[PATCH 4.4 284/312] l2tp: fix use-after-free during module unload

[Greg Kroah-Hartman]

From: Sabrina Dubroca <sd@xxxxxxxxxxxxxxx>

commit 2f86953e7436c9b9a4690909c5e2db24799e173b upstream.

Tunnel deletion is delayed by both a workqueue (l2tp_tunnel_delete -> wq
 -> l2tp_tunnel_del_work) and RCU (sk_destruct -> RCU ->
l2tp_tunnel_destruct).

By the time l2tp_tunnel_destruct() runs to destroy the tunnel and finish
destroying the socket, the private data reserved via the net_generic
mechanism has already been freed, but l2tp_tunnel_destruct() actually
uses this data.

Make sure tunnel deletion for the netns has completed before returning
from l2tp_exit_net() by first flushing the tunnel removal workqueue, and
then waiting for RCU callbacks to complete.

Fixes: 167eb17e0b17 ("l2tp: create tunnel sockets in the right namespace")
Signed-off-by: Sabrina Dubroca <sd@xxxxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 net/l2tp/l2tp_core.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1953,6 +1953,9 @@ static __net_exit void l2tp_exit_net(str
 		l2tp_tunnel_delete(tunnel);
 	}
 	rcu_read_unlock_bh();
+
+	flush_workqueue(l2tp_wq);
+	rcu_barrier();
 }
 
 static struct pernet_operations l2tp_net_ops = {