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From: Kees Cook <keescook@chromium.org>
Date: Tue, 25 Apr 2017 09:39:52 -0700
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Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling
To: PaX Team <pageexec@freemail.hu>
Cc: Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Eric Biggers <ebiggers3@gmail.com>,
        Christoph Hellwig <hch@infradead.org>,
        "axboe@kernel.dk" <axboe@kernel.dk>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Hans Liljestrand <ishkamiel@gmail.com>,
        David Windsor <dwindsor@gmail.com>, "x86@kernel.org" <x86@kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jann Horn <jann@thejh.net>, "David S. Miller" <davem@davemloft.net>,
        linux-arch <linux-arch@vger.kernel.org>,
        "kernel-hardening@lists.openwall.com" 
        <kernel-hardening@lists.openwall.com>
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On Tue, Apr 25, 2017 at 4:26 AM, PaX Team <pageexec@freemail.hu> wrote:
> INT_MAX threads would be needed when the leaking path is locked so
> that it can only be exercised once and you'll need to get normal
> (balanced) paths preempted just after the increment. if the leaking
> path is lockless (can be exercised in parallel without bounds) then
> 2 threads are enough where the one triggering the signed overflow
> would have to be preempted while the other one does INT_MAX increments
> and trigger the UAF. this is where the other mechanisms i talked about
> in the past become relevant: preemption or interrupts can be disabled
> or negative refcount values can be detected and acted upon (your blind
> copy-pasting effort passed upon this latter opportunity by not
> specializing the 'jo' into 'js' for the refcount case).

Well, it's not "blind" -- I'm trying to bring the code as-is to
upstream for discussion/examination with as little functional
differences as possible so it's easier to compare apples to apples.
(Which already resulted in more eyes looking at the code to find a bug
-- thanks Jann!) But yes, jo -> js hugely increases the coverage. I'll
make that change for v2.

Thanks!

-Kees

-- 
Kees Cook
Pixel Security