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Date: Tue, 5 Jan 2016 18:46:11 -0800
From: Laura Abbott <>
To: Kees Cook <>
Cc: Dave Hansen <>, Christoph Lameter <>,
 "" <>,
 Pekka Enberg <>, David Rientjes <>,
 Joonsoo Kim <>,
 Andrew Morton <>, Linux-MM <>,
 LKML <>
Subject: Re: [RFC][PATCH 6/7] mm: Add Kconfig option for
 slab sanitization

On 1/5/16 4:29 PM, Kees Cook wrote:
> On Tue, Dec 22, 2015 at 11:13 AM, Laura Abbott <> 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.


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