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

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Message-ID: <58FDDAC2.11341.175B5A99@pageexec.freemail.hu>
Date:   Mon, 24 Apr 2017 13:00:18 +0200
From:   "PaX Team" <pageexec@...email.hu>
To:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
CC:     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,
        Ingo Molnar <mingo@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jann Horn <jann@...jh.net>, davem@...emloft.net,
        linux-arch@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling

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).

> And is this impossible? Highly unlikely I'll grant you, but absolutely
> impossible?

here's my analysis from a while ago:
http://www.openwall.com/lists/kernel-hardening/2017/01/05/19

> Also, you forgot nr_cpus in your bound. Afaict the worst case here is
> O(nr_tasks + 3*nr_cpus).

what does nr_cpus have to do with winning the race?

> Because PaX does it, is not a correctness argument. And this really
> wants one.

heh, do you want to tell me about how checking for a 0 refcount prevents
exploiting a bug?