[PATCH net] rxrpc: Fix deadlock around release of dst cached on udp tunnel

[Vadim Fedorenko]

On 29.01.2021 16:44, David Howells wrote:
> AF_RXRPC sockets use UDP ports in encap mode.  This causes socket and dst
> from an incoming packet to get stolen and attached to the UDP socket from
> whence it is leaked when that socket is closed.
> 
> When a network namespace is removed, the wait for dst records to be cleaned
> up happens before the cleanup of the rxrpc and UDP socket, meaning that the
> wait never finishes.
> 
> Fix this by moving the rxrpc (and, by dependence, the afs) private
> per-network namespace registrations to the device group rather than subsys
> group.  This allows cached rxrpc local endpoints to be cleared and their
> UDP sockets closed before we try waiting for the dst records.
> 
> The symptom is that lines looking like the following:
> 
> 	unregister_netdevice: waiting for lo to become free
> 
> get emitted at regular intervals after running something like the
> referenced syzbot test.
> 
> Thanks to Vadim for tracking this down and work out the fix.

You missed the call to dst_release(sk->sk_rx_dst) in rxrpc_sock_destructor. 
Without it we are still leaking the dst.