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Message-ID: <CAGXu5jLD-ju_XHppVgDgUo1eSPoYKMqrzKS0PguvMAt5ddOoOw@mail.gmail.com> Date: Tue, 25 Jul 2017 12:11:08 -0700 From: Kees Cook <keescook@...omium.org> To: Li Kun <hw.likun@...wei.com> Cc: Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Christoph Hellwig <hch@...radead.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, Andrew Morton <akpm@...ux-foundation.org>, Jann Horn <jannh@...gle.com>, Eric Biggers <ebiggers3@...il.com>, Elena Reshetova <elena.reshetova@...el.com>, Hans Liljestrand <ishkamiel@...il.com>, Greg KH <gregkh@...uxfoundation.org>, Alexey Dobriyan <adobriyan@...il.com>, "Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com, Davidlohr Bueso <dave@...olabs.net>, Manfred Spraul <manfred@...orfullife.com>, "axboe@...nel.dk" <axboe@...nel.dk>, James Bottomley <James.Bottomley@...senpartnership.com>, "x86@...nel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>, "David S. Miller" <davem@...emloft.net>, Rik van Riel <riel@...hat.com>, LKML <linux-kernel@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH v8 3/3] x86/refcount: Implement fast refcount overflow protection On Tue, Jul 25, 2017 at 5:03 AM, Li Kun <hw.likun@...wei.com> wrote: > Hi Kees, > > > on 2017/7/25 2:35, Kees Cook wrote: >> >> +static __always_inline __must_check >> +int __refcount_add_unless(refcount_t *r, int a, int u) >> +{ >> + int c, new; >> + >> + c = atomic_read(&(r->refs)); >> + do { >> + if (unlikely(c == u)) >> + break; >> + >> + asm volatile("addl %2,%0\n\t" >> + REFCOUNT_CHECK_LT_ZERO >> + : "=r" (new) >> + : "0" (c), "ir" (a), >> + [counter] "m" (r->refs.counter) >> + : "cc", "cx"); >> + >> + } while (!atomic_try_cmpxchg(&(r->refs), &c, new)); >> + >> + return c; >> +} > > here when the result LT_ZERO, you will saturate the r->refs.counter and make > the > > atomic_try_cmpxchg(&(r->refs), &c, new) bound to fail first time. > > maybe we can just saturate the value of variable "new" ? Oh, good catch! Thanks. Actually, it's even worse than that: we'll exit this function without the refcount being correctly saturated. The final result will be INT_MIN / 2 + a. It's not terrible, but I should have noticed this in testing. (There was a gap in my testing for the _not_zero() overflows, which I've fixed now...) I'll figure this out or revert to the logic I was using in v6... -Kees -- Kees Cook Pixel Security
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