Date: Thu, 10 Nov 2016 12:56:09 -0800 From: Kees Cook <keescook@...omium.org> To: Peter Zijlstra <peterz@...radead.org>, Greg KH <gregkh@...uxfoundation.org> Cc: 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>, Will Deacon <will.deacon@....com>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC On Thu, Nov 10, 2016 at 12:37 PM, Peter Zijlstra <peterz@...radead.org> 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. Unfortunately, we have to do it this way because there are so many misuses of atomic_t, and they just keep appearing. We can't do opt-in protections for the kernel -- we need to protect atomic_t and opt OUT of the protection where it's not needed. We must change the kernel culture to making things secure-by-default. Without this, we're wasting our time and continuing to leave people vulnerable every time some new driver lands that refcounts with atomic_t. Since education is proven to not work, we have to harden the _infrastructure_ of the kernel, of which atomic_t is a part. > 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. Correct: everything must be audited in either case. However, making a mistake using opt-out means a DoS. Making a mistake using opt-in means an exploitable kernel escalation. We must have the courage to recognize this distinction. Right now, every refcount mistake is an exploitable kernel flaw. Reducing this to a DoS is a giant improvement. > 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. This has already been suggested in the past, and suffers from the same opt-in problem. I'll let Greg comment on it, though, as he's agreed with going opt-out in the past when reviewing this work. -Kees -- Kees Cook Nexus Security
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