Date: Thu, 10 Nov 2016 13:27:32 -0800 From: Kees Cook <keescook@...omium.org> To: David Windsor <dave@...gbits.org> Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, 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 1:23 PM, David Windsor <dave@...gbits.org> wrote: > 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. 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. >> 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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