenko
Software Engineer
Google Germany GmbH
Erika-Mann-Straße, 33
80636 München
Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg
Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()
On Wed, Jun 03, 2020 at 07:47:14PM +0200, gli...@google.com wrote:
> Under certain circumstances (we found this out running Docker on a
> Clang-built kernel with CONFIG_INIT_STACK_ALL) ovl_copy_xattr() may
> return uninitialized value of |error| from ovl_copy_xattr().
If we are returning uninitialized value of error, doesn't that mean
that somewhere in the function we are returning without setting error.
And that probably means that's a bug and we should fix it?
I am wondering if this is triggered by loop finishing because all
the xattr on the file are ovl_is_private_xattr(). In that case, we
will come out of the loop without setting error. This is in fact
success and we should return 0 instead of some random error?
Thanks
Vivek
> It is then returned by ovl_create() to lookup_open(), which casts it to
> an invalid dentry pointer, that can be further read or written by the
> lookup_open() callers.
>
> Signed-off-by: Alexander Potapenko
> Cc: Kees Cook
> Cc: Roy Yang
> Cc: # 4.1
>
> ---
>
> It's unclear to me whether error should be initially 0 or some error
> code (both seem to work), but I thought returning an error makes sense,
> as the situation wasn't anticipated by the code authors.
>
> The bug seem to date back to at least v4.1 where the annotation has been
> introduced (i.e. the compilers started noticing error could be used
> before being initialized). I hovever didn't try to prove that the
> problem is actually reproducible on such ancient kernels. We've seen it
> on a real machine running v4.4 as well.
> ---
> fs/overlayfs/copy_up.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index 9709cf22cab3..428d43e2d016 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -47,7 +47,7 @@ int ovl_copy_xattr(struct dentry *old, struct dentry *new)
> {
> ssize_t list_size, size, value_size = 0;
> char *buf, *name, *value = NULL;
> - int uninitialized_var(error);
> + int error = -EINVAL;
> size_t slen;
>
> if (!(old->d_inode->i_opflags & IOP_XATTR) ||
> --
> 2.27.0.rc2.251.g90737beb825-goog
>
Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()
On Wed, Jun 03, 2020 at 07:47:14PM +0200, gli...@google.com wrote:
> Under certain circumstances (we found this out running Docker on a
> Clang-built kernel with CONFIG_INIT_STACK_ALL) ovl_copy_xattr() may
> return uninitialized value of |error| from ovl_copy_xattr().
> It is then returned by ovl_create() to lookup_open(), which casts it to
> an invalid dentry pointer, that can be further read or written by the
> lookup_open() callers.
>
> Signed-off-by: Alexander Potapenko
Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1050405
Fixes: e4ad29fa0d22 ("ovl: use a minimal buffer in ovl_copy_xattr")
Cc: sta...@vger.kernel.org
Reviewed-by: Kees Cook
It seems the error isn't reported anywhere, so the value likely isn't
too important. -EINVAL seems sane to me.
Thought: should CONFIG_INIT_STACK_ALL=y disable uninitialized_var()?
$ git grep uninitialized_var | wc -l
300
We have evidence this is being used inappropriately and is masking bugs.
I would actually think it should should be removed globally, but it
seems especially important for CONFIG_INIT_STACK_ALL=y.
I've opened:
https://github.com/KSPP/linux/issues/81
--
Kees Cook
