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

[Kees Cook <keescook@chromium.org>]

On Fri, Jul 8, 2016 at 1:41 PM, Kees Cook <keescook@chromium.org> wrote:
> On Fri, Jul 8, 2016 at 12:20 PM, Christoph Lameter <cl@linux.com> 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;

-Kees


-- 
Kees Cook
Chrome OS & Brillo Security