Date: Wed, 20 Jan 2016 19:35:06 -0800 From: Laura Abbott <laura@...bott.name> To: Christoph Lameter <cl@...ux.com> Cc: Kees Cook <keescook@...omium.org>, Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>, Andrew Morton <akpm@...ux-foundation.org>, Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: [RFC][PATCH 0/7] Sanitization of slabs based on grsecurity/PaX On 1/13/16 7:49 PM, Laura Abbott wrote: > 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. > There are quite a few other checks which need to be skipped over as well, but I don't think skipping those are going to be sufficient to give an acceptable performance; a quick 'hackbench -g 20 -l 1000' shows at least a 3.5 second difference between just skipping all the checks+slab_debug=P and this series. The SLAB_DEBUG flags force everything to skip the CPU caches which is causing the slow down. I experimented with allowing the debugging to happen with CPU caches but I'm not convinced it's possible to do the checking on the fast path in a consistent manner without adding locking. Is it worth refactoring the debugging to be able to be used on cpu caches or should I take the approach here of having the clear be separate from free_debug_processing? Thanks, Laura
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