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Date: Mon, 24 Apr 2017 13:00:18 +0200
From: "PaX Team" <>
To: Kees Cook <>, Peter Zijlstra <>
CC:, Eric Biggers <>,
        Christoph Hellwig <>,
        "" <>,
        James Bottomley <>,
        Elena Reshetova <>,
        Hans Liljestrand <>,
        David Windsor <>,,
        Ingo Molnar <>, Arnd Bergmann <>,
        Greg Kroah-Hartman <>,
        Jann Horn <>,,,
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:

> 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?

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