Re: [kernel-hardening] Re: [PATCH 9/9] mm: SLUB hardened usercopy support

[Joonsoo Kim]

On Fri, Jul 08, 2016 at 04:48:38PM -0400, Kees Cook wrote:
> On Fri, Jul 8, 2016 at 1:41 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > On Fri, Jul 8, 2016 at 12:20 PM, Christoph Lameter <cl@xxxxxxxxx> wrote:
> >> On Fri, 8 Jul 2016, Kees Cook wrote:
> >>
> >>> Is check_valid_pointer() making sure the pointer is within the usable
> >>> size? It seemed like it was checking that it was within the slub
> >>> object (checks against s->size, wants it above base after moving
> >>> pointer to include redzone, etc).
> >>
> >> check_valid_pointer verifies that a pointer is pointing to the start of an
> >> object. It is used to verify the internal points that SLUB used and
> >> should not be modified to do anything different.
> >
> > Yup, no worries -- I won't touch it. :) I just wanted to verify my
> > understanding.
> >
> > And after playing a bit more, I see that the only thing to the left is
> > padding and redzone. SLUB layout, from what I saw:
> >
> > offset: what's there
> > -------
> > start: padding, redzone
> > red_left_pad: object itself
> > inuse: rest of metadata
> > size: start of next slub object
> >
> > (and object_size == inuse - red_left_pad)
> >
> > i.e. a pointer must be between red_left_pad and inuse, which is the
> > same as pointer - ref_left_pad being less than object_size.
> >
> > So, as found already, the position in the usercopy check needs to be
> > bumped down by red_left_pad, which is what Michael's fix does, so I'll
> > include it in the next version.
> 
> Actually, after some offline chats, I think this is better, since it
> makes sure the ptr doesn't end up somewhere weird before we start the
> calculations. This leaves the pointer as-is, but explicitly handles
> the redzone on the offset instead, with no wrapping, etc:
> 
>         /* Find offset within object. */
>         offset = (ptr - page_address(page)) % s->size;
> 
> +       /* Adjust for redzone and reject if within the redzone. */
> +       if (s->flags & SLAB_RED_ZONE) {
> +               if (offset < s->red_left_pad)
> +                       return s->name;
> +               offset -= s->red_left_pad;
> +       }
> +
>         /* Allow address range falling entirely within object size. */
>         if (offset <= s->object_size && n <= s->object_size - offset)
>                 return NULL;
> 

As Christoph saids, please use slab_ksize() rather than
s->object_size.

Otherwise, looks good to me.

Thanks.