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

[Kees Cook]

On Fri, Jul 8, 2016 at 9:45 AM, Christoph Lameter <cl@xxxxxxxxx> wrote:
> On Fri, 8 Jul 2016, Michael Ellerman wrote:
>
>> > I wonder if this code should be using size_from_object() instead of s->size?

BTW, I can't reproduce this on x86 yet...

>>
>> Hmm, not sure. Who's SLUB maintainer? :)
>
> Me.
>
> s->size is the size of the whole object including debugging info etc.
> ksize() gives you the actual usable size of an object.

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).

I think a potential problem with Michael's fix is that the ptr in
__check_heap_object() may not point at the _start_ of the usable
object, so doing the red zone shift isn't quite right.

This finds the ptr's offset within the slub object (since s->size is
the slub object size):

        offset = (ptr - page_address(page)) % s->size;

But this looks at object_size and doesn't take into account actual size:

        if (offset <= s->object_size && n <= s->object_size - offset)
                return NULL;

I think offset needs to be adjusted by the size of padding, which the
restore_red_left() call had the same effect, but may not cover all
padding conditions? I'm not sure.

Should it be:

        /* Find offset within slab object. */
        offset = (ptr - page_address(page)) % s->size;

        /* Adjust offset for meta data and padding. */
        offset -= s->size - s->object_size;

        /* Make sure offset and size are within bounds of the
allocation size. */
        if (offset <= s->object_size && n <= s->object_size - offset)
                return NULL;

?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security