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Date: Mon, 24 Apr 2017 13:15:53 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: PaX Team <pageexec@...email.hu>
Cc: Kees Cook <keescook@...omium.org>, 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 Mon, Apr 24, 2017 at 01:00:18PM +0200, PaX Team wrote:
> On 24 Apr 2017 at 10:32, Peter Zijlstra wrote:
> > 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?
The CPUs could each run nested softirq/hardirq/nmi context poking at the
refcount, irrespective of the (preempted) task context.
> > 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?
Not the point. All I said was that saying somebody else does it (anybody
else, doesn't matter it was you) isn't an argument for correctness.
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