linux-kernel - Re: [PATCH v6 0/2] x86: Implement fast refcount overflow protection

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Date:   Fri, 21 Jul 2017 09:50:20 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Kees Cook <keescook@...omium.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


* Kees Cook <keescook@...omium.org> wrote:

> 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).

Ok, that's pretty convincig - I'd suggest including a cicles row in the table
instead of an instructions row: number of instructions is indeed slightly
misleading in this case.

Thanks,

	Ingo