linux-kernel - Re: [PATCH] x86/refcount: Implement fast refcount

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Message-ID: <CAGXu5jLY0DpTWhL-CXOF15BvFvQ_4+_pg+vMbuvKqLu9tU_jQw@mail.gmail.com>
Date:   Mon, 24 Apr 2017 13:16:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Eric Biggers <ebiggers3@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jann Horn <jann@...jh.net>,
        "David S. Miller" <davem@...emloft.net>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        PaX Team <pageexec@...email.hu>
Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling

On Mon, Apr 24, 2017 at 1:32 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Apr 21, 2017 at 03:09:39PM -0700, Kees Cook wrote:
>> This patch ports the x86-specific atomic overflow handling from PaX's
>> PAX_REFCOUNT to the upstream refcount_t API. This is an updated version
>> from PaX that eliminates the saturation race condition by resetting the
>> atomic counter back to the INT_MAX saturation value on both overflow and
>> underflow. To win a race, a system would have to have INT_MAX threads
>> simultaneously overflow before the saturation handler runs.
>
> And is this impossible? Highly unlikely I'll grant you, but absolutely
> impossible?

I'll adjust the language. "Highly unlikely" is still better than
"trivially doable with a single thread". :)

> Also, you forgot nr_cpus in your bound. Afaict the worst case here is
> O(nr_tasks + 3*nr_cpus).
>
> Because PaX does it, is not a correctness argument. And this really
> wants one.

Sure, I didn't mean to imply anything other than a demonstration of
what PaX is doing (and that it's better than not having it). If we can
improve it, that's great.

-Kees

-- 
Kees Cook
Pixel Security