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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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