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

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Date:	Thu, 4 Apr 2013 14:00:03 -0700
From:	Kees Cook <keescook@...omium.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <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>
Subject: Re: [PATCH 3/3] x86: kernel base offset ASLR

On Thu, Apr 4, 2013 at 1:58 PM, H. Peter Anvin <hpa@...or.com> 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

>
> Kees Cook <keescook@...omium.org> wrote:
>
>>On Thu, Apr 4, 2013 at 1:12 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>> On 04/04/2013 01:07 PM, Kees Cook wrote:
>>>> However, the benefits of
>>>> this feature in certain environments exceed the perceived
>>weaknesses[2].
>>>
>>> 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 in
>>the future.
>>
>>-Kees
>>
>>--
>>Kees Cook
>>Chrome OS Security
>
> --
> Sent from my mobile phone. Please excuse brevity and lack of formatting.



--
Kees Cook
Chrome OS Security
--
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