From: Jason Baron <jbaron@akamai.com>
Date: Fri, 04 Dec 2015 17:11:05 +0000
Subject: Re: use-after-free in sctp_do_sm
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To: Joe Perches <joe@perches.com>, Dmitry Vyukov <dvyukov@google.com>
Cc: Aaron Conole <aconole@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, LKML <linux-kernel@vger.kernel.org>, Eric Dumazet <edumazet@google.com>, syzkaller <syzkaller@googlegroups.com>, Vladislav Yasevich <vyasevich@gmail.com>, linux-sctp@vger.kernel.org, netdev <netdev@vger.kernel.org>, Kostya Serebryany <kcc@google.com>, Alexander Potapenko <glider@google.com>, Sasha Levin <sasha.levin@oracle.com>



On 12/04/2015 12:03 PM, Joe Perches wrote:
> On Fri, 2015-12-04 at 11:47 -0500, Jason Baron wrote:
>> When DYNAMIC_DEBUG is enabled we have this wrapper from
>> include/linux/dynamic_debug.h:
>>
>> if (unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT))
>> 	<do debug stuff>
>>
>> So the compiler is not emitting the side-effects in this
>> case.
> 
> Huh?  Do I misunderstand what you are writing?

Yes, I wasn't terribly clear - I was trying to say that the
'side-effects', in this case the debug code and use-after-free, are
hidden behind the branch. They aren't invoked unless we enable the debug
statement.

Thanks,

-Jason

> 
> You are testing a variable that is not generally set
> so the call is not being performed in the general case,
> but the compiler can not elide the code.
> 
> If the variable was enabled via the control file, the
> __dynamic_pr_debug would be performed with the
> use-after-free.
>