Re: [RFC][PATCH 0/7] Sanitization of slabs based on grsecurity/PaX

From: Laura Abbott
Date: Wed Jan 13 2016 - 22:50:42 EST

On 1/8/16 6:07 AM, Christoph Lameter wrote:
On Thu, 7 Jan 2016, Laura Abbott wrote:

The slub_debug=P not only poisons it enables other consistency checks on the
slab as well, assuming my understanding of what check_object does is correct.
My hope was to have the poison part only and none of the consistency checks in
an attempt to mitigate performance issues. I misunderstood when the checks
actually run and how SLUB_DEBUG was used.

Ok I see that there pointer check is done without checking the
corresponding debug flag. Patch attached thar fixes it.

Another option would be to have a flag like SLAB_NO_SANITY_CHECK.
sanitization enablement would just be that and SLAB_POISON
in the debug options. The disadvantage to this approach would be losing
the sanitization for ->ctor caches (the grsecurity version works around this
by re-initializing with ->ctor, I haven't heard any feedback if this actually
acceptable) and not having some of the fast paths enabled
(assuming I'm understanding the code path correctly.) which would also
be a performance penalty

I think we simply need to fix the missing check there. There is already a
flag SLAB_DEBUG_FREE for the pointer checks.

The patch improves performance but the overall performance of these full
sanitization patches is still significantly better than slub_debug=P. I'll
put some effort into seeing if I can figure out where the slow down is
coming from.


Subject: slub: Only perform pointer checks in check_object when SLAB_DEBUG_FREE is set

Seems that check_object() always checks for pointer issues currently.

Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>

Index: linux/mm/slub.c
--- linux.orig/mm/slub.c
+++ linux/mm/slub.c
@@ -848,6 +848,9 @@ static int check_object(struct kmem_cach
return 1;

+ if (!(s->flags & SLAB_DEBUG_FREE))
+ return 1;
/* Check free pointer validity */
if (!check_valid_pointer(s, page, get_freepointer(s, p))) {
object_err(s, page, p, "Freepointer corrupt");