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Message-ID: <CAGXu5jKM_gThfVoTxPE8vX-9GhBMd+ZYbwky5jUKJNevYxd50Q@mail.gmail.com>
Date: Thu, 20 Jul 2017 15:53:25 -0700
From: Kees Cook <keescook@...omium.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: 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 v6 0/2] x86: Implement fast refcount overflow protection
On Thu, Jul 20, 2017 at 10:15 AM, Kees Cook <keescook@...omium.org> wrote:
> On Thu, Jul 20, 2017 at 2:11 AM, Ingo Molnar <mingo@...nel.org> wrote:
>> Could you please also create a tabulated quick-comparison of the three variants,
>> of all key properties, about behavior, feature and tradeoff differences?
>>
>> Something like:
>>
>> !ARCH_HAS_REFCOUNT ARCH_HAS_REFCOUNT=y REFCOUNT_FULL=y
>>
>> avg fast path instructions: 5 3 10
>> behavior on overflow: unsafe, silent safe, verbose safe, verbose
>> behavior on underflow: unsafe, silent unsafe, verbose unsafe, verbose
>> ...
>>
>> etc. - note that this table is just a quick mockup with wild guesses. (Please add
>> more comparisons of other aspects as well.)
>>
>> Such a comparison would make it easier for arch, subsystem and distribution
>> maintainers to decide on which variant to use/enable.
>
> Sure, I can write this up. I'm not sure "safe"/"unsafe" is quite that
> clean. The differences between -full and -fast are pretty subtle, but
> I think I can describe it using the updated LKDTM tests I've written
> to compare the two. There are conditions that -fast doesn't catch, but
> those cases aren't actually useful for the overflow defense.
>
> As for "avg fast path instructions", do you mean the resulting
> assembly for each refcount API function? I think it's going to look
> something like "1 2 45", but I'll write it up.
So, doing a worst-case timing of a loop of inc() to INT_MAX and then
dec_and_test() back to zero, I see this out of perf:
atomic
25255.114805 task-clock (msec)
82249267387 cycles
11208720041 instructions
refcount-fast
25259.577583 task-clock (msec)
82211446892 cycles
15486246572 instructions
refcount-full
44625.923432 task-clock (msec)
144814735193 cycles
105937495952 instructions
I'll still summarize all this in the v7 series, but I think that
really clarifies the differences: 1.5x more instructions in -fast, but
nearly identical cycles and clock. Using -full sees a large change (as
expected).
-Kees
--
Kees Cook
Pixel Security
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