Date: Thu, 10 Nov 2016 16:23:08 -0500 From: David Windsor <dave@...gbits.org> To: kernel-hardening@...ts.openwall.com Cc: Will Deacon <will.deacon@....com>, Greg KH <gregkh@...uxfoundation.org>, Peter Zijlstra <peterz@...radead.org>, Elena Reshetova <elena.reshetova@...el.com>, Arnd Bergmann <arnd@...db.de>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <h.peter.anvin@...el.com>, LKML <linux-kernel@...r.kernel.org> Subject: Re: Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC On Thu, Nov 10, 2016 at 4:01 PM, Kees Cook <keescook@...omium.org> wrote: > On Thu, Nov 10, 2016 at 12:48 PM, Will Deacon <will.deacon@....com> 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. > 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
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