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Message-ID: <20161111201943.GR3117@twins.programming.kicks-ass.net>
Date: Fri, 11 Nov 2016 21:19:43 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Kees Cook <keescook@...omium.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>,
Hans Liljestrand <ishkamiel@...il.com>,
David Windsor <dwindsor@...il.com>
Subject: Re: [RFC v4 PATCH 12/13] x86: implementation for HARDENED_ATOMIC
On Fri, Nov 11, 2016 at 10:00:27AM -0800, Kees Cook wrote:
> Agreed: the first thread will BUG and the second thread is still halfway to 0.
>
> On systems that panic on BUG, things are protected. For the rest of
> the systems, an alternative to "dec" on overflow is to sub (more than)
> NR_CPUS, to keep the saturation below the overflow level. This means
> that it is still detected (BUG) by at least 1 thread, and cannot reach
> 0 (to trigger the flaw) on all other threads, even if they all lose
> the race.
NR_CPUS has nothing to do with anything here. If this is a vcpu that got
scheduled out in between the inc and dec there can be an arbitrary
amount of other crap happening.
> To me, this seems better than taking the cmpxchg performance hit.
To me that shows you shouldn't be allowed near atomic_t. You cannot have
a non-atomic atomic op. That just doesn't happen.
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