netdev - Re: [PATCH v2 1/3] unix: fix use-after-free in unix_dgram

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Message-ID: <87d1w8f7mz.fsf@doppelsaurus.mobileactivedefense.com>
Date:	Wed, 21 Oct 2015 18:34:12 +0100
From:	Rainer Weikusat <rweikusat@...ileactivedefense.com>
To:	Jason Baron <jbaron@...mai.com>
Cc:	Rainer Weikusat <rweikusat@...ileactivedefense.com>,
	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, minipli@...glemail.com,
	normalperson@...t.net, eric.dumazet@...il.com,
	viro@...iv.linux.org.uk, davidel@...ilserver.org,
	dave@...olabs.net, olivier@...ras.ch, pageexec@...email.hu,
	torvalds@...ux-foundation.org, peterz@...radead.org
Subject: Re: [PATCH v2 1/3] unix: fix use-after-free in unix_dgram_poll()

Rainer Weikusat <rw@...pelsaurus.mobileactivedefense.com> writes:
> Jason Baron <jbaron@...mai.com> writes:
>> On 10/18/2015 04:58 PM, Rainer Weikusat wrote:

[...]

>> 1)
>>
>> In unix_peer_wake_relay() function, 'sk_wq' is an __rcu pointer and thus
>> it requires proper dereferencing. Something like:
>>
>> struct unix_sock *u;
>> struct socket_wq *wq;
>>
>> u = container_of(wait, struct unix_sock, wait);
>> rcu_read_lock();
>> wq = rcu_dereference(u->sk.sk_wq);
>> if (wq_has_sleeper(wq))
>> 	wake_up_interruptible_sync_poll(&wq->wait, key);
>> rcu_read_unlock();
>
> I think this may be unecessary

I consider this unnecessary now.

Rationale: The wait queue is allocated and freed in tandem with the
socket inode which means it will remain allocated until after the
protocol release function (unix_release with the bulk of the
implementation being in unix_release_sock) returned. As the connection
with the other socket is broken in unix_release_sock, any relayed wake
up must have completed before this time (since both operations take
place while holding the same wait queue lock).
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