Re: [kernel-hardening] [RFC][PATCH 6/7] mm: Add Kconfig option for slab sanitization

[Dave Hansen]

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)


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