Date: Thu, 10 Nov 2016 13:01:41 -0800 From: Kees Cook <keescook@...omium.org> To: Will Deacon <will.deacon@....com>, Greg KH <gregkh@...uxfoundation.org> Cc: Peter Zijlstra <peterz@...radead.org>, Elena Reshetova <elena.reshetova@...el.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.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: [RFC v4 PATCH 00/13] HARDENED_ATOMIC 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. 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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