On Wed, Jun 3, 2020 at 11:46 PM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>
> On Wed, Jun 03, 2020 at 07:47:14PM +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().
>
> 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@xxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Roy Yang <royyang@xxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx> # 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
Google Germany GmbH
Erika-Mann-Straße, 33
80636 München
Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg
