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

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Date:   Tue, 25 Apr 2017 09:36:17 -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:
> On 25 Apr 2017 at 0:01, Peter Zijlstra wrote:
>> 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.

Right, having a deterministic protection (checking for overflow) is
best since it stops all exploits using that path. Hoping that an
attacker is unlucky and hits a notification after they've already
landed their corruption is not a very useful defense. It certainly has
a non-zero value, but stopping overflow 100% is better. Especially
when we can do it with no meaningful change in performance which lets
us actually do the atomic_t -> refcount_t conversion everywhere.

-Kees

-- 
Kees Cook
Pixel Security