Messages in this thread | | | Subject | Re: [kernel-hardening] [RFC][PATCH 6/7] mm: Add Kconfig option for slab sanitization | From | Laura Abbott <> | Date | Tue, 5 Jan 2016 18:46:11 -0800 |
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On 1/5/16 4:29 PM, Kees Cook wrote: > On Tue, Dec 22, 2015 at 11:13 AM, Laura Abbott <laura@labbott.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. >
I was looking at the sanitization for the buddy allocator that exists in grsecurity and that does option #3 (zero at free, skip __GFP_ZERO). I'm going to look into adding that as an option for the slab allocator and see what the performance numbers show.
Thanks, Laura
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