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

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Date:	Tue, 22 Dec 2015 11:32:56 -0800
From:	Dave Hansen <dave.hansen@...el.com>
To:	Laura Abbott <laura@...bott.name>, 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: [kernel-hardening] [RFC][PATCH 6/7] mm: Add Kconfig option for
 slab sanitization

On 12/22/2015 11:13 AM, Laura Abbott wrote:
>> 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.

How does that scenario work?  Your patch description says:

> +	  Use-after-free bugs for structures containing
> +	  pointers can also be detected as dereferencing the sanitized pointer
> +	  will generate an access violation.

In the case that we wrote all zeros, we'd be accessing userspace at a
known place that we don't generally allow memory to be mapped anyway.
Could you elaborate on a scenario where zeros are weaker than a random
poison value?

In any case (if a poison value is superior to 0's), it's a balance
between performance vs. the likelihood of the poisoned value being
tripped over.

I think the performance impact of this feature is going to be *the*
major thing that keeps folks from using it in practice.  I'm trying to
suggest a way that you _might_ preserve some performance, and get more
folks to use it.

1. Keep information from leaking (doesn't matter which value we write)
2. Detect use-after-free bugs (0's are less likely to be detected???)
3. Preserve performance (0's are likely to preserve more performance)
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