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

Laura Abbott
2016-01-06T02:46:11Z

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