netdev - Re: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to refcount

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Date:   Wed, 22 Mar 2017 12:08:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        David Miller <davem@...emloft.net>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Network Development <netdev@...r.kernel.org>,
        bridge@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        James Morris <jmorris@...ei.org>,
        Patrick McHardy <kaber@...sh.net>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to refcount_t

On Tue, Mar 21, 2017 at 7:03 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2017-03-21 at 16:51 -0700, Kees Cook wrote:
>
>> Am I understanding you correctly that you'd want something like:
>>
>> refcount.h:
>> #ifdef UNPROTECTED_REFCOUNT
>> #define refcount_inc(x)   atomic_inc(x)
>> ...
>> #else
>> void refcount_inc(...
>> ...
>> #endif
>>
>> some/net.c:
>> #define UNPROTECTED_REFCOUNT
>> #include <refcount.h>
>>
>> or similar?
>
> At first, it could be something simple like that yes.
>
> Note that we might define two refcount_inc()  : One that does whole
> tests, and refcount_inc_relaxed() that might translate to atomic_inc()
> on non debug kernels.
>
> Then later, maybe provide a dynamic infrastructure so that we can
> dynamically force the full checks even for refcount_inc_relaxed() on say
> 1% of the hosts, to get better debug coverage ?

Well, this isn't about finding bugs in normal workflows. This is about
catching bugs that attackers have found and start exploiting to gain a
use-after-free primitive. The intention is for it to be always
enabled.

-Kees

-- 
Kees Cook
Pixel Security