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

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Date:   Mon, 24 Apr 2017 21:11:20 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     PaX Team <pageexec@...email.hu>,
        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: [kernel-hardening] Re: [PATCH] x86/refcount: Implement fast
 refcount_t handling

On Mon, 2017-04-24 at 15:37 -0700, Kees Cook wrote:
> On Mon, Apr 24, 2017 at 3:01 PM, Peter Zijlstra <peterz@...radead.org
> > 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.
> > 
> > > It should never happen, and if it does, there's a bug.
> > 
> > The very same is true of the overflow thing.
> > 
> > > However, what the refcount hardening protection is trying to do
> > > is
> > > protect again the exploitable condition: overflow.
> > 
> > Sure..
> > 
> > > 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).
> 
> Well, yes, but that's kind of my point. Detecting inc-from-0 is "too
> late" to offer a protection. It offers notification of a bug, rather
> than stopping an exploit from happening.

inc-from-0 could allow the attacker to gain access to
an object which gets allocated to a new user afterwards.

Certainly much less useful as an exploit, but still a
potential privilege escalation.