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" <>
To: Eric Northup <>
CC: Kees Cook <>,
        Linux Kernel Mailing List <>,,
        Thomas Gleixner <>, Ingo Molnar <>,
        the arch/x86 maintainers <>,
        Jarkko Sakkinen <>,
        Matthew Garrett <>,
        Matt Fleming <>,
        Dan Rosenberg <>,
        Julien Tinnes <>, Will Drewry <>
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.


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

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