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Date: Fri, 8 Jan 2016 08:07:44 -0600 (CST)
From: Christoph Lameter <>
To: Laura Abbott <>
cc: Kees Cook <>, Pekka Enberg <>, 
    David Rientjes <>, Joonsoo Kim <>, 
    Andrew Morton <>, Linux-MM <>, 
    LKML <>, 
    "" <>
Subject: Re: [RFC][PATCH 0/7] Sanitization of slabs based on grsecurity/PaX

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.

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 <>

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");

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