Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()

[Alexander Potapenko]

On Thu, Jun 4, 2020 at 5:57 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Thu, Jun 04, 2020 at 10:42:45AM +0200, glider@xxxxxxxxxx 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 <glider@xxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Roy Yang <royyang@xxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx> # 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 <keescook@xxxxxxxxxxxx>
>
> > 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: stable@xxxxxxxxxxxxxxx

Sounds good!