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Message-ID: <5679A0CB.3060707@labbott.name>
Date: Tue, 22 Dec 2015 11:13:15 -0800
From: Laura Abbott <laura@...bott.name>
To: Dave Hansen <dave.hansen@...el.com>, Christoph Lameter <cl@...ux.com>
Cc: 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@...ck.org,
linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [RFC][PATCH 6/7] mm: Add Kconfig option for
slab sanitization
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.
Thanks,
Laura
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