[kernel-hardening] Re: [RFC][PATCH 0/7] Sanitization of slabs based on grsecurity/PaX

Laura Abbott
2016-01-06T03:17:21Z

On 1/5/16 4:09 PM, Kees Cook wrote:
> On Tue, Dec 22, 2015 at 12:04 PM, Laura Abbott <laura@labbott.name> wrote:
>> On 12/22/15 8:08 AM, Christoph Lameter wrote:
>>>
>>> On Mon, 21 Dec 2015, Laura Abbott wrote:
>>>
>>>> The biggest change from PAX_MEMORY_SANTIIZE is that this feature
>>>> sanitizes
>>>> the SL[AOU]B allocators only. My plan is to work on the buddy allocator
>>>> santization after this series gets picked up. A side effect of this is
>>>> that allocations which go directly to the buddy allocator (i.e. large
>>>> allocations) aren't sanitized. I'd like feedback about whether it's worth
>>>> it to add sanitization on that path directly or just use the page
>>>> allocator sanitization when that comes in.
>
> This looks great! I love the added lkdtm tests, too. Very cool.
>
>>> I am not sure what the point of this patchset is. We have a similar effect
>>> to sanitization already in the allocators through two mechanisms:
>>>
>>> 1. Slab poisoning
>>> 2. Allocation with GFP_ZERO
>>>
>>> I do not think we need a third one. You could accomplish your goals much
>>> easier without this code churn by either
>>>
>>> 1. Improve the existing poisoning mechanism. Ensure that there are no
>>> gaps. Security sensitive kernel slab caches can then be created with
>>> the POISONING flag set. Maybe add a Kconfig flag that enables
>>> POISONING for each cache? What was the issue when you tried using
>>> posining for sanitization?
>>
>> The existing poisoning does work for sanitization but it's still a debug
>> feature. It seemed more appropriate to keep debug features and non-debug
>> features separate hence the separate option and configuration.
>
> What stuff is intertwined in the existing poisoning that makes it
> incompatible/orthogonal?
>

It's not the poisoning per se that's incompatible, it's how the poisoning is
set up. At least for slub, the current poisoning is part of SLUB_DEBUG which
enables other consistency checks on the allocator. Trying to pull out just
the poisoning for use when SLUB_DEBUG isn't on would result in roughly what
would be here anyway. I looked at trying to reuse some of the existing poisoning
and came to the conclusion it was less intrusive to the allocator to keep it
separate.

Thanks,
Laura