linux-kernel - Re: [PATCH 3/3] x86: kernel base offset ASLR

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Date:	Fri, 5 Apr 2013 10:04:18 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
	kernel-hardening@...ts.openwall.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Jarkko Sakkinen <jarkko.sakkinen@...el.com>,
	Matthew Garrett <mjg@...hat.com>,
	Matt Fleming <matt.fleming@...el.com>,
	Eric Northup <digitaleric@...gle.com>,
	Dan Rosenberg <drosenberg@...curity.com>,
	Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...omium.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3/3] x86: kernel base offset ASLR


* H. Peter Anvin <hpa@...or.com> wrote:

> I have to admit to being somewhat skeptical toward KASLR with only 8 
> bits of randomness.  There are at least two potential ways of 
> dramatically increasing the available randomness:
> 
> 1. actually compose the kernel of multiple independently relocatable
>    pieces (maybe chunk it on 2M boundaries or something.)
> 
> 2. compile the kernel as one of the memory models which can be executed
>    anywhere in the 64-bit address space.  The cost of this would have
>    to be quantified, of course.
> 
> The latter is particularly something that should be considered for the 
> LPF JIT, to defend against JIT spray attacks.

The cost of 64-bit RIPs is probably measurable both in cache footprint and 
in execution speed.

Doing that might make sense - but unless it's surprisingly cheap to do it, 
at least from a distro perspective, randomizing the kernel base using the 
existing compact address space would probably be the preferred option - 
even if a bigger build model was available.

Random runtime shuffling of the kernel image - is that possible with 
existing toolchains?

Thanks,

	Ingo
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