linux-kernel - Re: [PATCH] ovl: explicitly initialize error in ovl_copy

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Date:   Fri, 5 Jun 2020 11:54:27 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel@...r.kernel.org, royyang@...gle.com,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()

On Thu, Jun 4, 2020 at 5:57 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Thu, Jun 04, 2020 at 10:42:45AM +0200, glider@...gle.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 <glider@...gle.com>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: Roy Yang <royyang@...gle.com>
> > Cc: <stable@...r.kernel.org> # 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@...omium.org>
>
> > 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