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

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Date:   Tue, 25 Apr 2017 11:05:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.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: [PATCH] x86/refcount: Implement fast refcount_t handling

On Mon, Apr 24, 2017 at 03:37:32PM -0700, Kees Cook wrote:
> On Mon, Apr 24, 2017 at 3:01 PM, Peter Zijlstra <peterz@...radead.org> wrote:

> > 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.

Well, your setup (panic_on_warn et al) would have it panic the box. That
will effectively stop the exploit by virtue of stopping everything.

And warn/bug/panic etc.. are I think a better option that silently
letting it happen.