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; -Kees -- Kees Cook Chrome OS & Brillo Security -- To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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