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 <[email protected]>
cc: Eric Dumazet <[email protected]>
cc: Marc Dionne <[email protected]>
cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]/ [1]
Link: https://syzkaller.appspot.com/bug?extid=724378c4bb58f703b09a [2]
Reviewed-by: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

1 files changed, 2 insertions, 0 deletions

diff --git a/net/rxrpc/net_ns.c b/net/rxrpc/net_ns.c
index f15d6942da45..cc7e30733feb 100644
--- 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);