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Date: Thu, 04 Apr 2013 14:01:38 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Kees Cook <keescook@...omium.org>
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
What system monitoring? Most systems don't have much...
Kees Cook <keescook@...omium.org> wrote:
>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
--
Sent from my mobile phone. Please excuse brevity and lack of formatting.
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