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

[Kees Cook]

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.

Thanks!

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security