Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762389Ab3DDU6b (ORCPT ); Thu, 4 Apr 2013 16:58:31 -0400 Received: from terminus.zytor.com ([198.137.202.10]:57688 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762167Ab3DDU6a (ORCPT ); Thu, 4 Apr 2013 16:58:30 -0400 User-Agent: K-9 Mail for Android In-Reply-To: References: <1365106055-22939-1-git-send-email-keescook@chromium.org> <1365106055-22939-4-git-send-email-keescook@chromium.org> <515DDEC0.9000109@zytor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Subject: Re: [PATCH 3/3] x86: kernel base offset ASLR From: "H. Peter Anvin" Date: Thu, 04 Apr 2013 13:58:06 -0700 To: Kees Cook CC: LKML , "kernel-hardening@lists.openwall.com" , Thomas Gleixner , Ingo Molnar , "x86@kernel.org" , Jarkko Sakkinen , Matthew Garrett , Matt Fleming , Eric Northup , Dan Rosenberg , Julien Tinnes , Will Drewry Message-ID: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2021 Lines: 47 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... 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 >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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/