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Date: Tue, 1 Oct 2013 22:36:31 -0700
From: Kees Cook <keescook@...omium.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	"x86@...nel.org" <x86@...nel.org>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Aaron Durbin <adurbin@...gle.com>, 
	Eric Northup <digitaleric@...gle.com>, Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...gle.com>, 
	Mathias Krause <minipli@...glemail.com>, Zhang Yanfei <zhangyanfei@...fujitsu.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Arnaldo Carvalho de Melo <acme@...radead.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, 
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v6 0/7] Kernel base address randomization

On Tue, Oct 1, 2013 at 10:30 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 10/01/2013 10:25 PM, Ingo Molnar wrote:
>>
>> I mean, for example in an oops message we print data in words: the RIP,
>> other registers and stack contents. If any of these values lies within the
>> randomization range then we could de-randomize it.
>>
>> So instead of exposing randomized values, we could expose de-randomized
>> values.
>>
>> ( This isn't fool-proof: if some data value happens to lie within the
>>   random range spuriously then we'll incorrectly transform it. In the
>>   context of oops messages this should not be a big practical problem
>>   though. )
>>
>
> I don't agree that this isn't a big practical problem.  I often find it
> necessary to pick out "things that look like pointers".  Overall,
> derandomization would make it possible to get really confused when you
> have things like half a pointer overwritten.

I think reflecting the reality of the system is the correct way to go.
Attempting to do the derandomization on the live system seems
extremely fragile. It's much cleaner to have a "true" view of the
running system and work from there. I don't want to have to wonder if
my kernel is lying to me about where things are in memory any more
than it already does. :)

-Kees

-- 
Kees Cook
Chrome OS Security

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