From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936223AbcKJVkQ (ORCPT ); Thu, 10 Nov 2016 16:40:16 -0500 Received: from sender163-mail.zoho.com ([74.201.84.163]:21486 "EHLO sender163-mail.zoho.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755149AbcKJVkO (ORCPT ); Thu, 10 Nov 2016 16:40:14 -0500 MIME-Version: 1.0 In-Reply-To: References: <1478809488-18303-1-git-send-email-elena.reshetova@intel.com> <20161110203749.GV3117@twins.programming.kicks-ass.net> <20161110204838.GE17134@arm.com> From: David Windsor Date: Thu, 10 Nov 2016 16:39:49 -0500 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [kernel-hardening] Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC To: Kees Cook Cc: "kernel-hardening@lists.openwall.com" , Will Deacon , Greg KH , Peter Zijlstra , Elena Reshetova , 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 4:27 PM, Kees Cook wrote: > On Thu, Nov 10, 2016 at 1:23 PM, David Windsor wrote: >> On Thu, Nov 10, 2016 at 4:01 PM, Kees Cook wrote: >>> 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. >>> >> >> Discussions have been occurring since KSPP has begun: do we need a >> specialized type for reference counters? Oh, wait, we do: kref. >> Wait! kref is implemented with atomic_t. >> >> So, what? We obviously need an atomicity for a reference counter >> type. So, do we simply implement the HARDENED_ATOMIC protected >> version of atomic_t "inside" of kref and leave atomic_t alone? >> >> That would certainly reduce the number of users using atomic_t when >> they don't need a refcounter: kernel users using kref probably meant >> to use it as a reference counter, so wrap protection wouldn't cause a >> DoS. > > But it leaves all the newly added drivers that get it wrong (by not > using wrap-protected kref) exposed to privilege escalation. We have to > kill the entire class of vulnerability. It needs to be impossible to > get refcounting wrong from a pragmatic approach: we can't educate > everyone, so the infrastructure must be safe. > I completely agree with you: I'm just bringing to light now the arguments that were voiced about this (particularly by someone from the PaX/grsecurity team; I forget which thread). To address Peter's concerns about trivially DoS-ing a kernel, would it be feasible to still use atomic_t as a protected type, but change hardened_atomic_overflow() to not kill the process? Just for a time: we let this bake in distros for a while, fix all of the yet-to-be-identified wrappable users of atomic_t, then re-enable process killing? >>> 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 In-Reply-To: References: <1478809488-18303-1-git-send-email-elena.reshetova@intel.com> <20161110203749.GV3117@twins.programming.kicks-ass.net> <20161110204838.GE17134@arm.com> From: David Windsor Date: Thu, 10 Nov 2016 16:39:49 -0500 Message-ID: Content-Type: text/plain; charset=UTF-8 Subject: Re: [kernel-hardening] Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC To: Kees Cook Cc: "kernel-hardening@lists.openwall.com" , Will Deacon , Greg KH , Peter Zijlstra , Elena Reshetova , Arnd Bergmann , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , LKML List-ID: On Thu, Nov 10, 2016 at 4:27 PM, Kees Cook wrote: > On Thu, Nov 10, 2016 at 1:23 PM, David Windsor wrote: >> On Thu, Nov 10, 2016 at 4:01 PM, Kees Cook wrote: >>> 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. >>> >> >> Discussions have been occurring since KSPP has begun: do we need a >> specialized type for reference counters? Oh, wait, we do: kref. >> Wait! kref is implemented with atomic_t. >> >> So, what? We obviously need an atomicity for a reference counter >> type. So, do we simply implement the HARDENED_ATOMIC protected >> version of atomic_t "inside" of kref and leave atomic_t alone? >> >> That would certainly reduce the number of users using atomic_t when >> they don't need a refcounter: kernel users using kref probably meant >> to use it as a reference counter, so wrap protection wouldn't cause a >> DoS. > > But it leaves all the newly added drivers that get it wrong (by not > using wrap-protected kref) exposed to privilege escalation. We have to > kill the entire class of vulnerability. It needs to be impossible to > get refcounting wrong from a pragmatic approach: we can't educate > everyone, so the infrastructure must be safe. > I completely agree with you: I'm just bringing to light now the arguments that were voiced about this (particularly by someone from the PaX/grsecurity team; I forget which thread). To address Peter's concerns about trivially DoS-ing a kernel, would it be feasible to still use atomic_t as a protected type, but change hardened_atomic_overflow() to not kill the process? Just for a time: we let this bake in distros for a while, fix all of the yet-to-be-identified wrappable users of atomic_t, then re-enable process killing? >>> 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