ovpn: ensure sk is still valid during cleanup

Removing a peer while userspace attempts to close its transport
socket triggers a race condition resulting in the following
crash:

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000077: 0000 [#1] SMP KASAN
KASAN: null-ptr-deref in range [0x00000000000003b8-0x00000000000003bf]
CPU: 12 UID: 0 PID: 162 Comm: kworker/12:1 Tainted: G O 6.15.0-rc2-00635-g521139ac3840 #272 PREEMPT(full)
Tainted: [O]=OOT_MODULE
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-20240910_120124-localhost 04/01/2014
Workqueue: events ovpn_peer_keepalive_work [ovpn]
RIP: 0010:ovpn_socket_release+0x23c/0x500 [ovpn]
Code: ea 03 80 3c 02 00 0f 85 71 02 00 00 48 b8 00 00 00 00 00 fc ff df 4d 8b 64 24 18 49 8d bc 24 be 03 00 00 48 89 fa 48 c1 ea 03 <0f> b6 14 02 48 89 f8 83 e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 30
RSP: 0018:ffffc90000c9fb18 EFLAGS: 00010217
RAX: dffffc0000000000 RBX: ffff8881148d7940 RCX: ffffffff817787bb
RDX: 0000000000000077 RSI: 0000000000000008 RDI: 00000000000003be
RBP: ffffc90000c9fb30 R08: 0000000000000000 R09: fffffbfff0d3e840
R10: ffffffff869f4207 R11: 0000000000000000 R12: 0000000000000000
R13: ffff888115eb9300 R14: ffffc90000c9fbc8 R15: 000000000000000c
FS: 0000000000000000(0000) GS:ffff8882b0151000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f37266b6114 CR3: 00000000054a8000 CR4: 0000000000750ef0
PKRU: 55555554
Call Trace:
<TASK>
unlock_ovpn+0x8b/0xe0 [ovpn]
ovpn_peer_keepalive_work+0xe3/0x540 [ovpn]
? ovpn_peers_free+0x780/0x780 [ovpn]
? lock_acquire+0x56/0x70
? process_one_work+0x888/0x1740
process_one_work+0x933/0x1740
? pwq_dec_nr_in_flight+0x10b0/0x10b0
? move_linked_works+0x12d/0x2c0
? assign_work+0x163/0x270
worker_thread+0x4d6/0xd90
? preempt_count_sub+0x4c/0x70
? process_one_work+0x1740/0x1740
kthread+0x36c/0x710
? trace_preempt_on+0x8c/0x1e0
? kthread_is_per_cpu+0xc0/0xc0
? preempt_count_sub+0x4c/0x70
? _raw_spin_unlock_irq+0x36/0x60
? calculate_sigpending+0x7b/0xa0
? kthread_is_per_cpu+0xc0/0xc0
ret_from_fork+0x3a/0x80
? kthread_is_per_cpu+0xc0/0xc0
ret_from_fork_asm+0x11/0x20
</TASK>
Modules linked in: ovpn(O)

This happens because the peer deletion operation reaches
ovpn_socket_release() while ovpn_sock->sock (struct socket *)
and its sk member (struct sock *) are still both valid.
Here synchronize_rcu() is invoked, after which ovpn_sock->sock->sk
becomes NULL, due to the concurrent socket closing triggered
from userspace.

After having invoked synchronize_rcu(), ovpn_socket_release() will
attempt dereferencing ovpn_sock->sock->sk, triggering the crash
reported above.

The reason for accessing sk is that we need to retrieve its
protocol and continue the cleanup routine accordingly.

This crash can be easily produced by running openvpn userspace in
client mode with `--keepalive 10 20`, while entirely omitting this
option on the server side.
After 20 seconds ovpn will assume the peer (server) to be dead,
will start removing it and will notify userspace. The latter will
receive the notification and close the transport socket, thus
triggering the crash.

To fix the race condition for good, we need to refactor struct ovpn_socket.
Since ovpn is always only interested in the sock->sk member (struct sock *)
we can directly hold a reference to it, raher than accessing it via
its struct socket container.

This means changing "struct socket *ovpn_socket->sock" to
"struct sock *ovpn_socket->sk".

While acquiring a reference to sk, we can increase its refcounter
without affecting the socket close()/destroy() notification
(which we rely on when userspace closes a socket we are using).

By increasing sk's refcounter we know we can dereference it
in ovpn_socket_release() without incurring in any race condition
anymore.

ovpn_socket_release() will ultimately decrease the reference
counter.

Cc: Oleksandr Natalenko <[e-mail]>
Fixes: 11851cbd60ea ("ovpn: implement TCP transport")
Reported-by: Qingfang Deng <[e-mail]>
Closes: https://github.com/OpenVPN/ovpn-net-next/issues/1
Tested-by: Gert Doering <[e-mail]>
Link: https://www.mail-archive.com/[e-mail]/msg31575.html
Reviewed-by: Michal Swiatkowski <[e-mail]>
Signed-off-by: Antonio Quartulli <[e-mail]>

drivers/net/ovpn/io.c

···
 
 	rcu_read_lock();
 	sock = rcu_dereference(peer->sock);
-	if (sock && sock->sock->sk->sk_protocol == IPPROTO_UDP)
+	if (sock && sock->sk->sk_protocol == IPPROTO_UDP)
 		/* check if this peer changed local or remote endpoint */
 		ovpn_peer_endpoints_update(peer, skb);
 	rcu_read_unlock();
···
 	if (unlikely(!sock))
 		goto err_unlock;
 
-	switch (sock->sock->sk->sk_protocol) {
+	switch (sock->sk->sk_protocol) {
 	case IPPROTO_UDP:
-		ovpn_udp_send_skb(peer, sock->sock, skb);
+		ovpn_udp_send_skb(peer, sock->sk, skb);
 		break;
 	case IPPROTO_TCP:
-		ovpn_tcp_send_skb(peer, sock->sock, skb);
+		ovpn_tcp_send_skb(peer, sock->sk, skb);
 		break;
 	default:
 		/* no transport configured yet */

drivers/net/ovpn/netlink.c

···
 	/* when using a TCP socket the remote IP is not expected */
 	rcu_read_lock();
 	sock = rcu_dereference(peer->sock);
-	if (sock && sock->sock->sk->sk_protocol == IPPROTO_TCP &&
+	if (sock && sock->sk->sk_protocol == IPPROTO_TCP &&
 	    (attrs[OVPN_A_PEER_REMOTE_IPV4] ||
 	     attrs[OVPN_A_PEER_REMOTE_IPV6])) {
 		rcu_read_unlock();
···
 		goto err_unlock;
 	}
 
-	if (!net_eq(genl_info_net(info), sock_net(sock->sock->sk))) {
+	if (!net_eq(genl_info_net(info), sock_net(sock->sk))) {
 		id = peernet2id_alloc(genl_info_net(info),
-				      sock_net(sock->sock->sk),
+				      sock_net(sock->sk),
 				      GFP_ATOMIC);
 		if (nla_put_s32(skb, OVPN_A_PEER_SOCKET_NETNSID, id))
 			goto err_unlock;
 	}
-	local_port = inet_sk(sock->sock->sk)->inet_sport;
+	local_port = inet_sk(sock->sk)->inet_sport;
 	rcu_read_unlock();
 
 	if (nla_put_u32(skb, OVPN_A_PEER_ID, peer->id))
···
 		ret = -EINVAL;
 		goto err_unlock;
 	}
-	genlmsg_multicast_netns(&ovpn_nl_family, sock_net(sock->sock->sk),
-				msg, 0, OVPN_NLGRP_PEERS, GFP_ATOMIC);
+	genlmsg_multicast_netns(&ovpn_nl_family, sock_net(sock->sk), msg, 0,
+				OVPN_NLGRP_PEERS, GFP_ATOMIC);
 	rcu_read_unlock();
 
 	return 0;
···
 		ret = -EINVAL;
 		goto err_unlock;
 	}
-	genlmsg_multicast_netns(&ovpn_nl_family, sock_net(sock->sock->sk),
-				msg, 0, OVPN_NLGRP_PEERS, GFP_ATOMIC);
+	genlmsg_multicast_netns(&ovpn_nl_family, sock_net(sock->sk), msg, 0,
+				OVPN_NLGRP_PEERS, GFP_ATOMIC);
 	rcu_read_unlock();
 
 	return 0;

drivers/net/ovpn/peer.c

···
 
 	if (sk) {
 		ovpn_sock = rcu_access_pointer(peer->sock);
-		if (!ovpn_sock || ovpn_sock->sock->sk != sk) {
+		if (!ovpn_sock || ovpn_sock->sk != sk) {
 			spin_unlock_bh(&ovpn->lock);
 			ovpn_peer_put(peer);
 			return;
···
 		if (sk) {
 			rcu_read_lock();
 			ovpn_sock = rcu_dereference(peer->sock);
-			remove = ovpn_sock && ovpn_sock->sock->sk == sk;
+			remove = ovpn_sock && ovpn_sock->sk == sk;
 			rcu_read_unlock();
 		}
 

drivers/net/ovpn/socket.c

···
 	struct ovpn_socket *sock = container_of(kref, struct ovpn_socket,
 						refcount);
 
-	if (sock->sock->sk->sk_protocol == IPPROTO_UDP)
+	if (sock->sk->sk_protocol == IPPROTO_UDP)
 		ovpn_udp_socket_detach(sock);
-	else if (sock->sock->sk->sk_protocol == IPPROTO_TCP)
+	else if (sock->sk->sk_protocol == IPPROTO_TCP)
 		ovpn_tcp_socket_detach(sock);
 }
 
···
 	if (!sock)
 		return;
 
-	/* sanity check: we should not end up here if the socket
-	 * was already closed
-	 */
-	if (!sock->sock->sk) {
-		DEBUG_NET_WARN_ON_ONCE(1);
-		return;
-	}
-
 	/* Drop the reference while holding the sock lock to avoid
 	 * concurrent ovpn_socket_new call to mess up with a partially
 	 * detached socket.
···
 	 * Holding the lock ensures that a socket with refcnt 0 is fully
 	 * detached before it can be picked by a concurrent reader.
 	 */
-	lock_sock(sock->sock->sk);
+	lock_sock(sock->sk);
 	released = ovpn_socket_put(peer, sock);
-	release_sock(sock->sock->sk);
+	release_sock(sock->sk);
 
 	/* align all readers with sk_user_data being NULL */
 	synchronize_rcu();
 
 	/* following cleanup should happen with lock released */
 	if (released) {
-		if (sock->sock->sk->sk_protocol == IPPROTO_UDP) {
+		if (sock->sk->sk_protocol == IPPROTO_UDP) {
 			netdev_put(sock->ovpn->dev, &sock->dev_tracker);
-		} else if (sock->sock->sk->sk_protocol == IPPROTO_TCP) {
+		} else if (sock->sk->sk_protocol == IPPROTO_TCP) {
 			/* wait for TCP jobs to terminate */
 			ovpn_tcp_socket_wait_finish(sock);
 			ovpn_peer_put(sock->peer);
 		}
+		/* drop reference acquired in ovpn_socket_new() */
+		sock_put(sock->sk);
 		/* we can call plain kfree() because we already waited one RCU
 		 * period due to synchronize_rcu()
 		 */
···
 	return kref_get_unless_zero(&sock->refcount);
 }
 
-static int ovpn_socket_attach(struct ovpn_socket *sock, struct ovpn_peer *peer)
+static int ovpn_socket_attach(struct ovpn_socket *ovpn_sock,
+			      struct socket *sock,
+			      struct ovpn_peer *peer)
 {
-	if (sock->sock->sk->sk_protocol == IPPROTO_UDP)
-		return ovpn_udp_socket_attach(sock, peer->ovpn);
-	else if (sock->sock->sk->sk_protocol == IPPROTO_TCP)
-		return ovpn_tcp_socket_attach(sock, peer);
+	if (sock->sk->sk_protocol == IPPROTO_UDP)
+		return ovpn_udp_socket_attach(ovpn_sock, sock, peer->ovpn);
+	else if (sock->sk->sk_protocol == IPPROTO_TCP)
+		return ovpn_tcp_socket_attach(ovpn_sock, peer);
 
 	return -EOPNOTSUPP;
 }
···
 struct ovpn_socket *ovpn_socket_new(struct socket *sock, struct ovpn_peer *peer)
 {
 	struct ovpn_socket *ovpn_sock;
+	struct sock *sk = sock->sk;
 	int ret;
 
-	lock_sock(sock->sk);
+	lock_sock(sk);
 
 	/* a TCP socket can only be owned by a single peer, therefore there
 	 * can't be any other user
 	 */
-	if (sock->sk->sk_protocol == IPPROTO_TCP && sock->sk->sk_user_data) {
+	if (sk->sk_protocol == IPPROTO_TCP && sk->sk_user_data) {
 		ovpn_sock = ERR_PTR(-EBUSY);
 		goto sock_release;
 	}
···
 	/* a UDP socket can be shared across multiple peers, but we must make
 	 * sure it is not owned by something else
 	 */
-	if (sock->sk->sk_protocol == IPPROTO_UDP) {
-		u8 type = READ_ONCE(udp_sk(sock->sk)->encap_type);
+	if (sk->sk_protocol == IPPROTO_UDP) {
+		u8 type = READ_ONCE(udp_sk(sk)->encap_type);
 
 		/* socket owned by other encapsulation module */
 		if (type && type != UDP_ENCAP_OVPNINUDP) {
···
 		}
 
 		rcu_read_lock();
-		ovpn_sock = rcu_dereference_sk_user_data(sock->sk);
+		ovpn_sock = rcu_dereference_sk_user_data(sk);
 		if (ovpn_sock) {
 			/* socket owned by another ovpn instance, we can't use it */
 			if (ovpn_sock->ovpn != peer->ovpn) {
···
 		goto sock_release;
 	}
 
-	ovpn_sock->sock = sock;
+	ovpn_sock->sk = sk;
 	kref_init(&ovpn_sock->refcount);
 
-	ret = ovpn_socket_attach(ovpn_sock, peer);
+	/* the newly created ovpn_socket is holding reference to sk,
+	 * therefore we increase its refcounter.
+	 *
+	 * This ovpn_socket instance is referenced by all peers
+	 * using the same socket.
+	 *
+	 * ovpn_socket_release() will take care of dropping the reference.
+	 */
+	sock_hold(sk);
+
+	ret = ovpn_socket_attach(ovpn_sock, sock, peer);
 	if (ret < 0) {
+		sock_put(sk);
 		kfree(ovpn_sock);
 		ovpn_sock = ERR_PTR(ret);
 		goto sock_release;
···
 	/* TCP sockets are per-peer, therefore they are linked to their unique
 	 * peer
 	 */
-	if (sock->sk->sk_protocol == IPPROTO_TCP) {
+	if (sk->sk_protocol == IPPROTO_TCP) {
 		INIT_WORK(&ovpn_sock->tcp_tx_work, ovpn_tcp_tx_work);
 		ovpn_sock->peer = peer;
 		ovpn_peer_hold(peer);
-	} else if (sock->sk->sk_protocol == IPPROTO_UDP) {
+	} else if (sk->sk_protocol == IPPROTO_UDP) {
 		/* in UDP we only link the ovpn instance since the socket is
 		 * shared among multiple peers
 		 */
···
 			    GFP_KERNEL);
 	}
 
-	rcu_assign_sk_user_data(sock->sk, ovpn_sock);
+	rcu_assign_sk_user_data(sk, ovpn_sock);
 sock_release:
-	release_sock(sock->sk);
+	release_sock(sk);
 	return ovpn_sock;
 }

drivers/net/ovpn/socket.h

···
  * @ovpn: ovpn instance owning this socket (UDP only)
  * @dev_tracker: reference tracker for associated dev (UDP only)
  * @peer: unique peer transmitting over this socket (TCP only)
- * @sock: the low level sock object
+ * @sk: the low level sock object
  * @refcount: amount of contexts currently referencing this object
  * @work: member used to schedule release routine (it may block)
  * @tcp_tx_work: work for deferring outgoing packet processing (TCP only)
···
 		struct ovpn_peer *peer;
 	};
 
-	struct socket *sock;
+	struct sock *sk;
 	struct kref refcount;
 	struct work_struct work;
 	struct work_struct tcp_tx_work;

drivers/net/ovpn/tcp.c

···
 void ovpn_tcp_socket_detach(struct ovpn_socket *ovpn_sock)
 {
 	struct ovpn_peer *peer = ovpn_sock->peer;
-	struct socket *sock = ovpn_sock->sock;
+	struct sock *sk = ovpn_sock->sk;
 
 	strp_stop(&peer->tcp.strp);
 	skb_queue_purge(&peer->tcp.user_queue);
 
 	/* restore CBs that were saved in ovpn_sock_set_tcp_cb() */
-	sock->sk->sk_data_ready = peer->tcp.sk_cb.sk_data_ready;
-	sock->sk->sk_write_space = peer->tcp.sk_cb.sk_write_space;
-	sock->sk->sk_prot = peer->tcp.sk_cb.prot;
-	sock->sk->sk_socket->ops = peer->tcp.sk_cb.ops;
+	sk->sk_data_ready = peer->tcp.sk_cb.sk_data_ready;
+	sk->sk_write_space = peer->tcp.sk_cb.sk_write_space;
+	sk->sk_prot = peer->tcp.sk_cb.prot;
+	sk->sk_socket->ops = peer->tcp.sk_cb.ops;
 
-	rcu_assign_sk_user_data(sock->sk, NULL);
+	rcu_assign_sk_user_data(sk, NULL);
 }
 
 void ovpn_tcp_socket_wait_finish(struct ovpn_socket *sock)
···
 
 	sock = container_of(work, struct ovpn_socket, tcp_tx_work);
 
-	lock_sock(sock->sock->sk);
+	lock_sock(sock->sk);
 	if (sock->peer)
-		ovpn_tcp_send_sock(sock->peer, sock->sock->sk);
-	release_sock(sock->sock->sk);
+		ovpn_tcp_send_sock(sock->peer, sock->sk);
+	release_sock(sock->sk);
 }
 
 static void ovpn_tcp_send_sock_skb(struct ovpn_peer *peer, struct sock *sk,
···
 	ovpn_tcp_send_sock(peer, sk);
 }
 
-void ovpn_tcp_send_skb(struct ovpn_peer *peer, struct socket *sock,
+void ovpn_tcp_send_skb(struct ovpn_peer *peer, struct sock *sk,
 		       struct sk_buff *skb)
 {
 	u16 len = skb->len;
 
 	*(__be16 *)__skb_push(skb, sizeof(u16)) = htons(len);
 
-	spin_lock_nested(&sock->sk->sk_lock.slock, OVPN_TCP_DEPTH_NESTING);
-	if (sock_owned_by_user(sock->sk)) {
+	spin_lock_nested(&sk->sk_lock.slock, OVPN_TCP_DEPTH_NESTING);
+	if (sock_owned_by_user(sk)) {
 		if (skb_queue_len(&peer->tcp.out_queue) >=
 		    READ_ONCE(net_hotdata.max_backlog)) {
 			dev_dstats_tx_dropped(peer->ovpn->dev);
···
 		}
 		__skb_queue_tail(&peer->tcp.out_queue, skb);
 	} else {
-		ovpn_tcp_send_sock_skb(peer, sock->sk, skb);
+		ovpn_tcp_send_sock_skb(peer, sk, skb);
 	}
 unlock:
-	spin_unlock(&sock->sk->sk_lock.slock);
+	spin_unlock(&sk->sk_lock.slock);
 }
 
 static void ovpn_tcp_release(struct sock *sk)
···
 int ovpn_tcp_socket_attach(struct ovpn_socket *ovpn_sock,
 			   struct ovpn_peer *peer)
 {
-	struct socket *sock = ovpn_sock->sock;
 	struct strp_callbacks cb = {
 		.rcv_msg = ovpn_tcp_rcv,
 		.parse_msg = ovpn_tcp_parse,
···
 	int ret;
 
 	/* make sure no pre-existing encapsulation handler exists */
-	if (sock->sk->sk_user_data)
+	if (ovpn_sock->sk->sk_user_data)
 		return -EBUSY;
 
 	/* only a fully connected socket is expected. Connection should be
 	 * handled in userspace
 	 */
-	if (sock->sk->sk_state != TCP_ESTABLISHED) {
+	if (ovpn_sock->sk->sk_state != TCP_ESTABLISHED) {
 		net_err_ratelimited("%s: provided TCP socket is not in ESTABLISHED state: %d\n",
 				    netdev_name(peer->ovpn->dev),
-				    sock->sk->sk_state);
+				    ovpn_sock->sk->sk_state);
 		return -EINVAL;
 	}
 
-	ret = strp_init(&peer->tcp.strp, sock->sk, &cb);
+	ret = strp_init(&peer->tcp.strp, ovpn_sock->sk, &cb);
 	if (ret < 0) {
 		DEBUG_NET_WARN_ON_ONCE(1);
 		return ret;
···
 
 	INIT_WORK(&peer->tcp.defer_del_work, ovpn_tcp_peer_del_work);
 
-	__sk_dst_reset(sock->sk);
+	__sk_dst_reset(ovpn_sock->sk);
 	skb_queue_head_init(&peer->tcp.user_queue);
 	skb_queue_head_init(&peer->tcp.out_queue);
 
 	/* save current CBs so that they can be restored upon socket release */
-	peer->tcp.sk_cb.sk_data_ready = sock->sk->sk_data_ready;
-	peer->tcp.sk_cb.sk_write_space = sock->sk->sk_write_space;
-	peer->tcp.sk_cb.prot = sock->sk->sk_prot;
-	peer->tcp.sk_cb.ops = sock->sk->sk_socket->ops;
+	peer->tcp.sk_cb.sk_data_ready = ovpn_sock->sk->sk_data_ready;
+	peer->tcp.sk_cb.sk_write_space = ovpn_sock->sk->sk_write_space;
+	peer->tcp.sk_cb.prot = ovpn_sock->sk->sk_prot;
+	peer->tcp.sk_cb.ops = ovpn_sock->sk->sk_socket->ops;
 
 	/* assign our static CBs and prot/ops */
-	sock->sk->sk_data_ready = ovpn_tcp_data_ready;
-	sock->sk->sk_write_space = ovpn_tcp_write_space;
+	ovpn_sock->sk->sk_data_ready = ovpn_tcp_data_ready;
+	ovpn_sock->sk->sk_write_space = ovpn_tcp_write_space;
 
-	if (sock->sk->sk_family == AF_INET) {
-		sock->sk->sk_prot = &ovpn_tcp_prot;
-		sock->sk->sk_socket->ops = &ovpn_tcp_ops;
+	if (ovpn_sock->sk->sk_family == AF_INET) {
+		ovpn_sock->sk->sk_prot = &ovpn_tcp_prot;
+		ovpn_sock->sk->sk_socket->ops = &ovpn_tcp_ops;
 	} else {
-		sock->sk->sk_prot = &ovpn_tcp6_prot;
-		sock->sk->sk_socket->ops = &ovpn_tcp6_ops;
+		ovpn_sock->sk->sk_prot = &ovpn_tcp6_prot;
+		ovpn_sock->sk->sk_socket->ops = &ovpn_tcp6_ops;
 	}
 
 	/* avoid using task_frag */
-	sock->sk->sk_allocation = GFP_ATOMIC;
-	sock->sk->sk_use_task_frag = false;
+	ovpn_sock->sk->sk_allocation = GFP_ATOMIC;
+	ovpn_sock->sk->sk_use_task_frag = false;
 
 	/* enqueue the RX worker */
 	strp_check_rcv(&peer->tcp.strp);

drivers/net/ovpn/tcp.h

···
  * Required by the OpenVPN protocol in order to extract packets from
  * the TCP stream on the receiver side.
  */
-void ovpn_tcp_send_skb(struct ovpn_peer *peer, struct socket *sock, struct sk_buff *skb);
+void ovpn_tcp_send_skb(struct ovpn_peer *peer, struct sock *sk,
+		       struct sk_buff *skb);
 void ovpn_tcp_tx_work(struct work_struct *work);
 
 #endif /* _NET_OVPN_TCP_H_ */

drivers/net/ovpn/udp.c

···
 		return NULL;
 
 	/* make sure that sk matches our stored transport socket */
-	if (unlikely(!ovpn_sock->sock || sk != ovpn_sock->sock->sk))
+	if (unlikely(!ovpn_sock->sk || sk != ovpn_sock->sk))
 		return NULL;
 
 	return ovpn_sock;
···
 /**
  * ovpn_udp_send_skb - prepare skb and send it over via UDP
  * @peer: the destination peer
- * @sock: the RCU protected peer socket
+ * @sk: peer socket
  * @skb: the packet to send
  */
-void ovpn_udp_send_skb(struct ovpn_peer *peer, struct socket *sock,
+void ovpn_udp_send_skb(struct ovpn_peer *peer, struct sock *sk,
 		       struct sk_buff *skb)
 {
-	int ret = -1;
+	int ret;
 
 	skb->dev = peer->ovpn->dev;
 	/* no checksum performed at this layer */
 	skb->ip_summed = CHECKSUM_NONE;
 
-	/* get socket info */
-	if (unlikely(!sock)) {
-		net_warn_ratelimited("%s: no sock for remote peer %u\n",
-				     netdev_name(peer->ovpn->dev), peer->id);
-		goto out;
-	}
-
 	/* crypto layer -> transport (UDP) */
-	ret = ovpn_udp_output(peer, &peer->dst_cache, sock->sk, skb);
-out:
-	if (unlikely(ret < 0)) {
+	ret = ovpn_udp_output(peer, &peer->dst_cache, sk, skb);
+	if (unlikely(ret < 0))
 		kfree_skb(skb);
-		return;
-	}
 }
 
 static void ovpn_udp_encap_destroy(struct sock *sk)
···
 /**
  * ovpn_udp_socket_attach - set udp-tunnel CBs on socket and link it to ovpn
  * @ovpn_sock: socket to configure
+ * @sock: the socket container to be passed to setup_udp_tunnel_sock()
  * @ovpn: the openvp instance to link
  *
  * After invoking this function, the sock will be controlled by ovpn so that
···
  *
  * Return: 0 on success or a negative error code otherwise
  */
-int ovpn_udp_socket_attach(struct ovpn_socket *ovpn_sock,
+int ovpn_udp_socket_attach(struct ovpn_socket *ovpn_sock, struct socket *sock,
 			   struct ovpn_priv *ovpn)
 {
 	struct udp_tunnel_sock_cfg cfg = {
···
 		.encap_rcv = ovpn_udp_encap_recv,
 		.encap_destroy = ovpn_udp_encap_destroy,
 	};
-	struct socket *sock = ovpn_sock->sock;
 	struct ovpn_socket *old_data;
 	int ret;
 
 	/* make sure no pre-existing encapsulation handler exists */
 	rcu_read_lock();
-	old_data = rcu_dereference_sk_user_data(sock->sk);
+	old_data = rcu_dereference_sk_user_data(ovpn_sock->sk);
 	if (!old_data) {
 		/* socket is currently unused - we can take it */
 		rcu_read_unlock();
-		setup_udp_tunnel_sock(sock_net(sock->sk), sock, &cfg);
+		setup_udp_tunnel_sock(sock_net(ovpn_sock->sk), sock, &cfg);
 		return 0;
 	}
 
···
 	 * Unlikely TCP, a single UDP socket can be used to talk to many remote
 	 * hosts and therefore openvpn instantiates one only for all its peers
 	 */
-	if ((READ_ONCE(udp_sk(sock->sk)->encap_type) == UDP_ENCAP_OVPNINUDP) &&
+	if ((READ_ONCE(udp_sk(ovpn_sock->sk)->encap_type) == UDP_ENCAP_OVPNINUDP) &&
 	    old_data->ovpn == ovpn) {
 		netdev_dbg(ovpn->dev,
 			   "provided socket already owned by this interface\n");
···
  */
 void ovpn_udp_socket_detach(struct ovpn_socket *ovpn_sock)
 {
-	struct sock *sk = ovpn_sock->sock->sk;
+	struct sock *sk = ovpn_sock->sk;
 
 	/* Re-enable multicast loopback */
 	inet_set_bit(MC_LOOP, sk);

drivers/net/ovpn/udp.h

···
 struct ovpn_priv;
 struct socket;
 
-int ovpn_udp_socket_attach(struct ovpn_socket *ovpn_sock,
+int ovpn_udp_socket_attach(struct ovpn_socket *ovpn_sock, struct socket *sock,
 			   struct ovpn_priv *ovpn);
 void ovpn_udp_socket_detach(struct ovpn_socket *ovpn_sock);
 
-void ovpn_udp_send_skb(struct ovpn_peer *peer, struct socket *sock,
+void ovpn_udp_send_skb(struct ovpn_peer *peer, struct sock *sk,
 		       struct sk_buff *skb);
 
 #endif /* _NET_OVPN_UDP_H_ */