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Date: Tue, 25 Apr 2017 13:26:43 +0200
From: "PaX Team" <pageexec@...email.hu>
To: Kees Cook <keescook@...omium.org>, Peter Zijlstra <peterz@...radead.org>
CC: 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 25 Apr 2017 at 0:01, Peter Zijlstra wrote:
> On Mon, Apr 24, 2017 at 01:40:56PM -0700, Kees Cook wrote:
> > I think we're way off in the weeds here. The "cannot inc from 0" check
> > is about general sanity checks on refcounts.
>
> I disagree, although sanity check are good too.
exactly, an attacker doesn't care how a premature free occurs due
to reaching a 0 refcount, afterwards it's memory corruption time for
both old and new references regardless.
> > However, what the refcount hardening protection is trying to do is
> > protect again the exploitable condition: overflow.
>
> Sure..
underflow is also exploitable, it's just much harder to defend against
(there're no known practical solutions).
> > Inc-from-0 isn't an exploitable condition since in theory
> > the memory suddenly becomes correctly managed again.
>
> It does not. It just got free'ed. Nothing will stop the free from
> happening (or already having happened).
now hold this thought...
> How is the below not useful fodder for an exploit? It might be a less
> common bug, and perhaps a bit more fiddly to make work, but afaict its
> still a full use-after-free and therefore useful.
>
> ---
>
> Thread-A Thread-B
>
> if(dec_and_test(&obj->ref)) { // true, ref==0
>
> inc(&obj->ref) // ref: 0->1
>
> kfree(obj);
> }
... and tell me why an attacker would let Thread-B do that increment
(that you're trying to detect) *before* the underlying memory gets
reused and thus the 0 changed to something else? hint: he'll do everything
in his power to prevent that, either by winning the race or if there's
no race (no refcount users outside his control), he'll win every time.
IOW, checking for 0 is pointless and you kinda proved it yourself now.
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