Messages in this thread | | | From | Hannes Frederic Sowa <> | Subject | Re: [PATCH] unix: avoid use-after-free in ep_remove_wait_queue | Date | Wed, 11 Nov 2015 13:28:18 +0100 |
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Hello,
On Tue, Nov 10, 2015, at 22:55, Rainer Weikusat wrote: > An AF_UNIX datagram socket being the client in an n:1 association with > some server socket is only allowed to send messages to the server if the > receive queue of this socket contains at most sk_max_ack_backlog > datagrams. This implies that prospective writers might be forced to go > to sleep despite none of the message presently enqueued on the server > receive queue were sent by them. In order to ensure that these will be > woken up once space becomes again available, the present unix_dgram_poll > routine does a second sock_poll_wait call with the peer_wait wait queue > of the server socket as queue argument (unix_dgram_recvmsg does a wake > up on this queue after a datagram was received). This is inherently > problematic because the server socket is only guaranteed to remain alive > for as long as the client still holds a reference to it. In case the > connection is dissolved via connect or by the dead peer detection logic > in unix_dgram_sendmsg, the server socket may be freed despite "the > polling mechanism" (in particular, epoll) still has a pointer to the > corresponding peer_wait queue. There's no way to forcibly deregister a > wait queue with epoll. > > Based on an idea by Jason Baron, the patch below changes the code such > that a wait_queue_t belonging to the client socket is enqueued on the > peer_wait queue of the server whenever the peer receive queue full > condition is detected by either a sendmsg or a poll. A wake up on the > peer queue is then relayed to the ordinary wait queue of the client > socket via wake function. The connection to the peer wait queue is again > dissolved if either a wake up is about to be relayed or the client > socket reconnects or a dead peer is detected or the client socket is > itself closed. This enables removing the second sock_poll_wait from > unix_dgram_poll, thus avoiding the use-after-free, while still ensuring > that no blocked writer sleeps forever.
This whole patch seems pretty complicated to me.
Can't we just remove the unix_recvq_full checks alltogether and unify unix_dgram_poll with unix_poll?
If we want to be cautious we could simply make unix_max_dgram_qlen limit the number of skbs which are in flight from a sending socket. The skb destructor can then decrement this. This seems much simpler.
Would this work?
Thanks, Hannes
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