Merge branch 'l2tp-session-cleanup' into main

James Chapman says:

====================
l2tp: simplify tunnel and session cleanup

This series simplifies and improves l2tp tunnel and session cleanup.

 * refactor l2tp management code to not use the tunnel socket's
   sk_user_data. This allows the tunnel and its socket to be closed
   and freed without sequencing the two using the socket's sk_destruct
   hook.

 * export ip_flush_pending_frames and use it when closing l2tp_ip
   sockets.

 * move the work of closing all sessions in the tunnel to the work
   queue so that sessions are deleted using the same codepath whether
   they are closed by user API request or their parent tunnel is
   closing.

 * refactor l2tp_ppp pppox socket / session relationship to have the
   session keep the socket alive, not the other way around. Previously
   the pppox socket held a ref on the session, which complicated
   session delete by having to go through the pppox socket destructor.

 * free sessions and pppox sockets by rcu.

 * fix a possible tunnel refcount underflow.

 * avoid using rcu_barrier in net exit handler.
====================

Signed-off-by: David S. Miller <[email protected]>

8 files changed, 179 insertions, 168 deletions

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index b90d0f78ac80..8a10a7c67834 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1534,6 +1534,7 @@ void ip_flush_pending_frames(struct sock *sk)
 {
 	__ip_flush_pending_frames(sk, &sk->sk_write_queue, &inet_sk(sk)->cork.base);
 }
+EXPORT_SYMBOL_GPL(ip_flush_pending_frames);
 
 struct sk_buff *ip_make_skb(struct sock *sk,
 			    struct flowi4 *fl4,
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index c80ab3f26084..5d2068b6c778 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -137,9 +137,28 @@ static inline struct l2tp_net *l2tp_pernet(const struct net *net)
 
 static void l2tp_tunnel_free(struct l2tp_tunnel *tunnel)
 {
+	struct sock *sk = tunnel->sock;
+
 	trace_free_tunnel(tunnel);
-	sock_put(tunnel->sock);
-	/* the tunnel is freed in the socket destructor */
+
+	if (sk) {
+		/* Disable udp encapsulation */
+		switch (tunnel->encap) {
+		case L2TP_ENCAPTYPE_UDP:
+			/* No longer an encapsulation socket. See net/ipv4/udp.c */
+			WRITE_ONCE(udp_sk(sk)->encap_type, 0);
+			udp_sk(sk)->encap_rcv = NULL;
+			udp_sk(sk)->encap_destroy = NULL;
+			break;
+		case L2TP_ENCAPTYPE_IP:
+			break;
+		}
+
+		tunnel->sock = NULL;
+		sock_put(sk);
+	}
+
+	kfree_rcu(tunnel, rcu);
 }
 
 static void l2tp_session_free(struct l2tp_session *session)
@@ -147,18 +166,29 @@ static void l2tp_session_free(struct l2tp_session *session)
 	trace_free_session(session);
 	if (session->tunnel)
 		l2tp_tunnel_dec_refcount(session->tunnel);
-	kfree(session);
+	kfree_rcu(session, rcu);
 }
 
-struct l2tp_tunnel *l2tp_sk_to_tunnel(struct sock *sk)
+struct l2tp_tunnel *l2tp_sk_to_tunnel(const struct sock *sk)
 {
-	struct l2tp_tunnel *tunnel = sk->sk_user_data;
+	const struct net *net = sock_net(sk);
+	unsigned long tunnel_id, tmp;
+	struct l2tp_tunnel *tunnel;
+	struct l2tp_net *pn;
 
-	if (tunnel)
-		if (WARN_ON(tunnel->magic != L2TP_TUNNEL_MAGIC))
-			return NULL;
+	rcu_read_lock_bh();
+	pn = l2tp_pernet(net);
+	idr_for_each_entry_ul(&pn->l2tp_tunnel_idr, tunnel, tmp, tunnel_id) {
+		if (tunnel &&
+		    tunnel->sock == sk &&
+		    refcount_inc_not_zero(&tunnel->ref_count)) {
+			rcu_read_unlock_bh();
+			return tunnel;
+		}
+	}
+	rcu_read_unlock_bh();
 
-	return tunnel;
+	return NULL;
 }
 EXPORT_SYMBOL_GPL(l2tp_sk_to_tunnel);
 
@@ -249,7 +279,14 @@ struct l2tp_session *l2tp_v3_session_get(const struct net *net, struct sock *sk,
 
 		hash_for_each_possible_rcu(pn->l2tp_v3_session_htable, session,
 					   hlist, key) {
-			if (session->tunnel->sock == sk &&
+			/* session->tunnel may be NULL if another thread is in
+			 * l2tp_session_register and has added an item to
+			 * l2tp_v3_session_htable but hasn't yet added the
+			 * session to its tunnel's session_list.
+			 */
+			struct l2tp_tunnel *tunnel = READ_ONCE(session->tunnel);
+
+			if (tunnel && tunnel->sock == sk &&
 			    refcount_inc_not_zero(&session->ref_count)) {
 				rcu_read_unlock_bh();
 				return session;
@@ -382,12 +419,12 @@ static int l2tp_session_collision_add(struct l2tp_net *pn,
 
 	/* If existing session isn't already in the session hlist, add it. */
 	if (!hash_hashed(&session2->hlist))
-		hash_add(pn->l2tp_v3_session_htable, &session2->hlist,
-			 session2->hlist_key);
+		hash_add_rcu(pn->l2tp_v3_session_htable, &session2->hlist,
+			     session2->hlist_key);
 
 	/* Add new session to the hlist and collision list */
-	hash_add(pn->l2tp_v3_session_htable, &session1->hlist,
-		 session1->hlist_key);
+	hash_add_rcu(pn->l2tp_v3_session_htable, &session1->hlist,
+		     session1->hlist_key);
 	refcount_inc(&clist->ref_count);
 	l2tp_session_coll_list_add(clist, session1);
 
@@ -403,7 +440,7 @@ static void l2tp_session_collision_del(struct l2tp_net *pn,
 
 	lockdep_assert_held(&pn->l2tp_session_idr_lock);
 
-	hash_del(&session->hlist);
+	hash_del_rcu(&session->hlist);
 
 	if (clist) {
 		/* Remove session from its collision list. If there
@@ -437,6 +474,7 @@ int l2tp_session_register(struct l2tp_session *session,
 {
 	struct l2tp_net *pn = l2tp_pernet(tunnel->l2tp_net);
 	struct l2tp_session *other_session = NULL;
+	void *old = NULL;
 	u32 session_key;
 	int err;
 
@@ -477,15 +515,22 @@ int l2tp_session_register(struct l2tp_session *session,
 	}
 
 	l2tp_tunnel_inc_refcount(tunnel);
-	list_add(&session->list, &tunnel->session_list);
+	WRITE_ONCE(session->tunnel, tunnel);
+	list_add_rcu(&session->list, &tunnel->session_list);
 
+	/* this makes session available to lockless getters */
 	if (tunnel->version == L2TP_HDR_VER_3) {
 		if (!other_session)
-			idr_replace(&pn->l2tp_v3_session_idr, session, session_key);
+			old = idr_replace(&pn->l2tp_v3_session_idr, session, session_key);
 	} else {
-		idr_replace(&pn->l2tp_v2_session_idr, session, session_key);
+		old = idr_replace(&pn->l2tp_v2_session_idr, session, session_key);
 	}
 
+	/* old should be NULL, unless something removed or modified
+	 * the IDR entry after our idr_alloc_32 above (which shouldn't
+	 * happen).
+	 */
+	WARN_ON_ONCE(old);
 out:
 	spin_unlock_bh(&pn->l2tp_session_idr_lock);
 	spin_unlock_bh(&tunnel->list_lock);
@@ -792,7 +837,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
 		if (!session->lns_mode && !session->send_seq) {
 			trace_session_seqnum_lns_enable(session);
 			session->send_seq = 1;
-			l2tp_session_set_header_len(session, tunnel->version);
+			l2tp_session_set_header_len(session, tunnel->version,
+						    tunnel->encap);
 		}
 	} else {
 		/* No sequence numbers.
@@ -813,7 +859,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
 		if (!session->lns_mode && session->send_seq) {
 			trace_session_seqnum_lns_disable(session);
 			session->send_seq = 0;
-			l2tp_session_set_header_len(session, tunnel->version);
+			l2tp_session_set_header_len(session, tunnel->version,
+						    tunnel->encap);
 		} else if (session->send_seq) {
 			pr_debug_ratelimited("%s: recv data has no seq numbers when required. Discarding.\n",
 					     session->name);
@@ -1207,44 +1254,6 @@ EXPORT_SYMBOL_GPL(l2tp_xmit_skb);
  * Tinnel and session create/destroy.
  *****************************************************************************/
 
-/* Tunnel socket destruct hook.
- * The tunnel context is deleted only when all session sockets have been
- * closed.
- */
-static void l2tp_tunnel_destruct(struct sock *sk)
-{
-	struct l2tp_tunnel *tunnel = l2tp_sk_to_tunnel(sk);
-
-	if (!tunnel)
-		goto end;
-
-	/* Disable udp encapsulation */
-	switch (tunnel->encap) {
-	case L2TP_ENCAPTYPE_UDP:
-		/* No longer an encapsulation socket. See net/ipv4/udp.c */
-		WRITE_ONCE(udp_sk(sk)->encap_type, 0);
-		udp_sk(sk)->encap_rcv = NULL;
-		udp_sk(sk)->encap_destroy = NULL;
-		break;
-	case L2TP_ENCAPTYPE_IP:
-		break;
-	}
-
-	/* Remove hooks into tunnel socket */
-	write_lock_bh(&sk->sk_callback_lock);
-	sk->sk_destruct = tunnel->old_sk_destruct;
-	sk->sk_user_data = NULL;
-	write_unlock_bh(&sk->sk_callback_lock);
-
-	/* Call the original destructor */
-	if (sk->sk_destruct)
-		(*sk->sk_destruct)(sk);
-
-	kfree_rcu(tunnel, rcu);
-end:
-	return;
-}
-
 /* Remove an l2tp session from l2tp_core's lists. */
 static void l2tp_session_unhash(struct l2tp_session *session)
 {
@@ -1277,8 +1286,6 @@ static void l2tp_session_unhash(struct l2tp_session *session)
 
 		spin_unlock_bh(&pn->l2tp_session_idr_lock);
 		spin_unlock_bh(&tunnel->list_lock);
-
-		synchronize_rcu();
 	}
 }
 
@@ -1290,28 +1297,21 @@ static void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel)
 
 	spin_lock_bh(&tunnel->list_lock);
 	tunnel->acpt_newsess = false;
-	for (;;) {
-		session = list_first_entry_or_null(&tunnel->session_list,
-						   struct l2tp_session, list);
-		if (!session)
-			break;
-		l2tp_session_inc_refcount(session);
-		list_del_init(&session->list);
-		spin_unlock_bh(&tunnel->list_lock);
+	list_for_each_entry(session, &tunnel->session_list, list)
 		l2tp_session_delete(session);
-		spin_lock_bh(&tunnel->list_lock);
-		l2tp_session_dec_refcount(session);
-	}
 	spin_unlock_bh(&tunnel->list_lock);
 }
 
 /* Tunnel socket destroy hook for UDP encapsulation */
 static void l2tp_udp_encap_destroy(struct sock *sk)
 {
-	struct l2tp_tunnel *tunnel = l2tp_sk_to_tunnel(sk);
+	struct l2tp_tunnel *tunnel;
 
-	if (tunnel)
+	tunnel = l2tp_sk_to_tunnel(sk);
+	if (tunnel) {
 		l2tp_tunnel_delete(tunnel);
+		l2tp_tunnel_dec_refcount(tunnel);
+	}
 }
 
 static void l2tp_tunnel_remove(struct net *net, struct l2tp_tunnel *tunnel)
@@ -1494,7 +1494,6 @@ int l2tp_tunnel_create(int fd, int version, u32 tunnel_id, u32 peer_tunnel_id,
 	tunnel->tunnel_id = tunnel_id;
 	tunnel->peer_tunnel_id = peer_tunnel_id;
 
-	tunnel->magic = L2TP_TUNNEL_MAGIC;
 	sprintf(&tunnel->name[0], "tunl %u", tunnel_id);
 	spin_lock_init(&tunnel->list_lock);
 	tunnel->acpt_newsess = true;
@@ -1520,6 +1519,8 @@ EXPORT_SYMBOL_GPL(l2tp_tunnel_create);
 static int l2tp_validate_socket(const struct sock *sk, const struct net *net,
 				enum l2tp_encap_type encap)
 {
+	struct l2tp_tunnel *tunnel;
+
 	if (!net_eq(sock_net(sk), net))
 		return -EINVAL;
 
@@ -1533,8 +1534,11 @@ static int l2tp_validate_socket(const struct sock *sk, const struct net *net,
 	    (encap == L2TP_ENCAPTYPE_IP && sk->sk_protocol != IPPROTO_L2TP))
 		return -EPROTONOSUPPORT;
 
-	if (sk->sk_user_data)
+	tunnel = l2tp_sk_to_tunnel(sk);
+	if (tunnel) {
+		l2tp_tunnel_dec_refcount(tunnel);
 		return -EBUSY;
+	}
 
 	return 0;
 }
@@ -1573,12 +1577,10 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 	ret = l2tp_validate_socket(sk, net, tunnel->encap);
 	if (ret < 0)
 		goto err_inval_sock;
-	rcu_assign_sk_user_data(sk, tunnel);
 	write_unlock_bh(&sk->sk_callback_lock);
 
 	if (tunnel->encap == L2TP_ENCAPTYPE_UDP) {
 		struct udp_tunnel_sock_cfg udp_cfg = {
-			.sk_user_data = tunnel,
 			.encap_type = UDP_ENCAP_L2TPINUDP,
 			.encap_rcv = l2tp_udp_encap_recv,
 			.encap_err_rcv = l2tp_udp_encap_err_recv,
@@ -1588,8 +1590,6 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 		setup_udp_tunnel_sock(net, sock, &udp_cfg);
 	}
 
-	tunnel->old_sk_destruct = sk->sk_destruct;
-	sk->sk_destruct = &l2tp_tunnel_destruct;
 	sk->sk_allocation = GFP_ATOMIC;
 	release_sock(sk);
 
@@ -1636,23 +1636,37 @@ EXPORT_SYMBOL_GPL(l2tp_tunnel_delete);
 
 void l2tp_session_delete(struct l2tp_session *session)
 {
-	if (test_and_set_bit(0, &session->dead))
-		return;
+	if (!test_and_set_bit(0, &session->dead)) {
+		trace_delete_session(session);
+		l2tp_session_inc_refcount(session);
+		queue_work(l2tp_wq, &session->del_work);
+	}
+}
+EXPORT_SYMBOL_GPL(l2tp_session_delete);
+
+/* Workqueue session deletion function */
+static void l2tp_session_del_work(struct work_struct *work)
+{
+	struct l2tp_session *session = container_of(work, struct l2tp_session,
+						    del_work);
 
-	trace_delete_session(session);
 	l2tp_session_unhash(session);
 	l2tp_session_queue_purge(session);
 	if (session->session_close)
 		(*session->session_close)(session);
 
+	/* drop initial ref */
+	l2tp_session_dec_refcount(session);
+
+	/* drop workqueue ref */
 	l2tp_session_dec_refcount(session);
 }
-EXPORT_SYMBOL_GPL(l2tp_session_delete);
 
 /* We come here whenever a session's send_seq, cookie_len or
  * l2specific_type parameters are set.
  */
-void l2tp_session_set_header_len(struct l2tp_session *session, int version)
+void l2tp_session_set_header_len(struct l2tp_session *session, int version,
+				 enum l2tp_encap_type encap)
 {
 	if (version == L2TP_HDR_VER_2) {
 		session->hdr_len = 6;
@@ -1661,7 +1675,7 @@ void l2tp_session_set_header_len(struct l2tp_session *session, int version)
 	} else {
 		session->hdr_len = 4 + session->cookie_len;
 		session->hdr_len += l2tp_get_l2specific_len(session);
-		if (session->tunnel->encap == L2TP_ENCAPTYPE_UDP)
+		if (encap == L2TP_ENCAPTYPE_UDP)
 			session->hdr_len += 4;
 	}
 }
@@ -1675,7 +1689,6 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn
 	session = kzalloc(sizeof(*session) + priv_size, GFP_KERNEL);
 	if (session) {
 		session->magic = L2TP_SESSION_MAGIC;
-		session->tunnel = tunnel;
 
 		session->session_id = session_id;
 		session->peer_session_id = peer_session_id;
@@ -1699,6 +1712,7 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn
 		INIT_HLIST_NODE(&session->hlist);
 		INIT_LIST_HEAD(&session->clist);
 		INIT_LIST_HEAD(&session->list);
+		INIT_WORK(&session->del_work, l2tp_session_del_work);
 
 		if (cfg) {
 			session->pwtype = cfg->pw_type;
@@ -1713,7 +1727,7 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn
 			memcpy(&session->peer_cookie[0], &cfg->peer_cookie[0], cfg->peer_cookie_len);
 		}
 
-		l2tp_session_set_header_len(session, tunnel->version);
+		l2tp_session_set_header_len(session, tunnel->version, tunnel->encap);
 
 		refcount_set(&session->ref_count, 1);
 
@@ -1742,7 +1756,7 @@ static __net_init int l2tp_init_net(struct net *net)
 	return 0;
 }
 
-static __net_exit void l2tp_exit_net(struct net *net)
+static __net_exit void l2tp_pre_exit_net(struct net *net)
 {
 	struct l2tp_net *pn = l2tp_pernet(net);
 	struct l2tp_tunnel *tunnel = NULL;
@@ -1756,8 +1770,12 @@ static __net_exit void l2tp_exit_net(struct net *net)
 	rcu_read_unlock_bh();
 
 	if (l2tp_wq)
-		flush_workqueue(l2tp_wq);
-	rcu_barrier();
+		drain_workqueue(l2tp_wq);
+}
+
+static __net_exit void l2tp_exit_net(struct net *net)
+{
+	struct l2tp_net *pn = l2tp_pernet(net);
 
 	idr_destroy(&pn->l2tp_v2_session_idr);
 	idr_destroy(&pn->l2tp_v3_session_idr);
@@ -1767,6 +1785,7 @@ static __net_exit void l2tp_exit_net(struct net *net)
 static struct pernet_operations l2tp_net_ops = {
 	.init = l2tp_init_net,
 	.exit = l2tp_exit_net,
+	.pre_exit = l2tp_pre_exit_net,
 	.id   = &l2tp_net_id,
 	.size = sizeof(struct l2tp_net),
 };
diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index 8ac81bc1bc6f..c907687705b9 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -16,7 +16,6 @@
 #endif
 
 /* Random numbers used for internal consistency checks of tunnel and session structures */
-#define L2TP_TUNNEL_MAGIC	0x42114DDA
 #define L2TP_SESSION_MAGIC	0x0C04EB7D
 
 struct sk_buff;
@@ -67,6 +66,7 @@ struct l2tp_session_coll_list {
 struct l2tp_session {
 	int			magic;		/* should be L2TP_SESSION_MAGIC */
 	long			dead;
+	struct rcu_head		rcu;
 
 	struct l2tp_tunnel	*tunnel;	/* back pointer to tunnel context */
 	u32			session_id;
@@ -103,6 +103,7 @@ struct l2tp_session {
 	int			reorder_skip;	/* set if skip to next nr */
 	enum l2tp_pwtype	pwtype;
 	struct l2tp_stats	stats;
+	struct work_struct	del_work;
 
 	/* Session receive handler for data packets.
 	 * Each pseudowire implementation should implement this callback in order to
@@ -155,8 +156,6 @@ struct l2tp_tunnel_cfg {
  */
 #define L2TP_TUNNEL_NAME_MAX 20
 struct l2tp_tunnel {
-	int			magic;		/* Should be L2TP_TUNNEL_MAGIC */
-
 	unsigned long		dead;
 
 	struct rcu_head rcu;
@@ -176,7 +175,6 @@ struct l2tp_tunnel {
 	struct net		*l2tp_net;	/* the net we belong to */
 
 	refcount_t		ref_count;
-	void (*old_sk_destruct)(struct sock *sk);
 	struct sock		*sock;		/* parent socket */
 	int			fd;		/* parent fd, if tunnel socket was created
 						 * by userspace
@@ -260,7 +258,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
 int l2tp_udp_encap_recv(struct sock *sk, struct sk_buff *skb);
 
 /* Transmit path helpers for sending packets over the tunnel socket. */
-void l2tp_session_set_header_len(struct l2tp_session *session, int version);
+void l2tp_session_set_header_len(struct l2tp_session *session, int version,
+				 enum l2tp_encap_type encap);
 int l2tp_xmit_skb(struct l2tp_session *session, struct sk_buff *skb);
 
 /* Pseudowire management.
@@ -273,10 +272,7 @@ void l2tp_nl_unregister_ops(enum l2tp_pwtype pw_type);
 /* IOCTL helper for IP encap modules. */
 int l2tp_ioctl(struct sock *sk, int cmd, int *karg);
 
-/* Extract the tunnel structure from a socket's sk_user_data pointer,
- * validating the tunnel magic feather.
- */
-struct l2tp_tunnel *l2tp_sk_to_tunnel(struct sock *sk);
+struct l2tp_tunnel *l2tp_sk_to_tunnel(const struct sock *sk);
 
 static inline int l2tp_get_l2specific_len(struct l2tp_session *session)
 {
diff --git a/net/l2tp/l2tp_eth.c b/net/l2tp/l2tp_eth.c
index 8ba00ad433c2..cc8a3ce716e9 100644
--- a/net/l2tp/l2tp_eth.c
+++ b/net/l2tp/l2tp_eth.c
@@ -322,7 +322,7 @@ err_sess_dev:
 	l2tp_session_dec_refcount(session);
 	free_netdev(dev);
 err_sess:
-	kfree(session);
+	l2tp_session_dec_refcount(session);
 err:
 	return rc;
 }
diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index e48aa177d74c..f21dcbf3efd5 100644
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -235,14 +235,17 @@ static void l2tp_ip_close(struct sock *sk, long timeout)
 
 static void l2tp_ip_destroy_sock(struct sock *sk)
 {
-	struct l2tp_tunnel *tunnel = l2tp_sk_to_tunnel(sk);
-	struct sk_buff *skb;
+	struct l2tp_tunnel *tunnel;
 
-	while ((skb = __skb_dequeue_tail(&sk->sk_write_queue)) != NULL)
-		kfree_skb(skb);
+	lock_sock(sk);
+	ip_flush_pending_frames(sk);
+	release_sock(sk);
 
-	if (tunnel)
+	tunnel = l2tp_sk_to_tunnel(sk);
+	if (tunnel) {
 		l2tp_tunnel_delete(tunnel);
+		l2tp_tunnel_dec_refcount(tunnel);
+	}
 }
 
 static int l2tp_ip_bind(struct sock *sk, struct sockaddr *uaddr, int addr_len)
diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
index d217ff1f229e..3b0465f2d60d 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -246,14 +246,17 @@ static void l2tp_ip6_close(struct sock *sk, long timeout)
 
 static void l2tp_ip6_destroy_sock(struct sock *sk)
 {
-	struct l2tp_tunnel *tunnel = l2tp_sk_to_tunnel(sk);
+	struct l2tp_tunnel *tunnel;
 
 	lock_sock(sk);
 	ip6_flush_pending_frames(sk);
 	release_sock(sk);
 
-	if (tunnel)
+	tunnel = l2tp_sk_to_tunnel(sk);
+	if (tunnel) {
 		l2tp_tunnel_delete(tunnel);
+		l2tp_tunnel_dec_refcount(tunnel);
+	}
 }
 
 static int l2tp_ip6_bind(struct sock *sk, struct sockaddr *uaddr, int addr_len)
diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index d105030520f9..fc43ecbd128c 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -692,8 +692,10 @@ static int l2tp_nl_cmd_session_modify(struct sk_buff *skb, struct genl_info *inf
 		session->recv_seq = nla_get_u8(info->attrs[L2TP_ATTR_RECV_SEQ]);
 
 	if (info->attrs[L2TP_ATTR_SEND_SEQ]) {
+		struct l2tp_tunnel *tunnel = session->tunnel;
+
 		session->send_seq = nla_get_u8(info->attrs[L2TP_ATTR_SEND_SEQ]);
-		l2tp_session_set_header_len(session, session->tunnel->version);
+		l2tp_session_set_header_len(session, tunnel->version, tunnel->encap);
 	}
 
 	if (info->attrs[L2TP_ATTR_LNS_MODE])
diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 3596290047b2..90bf3a8ccab6 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -119,7 +119,6 @@ struct pppol2tp_session {
 	struct mutex		sk_lock;	/* Protects .sk */
 	struct sock __rcu	*sk;		/* Pointer to the session PPPoX socket */
 	struct sock		*__sk;		/* Copy of .sk, for cleanup */
-	struct rcu_head		rcu;		/* For asynchronous release */
 };
 
 static int pppol2tp_xmit(struct ppp_channel *chan, struct sk_buff *skb);
@@ -157,20 +156,16 @@ static inline struct l2tp_session *pppol2tp_sock_to_session(struct sock *sk)
 	if (!sk)
 		return NULL;
 
-	sock_hold(sk);
-	session = (struct l2tp_session *)(sk->sk_user_data);
-	if (!session) {
-		sock_put(sk);
-		goto out;
-	}
-	if (WARN_ON(session->magic != L2TP_SESSION_MAGIC)) {
-		session = NULL;
-		sock_put(sk);
-		goto out;
+	rcu_read_lock();
+	session = rcu_dereference_sk_user_data(sk);
+	if (session && refcount_inc_not_zero(&session->ref_count)) {
+		rcu_read_unlock();
+		WARN_ON_ONCE(session->magic != L2TP_SESSION_MAGIC);
+		return session;
 	}
+	rcu_read_unlock();
 
-out:
-	return session;
+	return NULL;
 }
 
 /*****************************************************************************
@@ -318,12 +313,12 @@ static int pppol2tp_sendmsg(struct socket *sock, struct msghdr *m,
 	l2tp_xmit_skb(session, skb);
 	local_bh_enable();
 
-	sock_put(sk);
+	l2tp_session_dec_refcount(session);
 
 	return total_len;
 
 error_put_sess:
-	sock_put(sk);
+	l2tp_session_dec_refcount(session);
 error:
 	return error;
 }
@@ -377,12 +372,12 @@ static int pppol2tp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
 	l2tp_xmit_skb(session, skb);
 	local_bh_enable();
 
-	sock_put(sk);
+	l2tp_session_dec_refcount(session);
 
 	return 1;
 
 abort_put_sess:
-	sock_put(sk);
+	l2tp_session_dec_refcount(session);
 abort:
 	/* Free the original skb */
 	kfree_skb(skb);
@@ -393,28 +388,31 @@ abort:
  * Session (and tunnel control) socket create/destroy.
  *****************************************************************************/
 
-static void pppol2tp_put_sk(struct rcu_head *head)
-{
-	struct pppol2tp_session *ps;
-
-	ps = container_of(head, typeof(*ps), rcu);
-	sock_put(ps->__sk);
-}
-
 /* Really kill the session socket. (Called from sock_put() if
  * refcnt == 0.)
  */
 static void pppol2tp_session_destruct(struct sock *sk)
 {
-	struct l2tp_session *session = sk->sk_user_data;
-
 	skb_queue_purge(&sk->sk_receive_queue);
 	skb_queue_purge(&sk->sk_write_queue);
+}
 
-	if (session) {
-		sk->sk_user_data = NULL;
-		if (WARN_ON(session->magic != L2TP_SESSION_MAGIC))
-			return;
+static void pppol2tp_session_close(struct l2tp_session *session)
+{
+	struct pppol2tp_session *ps;
+
+	ps = l2tp_session_priv(session);
+	mutex_lock(&ps->sk_lock);
+	ps->__sk = rcu_dereference_protected(ps->sk,
+					     lockdep_is_held(&ps->sk_lock));
+	RCU_INIT_POINTER(ps->sk, NULL);
+	mutex_unlock(&ps->sk_lock);
+	if (ps->__sk) {
+		/* detach socket */
+		rcu_assign_sk_user_data(ps->__sk, NULL);
+		sock_put(ps->__sk);
+
+		/* drop ref taken when we referenced socket via sk_user_data */
 		l2tp_session_dec_refcount(session);
 	}
 }
@@ -444,30 +442,13 @@ static int pppol2tp_release(struct socket *sock)
 
 	session = pppol2tp_sock_to_session(sk);
 	if (session) {
-		struct pppol2tp_session *ps;
-
 		l2tp_session_delete(session);
-
-		ps = l2tp_session_priv(session);
-		mutex_lock(&ps->sk_lock);
-		ps->__sk = rcu_dereference_protected(ps->sk,
-						     lockdep_is_held(&ps->sk_lock));
-		RCU_INIT_POINTER(ps->sk, NULL);
-		mutex_unlock(&ps->sk_lock);
-		call_rcu(&ps->rcu, pppol2tp_put_sk);
-
-		/* Rely on the sock_put() call at the end of the function for
-		 * dropping the reference held by pppol2tp_sock_to_session().
-		 * The last reference will be dropped by pppol2tp_put_sk().
-		 */
+		/* drop ref taken by pppol2tp_sock_to_session */
+		l2tp_session_dec_refcount(session);
 	}
 
 	release_sock(sk);
 
-	/* This will delete the session context via
-	 * pppol2tp_session_destruct() if the socket's refcnt drops to
-	 * zero.
-	 */
 	sock_put(sk);
 
 	return 0;
@@ -506,6 +487,7 @@ static int pppol2tp_create(struct net *net, struct socket *sock, int kern)
 		goto out;
 
 	sock_init_data(sock, sk);
+	sock_set_flag(sk, SOCK_RCU_FREE);
 
 	sock->state  = SS_UNCONNECTED;
 	sock->ops    = &pppol2tp_ops;
@@ -542,6 +524,7 @@ static void pppol2tp_session_init(struct l2tp_session *session)
 	struct pppol2tp_session *ps;
 
 	session->recv_skb = pppol2tp_recv;
+	session->session_close = pppol2tp_session_close;
 	if (IS_ENABLED(CONFIG_L2TP_DEBUGFS))
 		session->show = pppol2tp_show;
 
@@ -787,6 +770,8 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 			goto end;
 		}
 
+		drop_refcnt = true;
+
 		pppol2tp_session_init(session);
 		ps = l2tp_session_priv(session);
 		l2tp_session_inc_refcount(session);
@@ -795,10 +780,10 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 		error = l2tp_session_register(session, tunnel);
 		if (error < 0) {
 			mutex_unlock(&ps->sk_lock);
-			kfree(session);
+			l2tp_session_dec_refcount(session);
 			goto end;
 		}
-		drop_refcnt = true;
+
 		new_session = true;
 	}
 
@@ -830,12 +815,13 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 
 out_no_ppp:
 	/* This is how we get the session context from the socket. */
-	sk->sk_user_data = session;
+	sock_hold(sk);
+	rcu_assign_sk_user_data(sk, session);
 	rcu_assign_pointer(ps->sk, sk);
 	mutex_unlock(&ps->sk_lock);
 
 	/* Keep the reference we've grabbed on the session: sk doesn't expect
-	 * the session to disappear. pppol2tp_session_destruct() is responsible
+	 * the session to disappear. pppol2tp_session_close() is responsible
 	 * for dropping it.
 	 */
 	drop_refcnt = false;
@@ -891,7 +877,7 @@ static int pppol2tp_session_create(struct net *net, struct l2tp_tunnel *tunnel,
 	return 0;
 
 err_sess:
-	kfree(session);
+	l2tp_session_dec_refcount(session);
 err:
 	return error;
 }
@@ -1002,7 +988,7 @@ static int pppol2tp_getname(struct socket *sock, struct sockaddr *uaddr,
 
 	error = len;
 
-	sock_put(sk);
+	l2tp_session_dec_refcount(session);
 end:
 	return error;
 }
@@ -1205,7 +1191,8 @@ static int pppol2tp_session_setsockopt(struct sock *sk,
 			po->chan.hdrlen = val ? PPPOL2TP_L2TP_HDR_SIZE_SEQ :
 				PPPOL2TP_L2TP_HDR_SIZE_NOSEQ;
 		}
-		l2tp_session_set_header_len(session, session->tunnel->version);
+		l2tp_session_set_header_len(session, session->tunnel->version,
+					    session->tunnel->encap);
 		break;
 
 	case PPPOL2TP_SO_LNSMODE:
@@ -1274,7 +1261,7 @@ static int pppol2tp_setsockopt(struct socket *sock, int level, int optname,
 		err = pppol2tp_session_setsockopt(sk, session, optname, val);
 	}
 
-	sock_put(sk);
+	l2tp_session_dec_refcount(session);
 end:
 	return err;
 }
@@ -1395,7 +1382,7 @@ static int pppol2tp_getsockopt(struct socket *sock, int level, int optname,
 	err = 0;
 
 end_put_sess:
-	sock_put(sk);
+	l2tp_session_dec_refcount(session);
 end:
 	return err;
 }
@@ -1513,7 +1500,7 @@ static void pppol2tp_seq_tunnel_show(struct seq_file *m, void *v)
 
 	seq_printf(m, "\nTUNNEL '%s', %c %d\n",
 		   tunnel->name,
-		   (tunnel == tunnel->sock->sk_user_data) ? 'Y' : 'N',
+		   tunnel->sock ? 'Y' : 'N',
 		   refcount_read(&tunnel->ref_count) - 1);
 	seq_printf(m, " %08x %ld/%ld/%ld %ld/%ld/%ld\n",
 		   0,