From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964963AbcKJVBp (ORCPT ); Thu, 10 Nov 2016 16:01:45 -0500 Received: from mail-wm0-f48.google.com ([74.125.82.48]:33773 "EHLO mail-wm0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932614AbcKJVBo (ORCPT ); Thu, 10 Nov 2016 16:01:44 -0500 MIME-Version: 1.0 In-Reply-To: <20161110204838.GE17134@arm.com> References: <1478809488-18303-1-git-send-email-elena.reshetova@intel.com> <20161110203749.GV3117@twins.programming.kicks-ass.net> <20161110204838.GE17134@arm.com> From: Kees Cook Date: Thu, 10 Nov 2016 13:01:41 -0800 X-Google-Sender-Auth: Z_rRILT_Y4g0w_YdIbRO039WTAg Message-ID: Subject: Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC To: Will Deacon , Greg KH Cc: Peter Zijlstra , Elena Reshetova , "kernel-hardening@lists.openwall.com" , Arnd Bergmann , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 10, 2016 at 12:48 PM, Will Deacon wrote: > On Thu, Nov 10, 2016 at 09:37:49PM +0100, Peter Zijlstra wrote: >> On Thu, Nov 10, 2016 at 10:24:35PM +0200, Elena Reshetova wrote: >> > This series brings the PaX/Grsecurity PAX_REFCOUNT >> > feature support to the upstream kernel. All credit for the >> > feature goes to the feature authors. >> > >> > The name of the upstream feature is HARDENED_ATOMIC >> > and it is configured using CONFIG_HARDENED_ATOMIC and >> > HAVE_ARCH_HARDENED_ATOMIC. >> > >> > This series only adds x86 support; other architectures are expected >> > to add similar support gradually. >> > >> > More information about the feature can be found in the following >> > commit messages. >> >> No, this should be here. As it stands this is completely without >> content. >> >> In any case, NAK on this approach. Its the wrong way around. >> >> _IF_ you want to do a non-wrapping variant, it must not be the default. >> >> Since you need to audit every single atomic_t user in the kernel anyway, >> it doesn't matter. But changing atomic_t to non-wrap by default is not >> robust, if you forgot one, you can then trivially dos the kernel. > > Completely agreed. > > Whilst I understand that you're addressing an important and commonly > exploited vulnerability, this really needs to be opt-in rather than > opt-out given the prevalence of atomic_t users in the kernel. Having a > "hardened" kernel that does the wrong thing is useless. I (obviously) disagree. It's not useless. Such a kernel is totally safe against refcount errors and would be exposed to DoS issues only where mistakes were made. This is the fundamental shift here: - we already have exploitable privilege escalation refcount flaws on a regular basis - this changes things to have zero exploitable refcount flaws now and into the future - the risk is bugs leading to DoS instead of the risk of exploitable flaws That's the real trade. >> That said, I still don't much like this. >> >> I would much rather you make kref useful and use that. It still means >> you get to audit all refcounts in the kernel, but hey, you had to do >> that anyway. > > What needs to happen to kref to make it useful? Like many others, I've > been guilty of using atomic_t for refcounts in the past. That's the point: expecting everyone to get this right and not miss mistake from now into the future is not a solution. This solves the privilege escalation issue for refcounts now and forever. -Kees -- Kees Cook Nexus Security From mboxrd@z Thu Jan 1 00:00:00 1970 Reply-To: kernel-hardening@lists.openwall.com MIME-Version: 1.0 Sender: keescook@google.com In-Reply-To: <20161110204838.GE17134@arm.com> References: <1478809488-18303-1-git-send-email-elena.reshetova@intel.com> <20161110203749.GV3117@twins.programming.kicks-ass.net> <20161110204838.GE17134@arm.com> From: Kees Cook Date: Thu, 10 Nov 2016 13:01:41 -0800 Message-ID: Content-Type: text/plain; charset=UTF-8 Subject: [kernel-hardening] Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC To: Will Deacon , Greg KH Cc: Peter Zijlstra , Elena Reshetova , "kernel-hardening@lists.openwall.com" , Arnd Bergmann , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , LKML List-ID: On Thu, Nov 10, 2016 at 12:48 PM, Will Deacon wrote: > On Thu, Nov 10, 2016 at 09:37:49PM +0100, Peter Zijlstra wrote: >> On Thu, Nov 10, 2016 at 10:24:35PM +0200, Elena Reshetova wrote: >> > This series brings the PaX/Grsecurity PAX_REFCOUNT >> > feature support to the upstream kernel. All credit for the >> > feature goes to the feature authors. >> > >> > The name of the upstream feature is HARDENED_ATOMIC >> > and it is configured using CONFIG_HARDENED_ATOMIC and >> > HAVE_ARCH_HARDENED_ATOMIC. >> > >> > This series only adds x86 support; other architectures are expected >> > to add similar support gradually. >> > >> > More information about the feature can be found in the following >> > commit messages. >> >> No, this should be here. As it stands this is completely without >> content. >> >> In any case, NAK on this approach. Its the wrong way around. >> >> _IF_ you want to do a non-wrapping variant, it must not be the default. >> >> Since you need to audit every single atomic_t user in the kernel anyway, >> it doesn't matter. But changing atomic_t to non-wrap by default is not >> robust, if you forgot one, you can then trivially dos the kernel. > > Completely agreed. > > Whilst I understand that you're addressing an important and commonly > exploited vulnerability, this really needs to be opt-in rather than > opt-out given the prevalence of atomic_t users in the kernel. Having a > "hardened" kernel that does the wrong thing is useless. I (obviously) disagree. It's not useless. Such a kernel is totally safe against refcount errors and would be exposed to DoS issues only where mistakes were made. This is the fundamental shift here: - we already have exploitable privilege escalation refcount flaws on a regular basis - this changes things to have zero exploitable refcount flaws now and into the future - the risk is bugs leading to DoS instead of the risk of exploitable flaws That's the real trade. >> That said, I still don't much like this. >> >> I would much rather you make kref useful and use that. It still means >> you get to audit all refcounts in the kernel, but hey, you had to do >> that anyway. > > What needs to happen to kref to make it useful? Like many others, I've > been guilty of using atomic_t for refcounts in the past. That's the point: expecting everyone to get this right and not miss mistake from now into the future is not a solution. This solves the privilege escalation issue for refcounts now and forever. -Kees -- Kees Cook Nexus Security