linux-kernel - Re: use-after-free in sctp_do

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Date:	Fri, 4 Dec 2015 12:11:05 -0500
From:	Jason Baron <jbaron@...mai.com>
To:	Joe Perches <joe@...ches.com>, Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Aaron Conole <aconole@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	syzkaller <syzkaller@...glegroups.com>,
	Vladislav Yasevich <vyasevich@...il.com>,
	linux-sctp@...r.kernel.org, netdev <netdev@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: use-after-free in sctp_do_sm



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.
> 
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