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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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