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Subject[PATCH 3.16 246/346] l2tp: fix use-after-free during module unload
3.16.39-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Sabrina Dubroca <sd@queasysnail.net>

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@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
net/l2tp/l2tp_core.c | 3 +++
1 file changed, 3 insertions(+)

--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1892,6 +1892,9 @@ static __net_exit void l2tp_exit_net(str
(void)l2tp_tunnel_delete(tunnel);
}
rcu_read_unlock_bh();
+
+ flush_workqueue(l2tp_wq);
+ rcu_barrier();
}

static struct pernet_operations l2tp_net_ops = {
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