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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Thu, 4 Jun 2020 10:57:24 +0200
Message-ID: <CAJfpegv5W9BnCFGc2jWxCGS_RcqT0LFxw5ke2Z2XbCotokdUWw@mail.gmail.com>
Subject: Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()
To:     Alexander Potapenko <glider@google.com>
Cc:     Vivek Goyal <vgoyal@redhat.com>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>,
        royyang@google.com, stable <stable@vger.kernel.org>
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On Thu, Jun 4, 2020 at 10:43 AM <glider@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 <glider@google.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Roy Yang <royyang@google.com>
> Cc: <stable@vger.kernel.org> # 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