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

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Date:	Tue, 5 Jan 2016 16:29:21 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Laura Abbott <laura@...bott.name>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Christoph Lameter <cl@...ux.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [kernel-hardening] [RFC][PATCH 6/7] mm: Add Kconfig option for
 slab sanitization

On Tue, Dec 22, 2015 at 11:13 AM, Laura Abbott <laura@...bott.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
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