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

[Laura Abbott]

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.

Thanks,
Laura

> 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");