rxrpc: Restore removed timer deletion

A recent patch[1] from Eric Dumazet flipped the order in which the
keepalive timer and the keepalive worker were cancelled in order to fix a
syzbot reported issue[2].  Unfortunately, this enables the mirror image bug
whereby the timer races with rxrpc_exit_net(), restarting the worker after
it has been cancelled:

	CPU 1		CPU 2
	===============	=====================
			if (rxnet->live)
			<INTERRUPT>
	rxnet->live = false;
 	cancel_work_sync(&rxnet->peer_keepalive_work);
			rxrpc_queue_work(&rxnet->peer_keepalive_work);
	del_timer_sync(&rxnet->peer_keepalive_timer);

Fix this by restoring the removed del_timer_sync() so that we try to remove
the timer twice.  If the timer runs again, it should see ->live == false
and not restart the worker.

Fixes: 1946014ca3b1 ("rxrpc: fix a race in rxrpc_exit_net()")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Eric Dumazet <edumazet@google.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/20220404183439.3537837-1-eric.dumazet@gmail.com/
Link: https://syzkaller.appspot.com/bug?extid=724378c4bb58f703b09a
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/rxrpc/net_ns.c b/net/rxrpc/net_ns.c
index f15d694..cc7e307 100644 (file)
--- a/net/rxrpc/net_ns.c
+++ b/net/rxrpc/net_ns.c
@@ -113,7 +113,9 @@ static __net_exit void rxrpc_exit_net(struct net *net)
        struct rxrpc_net *rxnet = rxrpc_net(net);
 
        rxnet->live = false;
+       del_timer_sync(&rxnet->peer_keepalive_timer);
        cancel_work_sync(&rxnet->peer_keepalive_work);
+       /* Remove the timer again as the worker may have restarted it. */
        del_timer_sync(&rxnet->peer_keepalive_timer);
        rxrpc_destroy_all_calls(rxnet);
        rxrpc_destroy_all_connections(rxnet);