Patch "l2tp: fix race condition in l2tp_tunnel_delete" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

l2tp: fix race condition in l2tp_tunnel_delete

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
l2tp-fix-race-condition-in-l2tp_tunnel_delete.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Sabrina Dubroca <[email protected]>
Date: Tue, 26 Sep 2017 16:16:43 +0200
Subject: l2tp: fix race condition in l2tp_tunnel_delete

From: Sabrina Dubroca <[email protected]>

[ Upstream commit 62b982eeb4589b2e6d7c01a90590e3a4c2b2ca19 ]

If we try to delete the same tunnel twice, the first delete operation does a lookup (l2tp_tunnel_get), finds the tunnel, calls l2tp_tunnel_delete, which queues it for deletion by l2tp_tunnel_del_work.

The second delete operation also finds the tunnel and calls l2tp_tunnel_delete. If the workqueue has already fired and started running l2tp_tunnel_del_work, then l2tp_tunnel_delete will queue the same tunnel a second time, and try to free the socket again.

Add a dead flag to prevent firing the workqueue twice. Then we can remove the check of queue_work's result that was meant to prevent that race but doesn't.

Reproducer:

ip l2tp add tunnel tunnel_id 3000 peer_tunnel_id 4000 local 192.168.0.2 remote 192.168.0.1 encap udp udp_sport 5000 udp_dport 6000 ip l2tp add session name l2tp1 tunnel_id 3000 session_id 1000 peer_session_id 2000
ip link set l2tp1 up
ip l2tp del tunnel tunnel_id 3000
ip l2tp del tunnel tunnel_id 3000

---
 net/l2tp/l2tp_core.c |   10 ++++------
 net/l2tp/l2tp_core.h |    5 ++++-
 2 files changed, 8 insertions(+), 7 deletions(-)

@@ -1737,14 +1737,12 @@ EXPORT_SYMBOL_GPL(l2tp_tunnel_create);
 
 /* This function is used by the netlink TUNNEL_DELETE command.
  */
-int l2tp_tunnel_delete(struct l2tp_tunnel *tunnel)
+void l2tp_tunnel_delete(struct l2tp_tunnel *tunnel)
 {
-	l2tp_tunnel_inc_refcount(tunnel);
-	if (false == queue_work(l2tp_wq, &tunnel->del_work)) {
-		l2tp_tunnel_dec_refcount(tunnel);
-		return 1;
+	if (!test_and_set_bit(0, &tunnel->dead)) {
+		l2tp_tunnel_inc_refcount(tunnel);
+		queue_work(l2tp_wq, &tunnel->del_work);
 	}
-	return 0;
 }
 EXPORT_SYMBOL_GPL(l2tp_tunnel_delete);
 

@@ -169,6 +169,9 @@ struct l2tp_tunnel_cfg {
 
 struct l2tp_tunnel {
 	int			magic;		/* Should be L2TP_TUNNEL_MAGIC */
+
+	unsigned long		dead;
+
 	struct rcu_head rcu;
 	rwlock_t		hlist_lock;	/* protect session_hlist */
 	struct hlist_head	session_hlist[L2TP_HASH_SIZE];

@@ -257,7 +260,7 @@ int l2tp_tunnel_create(struct net *net,
 		       u32 peer_tunnel_id, struct l2tp_tunnel_cfg *cfg,
 		       struct l2tp_tunnel **tunnelp);
 void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel);
-int l2tp_tunnel_delete(struct l2tp_tunnel *tunnel);
+void l2tp_tunnel_delete(struct l2tp_tunnel *tunnel);
 struct l2tp_session *l2tp_session_create(int priv_size,
 					 struct l2tp_tunnel *tunnel,
 					 u32 session_id, u32 peer_session_id,

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/l2tp-fix-race-condition-in-l2tp_tunnel_delete.patch