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

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Message-ID: <CAGXu5jLDybHn_K=vpXOhrsBE_-EtPA+jbXCqHmm3tUgbK24BOw@mail.gmail.com>
Date:   Mon, 24 Apr 2017 13:33:11 -0700
From:   Kees Cook <keescook@...omium.org>
To:     PaX Team <pageexec@...email.hu>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>
Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling

On Mon, Apr 24, 2017 at 4:00 AM, PaX Team <pageexec@...email.hu> wrote:
> On 24 Apr 2017 at 10:32, Peter Zijlstra 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.
>
> note that the above is wrong (and even contradicting itself and the code).

True, this changelog could be more accurate (it resets to INT_MAX on
overflow and INT_MIN on underflow). I think I'm right in saying that a
system would need INT_MAX threads running a refcount_inc() (and a
refcount_dec_and_test() at exactly the right moment) before the reset
handler got scheduled, though, yes?

I'll attempt to clarify this.

-Kees

-- 
Kees Cook
Pixel Security