Openwall GNU/*/Linux - a small security-enhanced Linux distro for servers
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Date: Thu, 04 Apr 2013 18:08:53 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Eric Northup <digitaleric@...gle.com>
CC: Kees Cook <keescook@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...el.com>,
        Matthew Garrett <mjg@...hat.com>,
        Matt Fleming <matt.fleming@...el.com>,
        Dan Rosenberg <drosenberg@...curity.com>,
        Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...omium.org>
Subject: Re: [PATCH 3/3] x86: kernel base offset ASLR

On 04/04/2013 01:47 PM, Eric Northup wrote:
>>
>> 1. actually compose the kernel of multiple independently relocatable
>>    pieces (maybe chunk it on 2M boundaries or something.)
> 
> Without increasing the entropy bits, does this actually increase the #
> of tries necessary for an attacker to guess correctly?  It
> dramatically increases the number of possible configurations of kernel
> address space, but for any given piece there are only 256 possible
> locations.
> 

The 2M chunk was a red herring; one would of course effectively pack
blocks together, probably packed back to back, in random order.

>> 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.
> 
> I attempted to do this, but was limited by my knowledge of the
> toolchain.  I would welcome help or suggestions!

Start by looking at the ABI document.  I suspect what we need is some
variant of the small PIC model.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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