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

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Message-ID: <87two93ig8.fsf@doppelsaurus.mobileactivedefense.com>
Date:	Wed, 25 Nov 2015 20:57:59 +0000
From:	Rainer Weikusat <rweikusat@...ileactivedefense.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Rainer Weikusat <rweikusat@...ileactivedefense.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	"David S. Miller" <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Ying Xue <ying.xue@...driver.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: use-after-free in sock_wake_async

Eric Dumazet <eric.dumazet@...il.com> writes:
> On Wed, 2015-11-25 at 11:50 -0800, Eric Dumazet wrote:
>
>> >                 other->sk_data_ready(other);
>> > +               unix_state_unlock(other);
>
>
> Also, problem with such construct is that we wakeup a thread that will
> block on the lock we hold.
>
> Beauty of sk_data_ready() is to call it once we hold no lock any more,
> to enable another cpu to immediately proceed.
>
> In this case, 'other' can not disappear, so it should be safe.

I do agree that keeping the ->sk_data_ready outside of the lock will
very likely have performance advantages. That's just something I
wouldn't have undertaken because I'd be reluctant to make a fairly
complicated change to a lot of code in order to improve performance
unless performance was actually found to be lacking and because it would
step onto to many different people's turf.
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