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Date:   Thu, 4 Jun 2020 09:00:37 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     Alexander Potapenko <glider@google.com>,
        Vivek Goyal <vgoyal@redhat.com>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel@vger.kernel.org, royyang@google.com,
        stable <stable@vger.kernel.org>
Subject: Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()
Message-ID: <202006040858.3BF48FCF63@keescook>
References: <20200604084245.161480-1-glider@google.com>
 <CAJfpegv5W9BnCFGc2jWxCGS_RcqT0LFxw5ke2Z2XbCotokdUWw@mail.gmail.com>
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On Thu, Jun 04, 2020 at 10:57:24AM +0200, Miklos Szeredi wrote:
> 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/

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 <keescook@chromium.org>

(and adjust the CC field to drop the "# 4.1" so tools can figure it
out?)

Thanks!

-- 
Kees Cook