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

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Date:   Thu, 4 Jun 2020 10:28:50 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     miklos@...redi.hu, linux-unionfs@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Roy Yang <royyang@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()

On Wed, Jun 3, 2020 at 11:46 PM Vivek Goyal <vgoyal@...hat.com> wrote:
>
> On Wed, Jun 03, 2020 at 07:47:14PM +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().
>
> 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 <glider@...gle.com>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: Roy Yang <royyang@...gle.com>
> > Cc: <stable@...r.kernel.org> # 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

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