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

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Date:   Tue, 25 Apr 2017 09:39:52 -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 Tue, Apr 25, 2017 at 4:26 AM, PaX Team <pageexec@...email.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