linux-kernel - Re: [PATCH v4 1/3] net: unix: fix use-after-free in unix_dgram

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Date:	Fri, 09 Oct 2015 16:38:29 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Jason Baron <jbaron@...mai.com>, davem@...emloft.net
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	minipli@...glemail.com, normalperson@...t.net,
	eric.dumazet@...il.com, rweikusat@...ileactivedefense.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,
	joe@...ches.com
Subject: Re: [PATCH v4 1/3] net: unix: fix use-after-free in unix_dgram_poll()

Hi,

Jason Baron <jbaron@...mai.com> writes:

> The unix_dgram_poll() routine calls sock_poll_wait() not only for the wait
> queue associated with the socket s that we are poll'ing against, but also calls
> sock_poll_wait() for a remote peer socket p, if it is connected. Thus,
> if we call poll()/select()/epoll() for the socket s, there are then
> a couple of code paths in which the remote peer socket p and its associated
> peer_wait queue can be freed before poll()/select()/epoll() have a chance
> to remove themselves from the remote peer socket.
>
> The way that remote peer socket can be freed are:
>
> 1. If s calls connect() to a connect to a new socket other than p, it will
> drop its reference on p, and thus a close() on p will free it.
>
> 2. If we call close on p(), then a subsequent sendmsg() from s, will drop
> the final reference to p, allowing it to be freed.
>
> Address this issue, by reverting unix_dgram_poll() to only register with
> the wait queue associated with s and register a callback with the remote peer
> socket on connect() that will wake up the wait queue associated with s. If
> scenarios 1 or 2 occur above we then simply remove the callback from the
> remote peer. This then presents the expected semantics to poll()/select()/
> epoll().
>
> I've implemented this for sock-type, SOCK_RAW, SOCK_DGRAM, and SOCK_SEQPACKET
> but not for SOCK_STREAM, since SOCK_STREAM does not use unix_dgram_poll().
>
> Introduced in commit ec0d215f9420 ("af_unix: fix 'poll for write'/connected
> DGRAM sockets").
>
> Tested-by: Mathias Krause <minipli@...glemail.com>
> Signed-off-by: Jason Baron <jbaron@...mai.com>

While I think this approach works, I haven't seen where the current code
leaks a reference. Assignment to unix_peer(sk) in general take spin_lock
and increment refcount. Are there bugs at the two places you referred
to?

Is an easier fix just to use atomic_inc_not_zero(&sk->sk_refcnt) in
unix_peer_get() which could also help other places?

Thanks,
Hannes
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