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Date: Thu, 4 Apr 2013 14:04:44 -0700
From: Eric Northup <>
To: "H. Peter Anvin" <>
Cc: Kees Cook <>, LKML <>, 
	"" <>, Thomas Gleixner <>, 
	Ingo Molnar <>, "" <>, 
	Jarkko Sakkinen <>, Matthew Garrett <>, 
	Matt Fleming <>, Dan Rosenberg <>, 
	Julien Tinnes <>, Will Drewry <>
Subject: Re: [PATCH 3/3] x86: kernel base offset ASLR

On Thu, Apr 4, 2013 at 2:01 PM, H. Peter Anvin <> wrote:
> What system monitoring?  Most systems don't have much...

The security of an unmonitored system is going to be much lower than
of a well-monitored system.  That's true independent of whether kASLR
is deployed.

> Kees Cook <> wrote:
>>On Thu, Apr 4, 2013 at 1:58 PM, H. Peter Anvin <> wrote:
>>> It seems to me that you are assuming that the attacker is targeting a
>>specific system, but a bot might as well target 256 different systems
>>and see what sticks...
>>Certainly, but system monitoring will show 255 crashed machines, which
>>is a huge blip on any radar. :)
>>> Kees Cook <> wrote:
>>>>On Thu, Apr 4, 2013 at 1:12 PM, H. Peter Anvin <> wrote:
>>>>> On 04/04/2013 01:07 PM, Kees Cook wrote:
>>>>>> However, the benefits of
>>>>>> this feature in certain environments exceed the perceived
>>>>> Could you clarify?
>>>>I would summarize the discussion of KASLR weaknesses into to two
>>>>general observations:
>>>>1- it depends on address location secrecy and leaks are common/easy.
>>>>2- it has low entropy so attack success rates may be high.
>>>>For "1", as Julien mentions, remote attacks and attacks from a
>>>>significantly contained process (via seccomp-bpf) minimizes the leak
>>>>exposure. For local attacks, cache timing attacks and other things
>>>>also exist, but the ASLR can be improved to defend against that too.
>>>>So, KASLR is useful on systems that are virtualization hosts,
>>>>providing remote services, or running locally confined processes.
>>>>For "2", I think that the comparison to userspace ASLR entropy isn't
>>>>as direct. For userspace, most systems don't tend to have any kind of
>>>>watchdog on segfaulting processes, so a remote attacker could just
>>>>keep trying an attack until they got lucky, in which case low entropy
>>>>is a serious problem. In the case of KASLR, a single attack failure
>>>>means the system goes down, which makes mounting an attack much more
>>>>difficult. I think 8 bits is fine to start with, and I think start
>>>>with a base offset ASLR is a good first step. We can improve things
>>>>the future.
>>>>Kees Cook
>>>>Chrome OS Security
>>> --
>>> Sent from my mobile phone. Please excuse brevity and lack of
>>Kees Cook
>>Chrome OS Security
> --
> Sent from my mobile phone. Please excuse brevity and lack of formatting.

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