Date: Tue, 5 Jan 2016 16:29:21 -0800 From: Kees Cook <keescook@...omium.org> To: Laura Abbott <laura@...bott.name> Cc: Dave Hansen <dave.hansen@...el.com>, Christoph Lameter <cl@...ux.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, 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> Subject: Re: [RFC][PATCH 6/7] mm: Add Kconfig option for slab sanitization On Tue, Dec 22, 2015 at 11:13 AM, Laura Abbott <laura@...bott.name> wrote: > On 12/22/15 10:19 AM, Dave Hansen wrote: >> >> On 12/22/2015 10:08 AM, Christoph Lameter wrote: >>> >>> On Tue, 22 Dec 2015, Dave Hansen wrote: >>>>> >>>>> Why would you use zeros? The point is just to clear the information >>>>> right? >>>>> The regular poisoning does that. >>>> >>>> >>>> It then allows you to avoid the zeroing at allocation time. >>> >>> >>> Well much of the code is expecting a zeroed object from the allocator and >>> its zeroed at that time. Zeroing makes the object cache hot which is an >>> important performance aspect. >> >> >> Yes, modifying this behavior has a performance impact. It absolutely >> needs to be evaluated, and I wouldn't want to speculate too much on how >> good or bad any of the choices are. >> >> Just to reiterate, I think we have 3 real choices here: >> >> 1. Zero at alloc, only when __GFP_ZERO >> (behavior today) >> 2. Poison at free, also Zero at alloc (when __GFP_ZERO) >> (this patch's proposed behavior, also what current poisoning does, >> doubles writes) >> 3. Zero at free, *don't* Zero at alloc (when __GFP_ZERO) >> (what I'm suggesting, possibly less perf impact vs. #2) >> >> > > poisoning with non-zero memory makes it easier to determine that the error > came from accessing the sanitized memory vs. some other case. I don't think > the feature would be as strong if the memory was only zeroed vs. some other > data value. I would tend to agree. If there are significant perf improvements for "3" above, that should be easy to add on later as another choice. -Kees -- Kees Cook Chrome OS & Brillo Security
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