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

Kees Cook
2016-01-06T00:29:21Z

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.

-Kees

--
Kees Cook
Chrome OS & Brillo Security