Message-Id: <20250227-b4-ovpn-v20-12-93f363310834@openvpn.net>
Date: Thu, 27 Feb 2025 02:21:37 +0100
From: Antonio Quartulli <antonio@...nvpn.net>
To: netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Donald Hunter <donald.hunter@...il.com>, 
 Antonio Quartulli <antonio@...nvpn.net>, Shuah Khan <shuah@...nel.org>, 
 sd@...asysnail.net, ryazanov.s.a@...il.com, 
 Andrew Lunn <andrew+netdev@...n.ch>
Cc: Simon Horman <horms@...nel.org>, linux-kernel@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, Xiao Liang <shaw.leon@...il.com>, 
 David Ahern <dsahern@...nel.org>
Subject: [PATCH net-next v20 12/25] ovpn: implement TCP transport

With this change ovpn is allowed to communicate to peers also via TCP.
Parsing of incoming messages is implemented through the strparser API.

Note that ovpn redefines sk_prot and sk_socket->ops for the TCP socket
used to communicate with the peer.
For this reason it needs to access inet6_stream_ops, which is declared
as extern in the IPv6 module, but it is not fully exported.

Therefore this patch is also adding EXPORT_SYMBOL_GPL(inet6_stream_ops)
to net/ipv6/af_inet6.c.

Moreover export tcp_release_cb by means of EXPORT_SYMBOL instead of
EXPORT_IPV6_MOD, so that other modules can use it, even if IPV6 is
not compiled in.

Cc: David Ahern <dsahern@...nel.org>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>
Cc: Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>
Signed-off-by: Antonio Quartulli <antonio@...nvpn.net>
---
 drivers/net/Kconfig         |   1 +
 drivers/net/ovpn/Makefile   |   1 +
 drivers/net/ovpn/io.c       |   4 +
 drivers/net/ovpn/main.c     |   2 +
 drivers/net/ovpn/ovpnpriv.h |   1 +
 drivers/net/ovpn/peer.h     |  33 +++
 drivers/net/ovpn/socket.c   |  41 +++-
 drivers/net/ovpn/socket.h   |   6 +
 drivers/net/ovpn/tcp.c      | 567 ++++++++++++++++++++++++++++++++++++++++++++
 drivers/net/ovpn/tcp.h      |  36 +++
 net/ipv4/tcp_output.c       |   2 +-
 net/ipv6/af_inet6.c         |   1 +
 12 files changed, 685 insertions(+), 10 deletions(-)

diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index 51d77f3c0848c3c9425b586c6a90cff99a744390..754476cf6dc4774310205dd34f1124795fd2c4f7 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -125,6 +125,7 @@ config OVPN
 	select CRYPTO_AES
 	select CRYPTO_GCM
 	select CRYPTO_CHACHA20POLY1305
+	select STREAM_PARSER
 	help
 	  This module enhances the performance of the OpenVPN userspace software
 	  by offloading the data channel processing to kernelspace.
diff --git a/drivers/net/ovpn/Makefile b/drivers/net/ovpn/Makefile
index 04c3345807c5d759daf65cc80a290f784dbf5588..229be66167e1fe8e5e4c8f475343b544d7ee694e 100644
--- a/drivers/net/ovpn/Makefile
+++ b/drivers/net/ovpn/Makefile
@@ -18,4 +18,5 @@ ovpn-y += peer.o
 ovpn-y += pktid.o
 ovpn-y += socket.o
 ovpn-y += stats.o
+ovpn-y += tcp.o
 ovpn-y += udp.o
diff --git a/drivers/net/ovpn/io.c b/drivers/net/ovpn/io.c
index 50dc2e4c03f01b02bdf616473b755b6e1e6b57f7..729f49ff6ce8001c2bbe804db0a617a2cc8965a8 100644
--- a/drivers/net/ovpn/io.c
+++ b/drivers/net/ovpn/io.c
@@ -22,6 +22,7 @@
 #include "crypto_aead.h"
 #include "netlink.h"
 #include "proto.h"
+#include "tcp.h"
 #include "udp.h"
 #include "skb.h"
 #include "socket.h"
@@ -223,6 +224,9 @@ void ovpn_encrypt_post(void *data, int ret)
 	case IPPROTO_UDP:
 		ovpn_udp_send_skb(peer, sock->sock, skb);
 		break;
+	case IPPROTO_TCP:
+		ovpn_tcp_send_skb(peer, sock->sock, skb);
+		break;
 	default:
 		/* no transport configured yet */
 		goto err_unlock;
diff --git a/drivers/net/ovpn/main.c b/drivers/net/ovpn/main.c
index 241f2345298ab149d79b2dc04a1e52cfcb52d12c..3498c0dafb1ee64b974a77dae882447e317352fd 100644
--- a/drivers/net/ovpn/main.c
+++ b/drivers/net/ovpn/main.c
@@ -22,6 +22,7 @@
 #include "io.h"
 #include "peer.h"
 #include "proto.h"
+#include "tcp.h"
 #include "udp.h"
 
 static int ovpn_net_init(struct net_device *dev)
@@ -239,6 +240,7 @@ static int __init ovpn_init(void)
 	}
 
 	ovpn_udp_init();
+	ovpn_tcp_init();
 
 	return 0;
 
diff --git a/drivers/net/ovpn/ovpnpriv.h b/drivers/net/ovpn/ovpnpriv.h
index 9d0640e9c71e7fd494e3d9df155732bd5d82463e..2e3f4baf305f0b37b474d7b7d94751aa4af8a2ea 100644
--- a/drivers/net/ovpn/ovpnpriv.h
+++ b/drivers/net/ovpn/ovpnpriv.h
@@ -10,6 +10,7 @@
 #ifndef _NET_OVPN_OVPNSTRUCT_H_
 #define _NET_OVPN_OVPNSTRUCT_H_
 
+#include <linux/workqueue.h>
 #include <net/gro_cells.h>
 #include <uapi/linux/if_link.h>
 #include <uapi/linux/ovpn.h>
diff --git a/drivers/net/ovpn/peer.h b/drivers/net/ovpn/peer.h
index 2453d39ce327c6d174cfb35fe5430865b32c2efe..777a0b24843ed03e83ae8821837103a979e2c6de 100644
--- a/drivers/net/ovpn/peer.h
+++ b/drivers/net/ovpn/peer.h
@@ -11,6 +11,7 @@
 #define _NET_OVPN_OVPNPEER_H_
 
 #include <net/dst_cache.h>
+#include <net/strparser.h>
 
 #include "crypto.h"
 #include "socket.h"
@@ -25,6 +26,18 @@
  * @vpn_addrs.ipv4: IPv4 assigned to peer on the tunnel
  * @vpn_addrs.ipv6: IPv6 assigned to peer on the tunnel
  * @sock: the socket being used to talk to this peer
+ * @tcp: keeps track of TCP specific state
+ * @tcp.strp: stream parser context (TCP only)
+ * @tcp.user_queue: received packets that have to go to userspace (TCP only)
+ * @tcp.out_queue: packets on hold while socket is taken by user (TCP only)
+ * @tcp.tx_in_progress: true if TX is already ongoing (TCP only)
+ * @tcp.out_msg.skb: packet scheduled for sending (TCP only)
+ * @tcp.out_msg.offset: offset where next send should start (TCP only)
+ * @tcp.out_msg.len: remaining data to send within packet (TCP only)
+ * @tcp.sk_cb.sk_data_ready: pointer to original cb (TCP only)
+ * @tcp.sk_cb.sk_write_space: pointer to original cb (TCP only)
+ * @tcp.sk_cb.prot: pointer to original prot object (TCP only)
+ * @tcp.sk_cb.ops: pointer to the original prot_ops object (TCP only)
  * @crypto: the crypto configuration (ciphers, keys, etc..)
  * @dst_cache: cache for dst_entry used to send to peer
  * @bind: remote peer binding
@@ -45,6 +58,26 @@ struct ovpn_peer {
 		struct in6_addr ipv6;
 	} vpn_addrs;
 	struct ovpn_socket __rcu *sock;
+
+	struct {
+		struct strparser strp;
+		struct sk_buff_head user_queue;
+		struct sk_buff_head out_queue;
+		bool tx_in_progress;
+
+		struct {
+			struct sk_buff *skb;
+			int offset;
+			int len;
+		} out_msg;
+
+		struct {
+			void (*sk_data_ready)(struct sock *sk);
+			void (*sk_write_space)(struct sock *sk);
+			struct proto *prot;
+			const struct proto_ops *ops;
+		} sk_cb;
+	} tcp;
 	struct ovpn_crypto_state crypto;
 	struct dst_cache dst_cache;
 	struct ovpn_bind __rcu *bind;
diff --git a/drivers/net/ovpn/socket.c b/drivers/net/ovpn/socket.c
index 6de1e8c85a3d35a5d4f179d595d2409aa34cdc01..daf69154ee13c6755d4734f3ddd9a3004cf4288c 100644
--- a/drivers/net/ovpn/socket.c
+++ b/drivers/net/ovpn/socket.c
@@ -16,6 +16,7 @@
 #include "io.h"
 #include "peer.h"
 #include "socket.h"
+#include "tcp.h"
 #include "udp.h"
 
 static void ovpn_socket_release_kref(struct kref *kref)
@@ -23,12 +24,10 @@ static void ovpn_socket_release_kref(struct kref *kref)
 	struct ovpn_socket *sock = container_of(kref, struct ovpn_socket,
 						refcount);
 
-	if (sock->sock->sk->sk_protocol == IPPROTO_UDP) {
+	if (sock->sock->sk->sk_protocol == IPPROTO_UDP)
 		ovpn_udp_socket_detach(sock);
-		netdev_put(sock->ovpn->dev, &sock->dev_tracker);
-	}
-
-	kfree_rcu(sock, rcu);
+	else if (sock->sock->sk->sk_protocol == IPPROTO_TCP)
+		ovpn_tcp_socket_detach(sock);
 }
 
 /**
@@ -38,10 +37,12 @@ static void ovpn_socket_release_kref(struct kref *kref)
  *
  * This function is only used internally. Users willing to release
  * references to the ovpn_socket should use ovpn_socket_release()
+ *
+ * Return: true if the socket was released, false otherwise
  */
-static void ovpn_socket_put(struct ovpn_peer *peer, struct ovpn_socket *sock)
+static bool ovpn_socket_put(struct ovpn_peer *peer, struct ovpn_socket *sock)
 {
-	kref_put(&sock->refcount, ovpn_socket_release_kref);
+	return kref_put(&sock->refcount, ovpn_socket_release_kref);
 }
 
 /**
@@ -65,6 +66,7 @@ static void ovpn_socket_put(struct ovpn_peer *peer, struct ovpn_socket *sock)
 void ovpn_socket_release(struct ovpn_peer *peer)
 {
 	struct ovpn_socket *sock;
+	bool released;
 
 	might_sleep();
 
@@ -94,11 +96,23 @@ void ovpn_socket_release(struct ovpn_peer *peer)
 	 * detached before it can be picked by a concurrent reader.
 	 */
 	lock_sock(sock->sock->sk);
-	ovpn_socket_put(peer, sock);
+	released = ovpn_socket_put(peer, sock);
 	release_sock(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) {
+			netdev_put(sock->ovpn->dev, &sock->dev_tracker);
+		} else if (sock->sock->sk->sk_protocol == IPPROTO_TCP) {
+			/* wait for TCP jobs to terminate */
+			ovpn_tcp_socket_wait_finish(sock);
+			ovpn_peer_put(sock->peer);
+		}
+		kfree_rcu(sock, rcu);
+	}
 }
 
 static bool ovpn_socket_hold(struct ovpn_socket *sock)
@@ -110,6 +124,8 @@ static int ovpn_socket_attach(struct ovpn_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);
 
 	return -EOPNOTSUPP;
 }
@@ -202,7 +218,14 @@ struct ovpn_socket *ovpn_socket_new(struct socket *sock, struct ovpn_peer *peer)
 		goto sock_release;
 	}
 
-	if (sock->sk->sk_protocol == IPPROTO_UDP) {
+	/* TCP sockets are per-peer, therefore they are linked to their unique
+	 * peer
+	 */
+	if (sock->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) {
 		/* in UDP we only link the ovpn instance since the socket is
 		 * shared among multiple peers
 		 */
diff --git a/drivers/net/ovpn/socket.h b/drivers/net/ovpn/socket.h
index e5b94afe0cfa59ecd41007779456d083d8c1555f..3e361394261afec8f999342b2b0ce7983f8539bd 100644
--- a/drivers/net/ovpn/socket.h
+++ b/drivers/net/ovpn/socket.h
@@ -22,8 +22,11 @@ struct ovpn_peer;
  * @ovpn: ovpn instance owning this socket (UDP only)
  * @dev_tracker: reference tracker for associated dev (UDP only)
  * @udp_prot: pointer to the original socket sk_proto (UDP only)
+ * @peer: unique peer transmitting over this socket (TCP only)
  * @sock: 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)
  * @rcu: member used to schedule RCU destructor callback
  */
 struct ovpn_socket {
@@ -33,10 +36,13 @@ struct ovpn_socket {
 			netdevice_tracker dev_tracker;
 			struct proto *udp_prot;
 		};
+		struct ovpn_peer *peer;
 	};
 
 	struct socket *sock;
 	struct kref refcount;
+	struct work_struct work;
+	struct work_struct tcp_tx_work;
 	struct rcu_head rcu;
 };
 
diff --git a/drivers/net/ovpn/tcp.c b/drivers/net/ovpn/tcp.c
new file mode 100644
index 0000000000000000000000000000000000000000..2d343bce477156a9f60ce92d18c815b7c832d2af
--- /dev/null
+++ b/drivers/net/ovpn/tcp.c
@@ -0,0 +1,567 @@
+// SPDX-License-Identifier: GPL-2.0
+/*  OpenVPN data channel offload
+ *
+ *  Copyright (C) 2019-2025 OpenVPN, Inc.
+ *
+ *  Author:	Antonio Quartulli <antonio@...nvpn.net>
+ */
+
+#include <linux/skbuff.h>
+#include <net/hotdata.h>
+#include <net/inet_common.h>
+#include <net/ipv6.h>
+#include <net/tcp.h>
+#include <net/transp_v6.h>
+#include <net/route.h>
+#include <trace/events/sock.h>
+
+#include "ovpnpriv.h"
+#include "main.h"
+#include "io.h"
+#include "peer.h"
+#include "proto.h"
+#include "skb.h"
+#include "tcp.h"
+
+#define OVPN_TCP_DEPTH_NESTING	2
+#if OVPN_TCP_DEPTH_NESTING == SINGLE_DEPTH_NESTING
+#error "OVPN TCP requires its own lockdep subclass"
+#endif
+
+static struct proto ovpn_tcp_prot __ro_after_init;
+static struct proto_ops ovpn_tcp_ops __ro_after_init;
+static struct proto ovpn_tcp6_prot __ro_after_init;
+static struct proto_ops ovpn_tcp6_ops __ro_after_init;
+
+static int ovpn_tcp_parse(struct strparser *strp, struct sk_buff *skb)
+{
+	struct strp_msg *rxm = strp_msg(skb);
+	__be16 blen;
+	u16 len;
+	int err;
+
+	/* when packets are written to the TCP stream, they are prepended with
+	 * two bytes indicating the actual packet size.
+	 * Here we read those two bytes and move the skb data pointer to the
+	 * beginning of the packet
+	 */
+
+	if (skb->len < rxm->offset + 2)
+		return 0;
+
+	err = skb_copy_bits(skb, rxm->offset, &blen, sizeof(blen));
+	if (err < 0)
+		return err;
+
+	len = be16_to_cpu(blen);
+	if (len < 2)
+		return -EINVAL;
+
+	return len + 2;
+}
+
+/* queue skb for sending to userspace via recvmsg on the socket */
+static void ovpn_tcp_to_userspace(struct ovpn_peer *peer, struct sock *sk,
+				  struct sk_buff *skb)
+{
+	skb_set_owner_r(skb, sk);
+	memset(skb->cb, 0, sizeof(skb->cb));
+	skb_queue_tail(&peer->tcp.user_queue, skb);
+	peer->tcp.sk_cb.sk_data_ready(sk);
+}
+
+static void ovpn_tcp_rcv(struct strparser *strp, struct sk_buff *skb)
+{
+	struct ovpn_peer *peer = container_of(strp, struct ovpn_peer, tcp.strp);
+	struct strp_msg *msg = strp_msg(skb);
+	size_t pkt_len = msg->full_len - 2;
+	size_t off = msg->offset + 2;
+	u8 opcode;
+
+	/* ensure skb->data points to the beginning of the openvpn packet */
+	if (!pskb_pull(skb, off)) {
+		net_warn_ratelimited("%s: packet too small for peer %u\n",
+				     netdev_name(peer->ovpn->dev), peer->id);
+		goto err;
+	}
+
+	/* strparser does not trim the skb for us, therefore we do it now */
+	if (pskb_trim(skb, pkt_len) != 0) {
+		net_warn_ratelimited("%s: trimming skb failed for peer %u\n",
+				     netdev_name(peer->ovpn->dev), peer->id);
+		goto err;
+	}
+
+	/* we need the first byte of data to be accessible
+	 * to extract the opcode and the key ID later on
+	 */
+	if (!pskb_may_pull(skb, OVPN_OPCODE_SIZE)) {
+		net_warn_ratelimited("%s: packet too small to fetch opcode for peer %u\n",
+				     netdev_name(peer->ovpn->dev), peer->id);
+		goto err;
+	}
+
+	/* DATA_V2 packets are handled in kernel, the rest goes to user space */
+	opcode = ovpn_opcode_from_skb(skb, 0);
+	if (unlikely(opcode != OVPN_DATA_V2)) {
+		if (opcode == OVPN_DATA_V1) {
+			net_warn_ratelimited("%s: DATA_V1 detected on the TCP stream\n",
+					     netdev_name(peer->ovpn->dev));
+			goto err;
+		}
+
+		/* The packet size header must be there when sending the packet
+		 * to userspace, therefore we put it back
+		 */
+		skb_push(skb, 2);
+		ovpn_tcp_to_userspace(peer, strp->sk, skb);
+		return;
+	}
+
+	/* hold reference to peer as required by ovpn_recv().
+	 *
+	 * NOTE: in this context we should already be holding a reference to
+	 * this peer, therefore ovpn_peer_hold() is not expected to fail
+	 */
+	if (WARN_ON(!ovpn_peer_hold(peer)))
+		goto err;
+
+	ovpn_recv(peer, skb);
+	return;
+err:
+	dev_core_stats_rx_dropped_inc(peer->ovpn->dev);
+	kfree_skb(skb);
+	ovpn_peer_del(peer, OVPN_DEL_PEER_REASON_TRANSPORT_ERROR);
+}
+
+static int ovpn_tcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
+			    int flags, int *addr_len)
+{
+	int err = 0, off, copied = 0, ret;
+	struct ovpn_socket *sock;
+	struct ovpn_peer *peer;
+	struct sk_buff *skb;
+
+	rcu_read_lock();
+	sock = rcu_dereference_sk_user_data(sk);
+	if (unlikely(!sock || !sock->peer || !ovpn_peer_hold(sock->peer))) {
+		rcu_read_unlock();
+		return -EBADF;
+	}
+	peer = sock->peer;
+	rcu_read_unlock();
+
+	skb = __skb_recv_datagram(sk, &peer->tcp.user_queue, flags, &off, &err);
+	if (!skb) {
+		if (err == -EAGAIN && sk->sk_shutdown & RCV_SHUTDOWN) {
+			ret = 0;
+			goto out;
+		}
+		ret = err;
+		goto out;
+	}
+
+	copied = len;
+	if (copied > skb->len)
+		copied = skb->len;
+	else if (copied < skb->len)
+		msg->msg_flags |= MSG_TRUNC;
+
+	err = skb_copy_datagram_msg(skb, 0, msg, copied);
+	if (unlikely(err)) {
+		kfree_skb(skb);
+		ret = err;
+		goto out;
+	}
+
+	if (flags & MSG_TRUNC)
+		copied = skb->len;
+	kfree_skb(skb);
+	ret = copied;
+out:
+	ovpn_peer_put(peer);
+	return ret;
+}
+
+void ovpn_tcp_socket_detach(struct ovpn_socket *ovpn_sock)
+{
+	struct ovpn_peer *peer = ovpn_sock->peer;
+	struct socket *sock = ovpn_sock->sock;
+
+	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;
+
+	rcu_assign_sk_user_data(sock->sk, NULL);
+}
+
+void ovpn_tcp_socket_wait_finish(struct ovpn_socket *sock)
+{
+	struct ovpn_peer *peer = sock->peer;
+
+	cancel_work_sync(&sock->tcp_tx_work);
+	strp_done(&peer->tcp.strp);
+
+	skb_queue_purge(&peer->tcp.out_queue);
+	kfree_skb(peer->tcp.out_msg.skb);
+	peer->tcp.out_msg.skb = NULL;
+}
+
+static void ovpn_tcp_send_sock(struct ovpn_peer *peer, struct sock *sk)
+{
+	struct sk_buff *skb = peer->tcp.out_msg.skb;
+
+	if (!skb)
+		return;
+
+	if (peer->tcp.tx_in_progress)
+		return;
+
+	peer->tcp.tx_in_progress = true;
+
+	do {
+		int ret = skb_send_sock_locked(sk, skb,
+					       peer->tcp.out_msg.offset,
+					       peer->tcp.out_msg.len);
+		if (unlikely(ret < 0)) {
+			if (ret == -EAGAIN)
+				goto out;
+
+			net_warn_ratelimited("%s: TCP error to peer %u: %d\n",
+					     netdev_name(peer->ovpn->dev),
+					     peer->id, ret);
+
+			/* in case of TCP error we can't recover the VPN
+			 * stream therefore we abort the connection
+			 */
+			ovpn_peer_del(peer,
+				      OVPN_DEL_PEER_REASON_TRANSPORT_ERROR);
+			break;
+		}
+
+		peer->tcp.out_msg.len -= ret;
+		peer->tcp.out_msg.offset += ret;
+	} while (peer->tcp.out_msg.len > 0);
+
+	if (!peer->tcp.out_msg.len) {
+		preempt_disable();
+		dev_sw_netstats_tx_add(peer->ovpn->dev, 1, skb->len);
+		preempt_enable();
+	}
+
+	kfree_skb(peer->tcp.out_msg.skb);
+	peer->tcp.out_msg.skb = NULL;
+	peer->tcp.out_msg.len = 0;
+	peer->tcp.out_msg.offset = 0;
+
+out:
+	peer->tcp.tx_in_progress = false;
+}
+
+void ovpn_tcp_tx_work(struct work_struct *work)
+{
+	struct ovpn_socket *sock;
+
+	sock = container_of(work, struct ovpn_socket, tcp_tx_work);
+
+	lock_sock(sock->sock->sk);
+	if (sock->peer)
+		ovpn_tcp_send_sock(sock->peer, sock->sock->sk);
+	release_sock(sock->sock->sk);
+}
+
+static void ovpn_tcp_send_sock_skb(struct ovpn_peer *peer, struct sock *sk,
+				   struct sk_buff *skb)
+{
+	if (peer->tcp.out_msg.skb)
+		ovpn_tcp_send_sock(peer, sk);
+
+	if (peer->tcp.out_msg.skb) {
+		dev_core_stats_tx_dropped_inc(peer->ovpn->dev);
+		kfree_skb(skb);
+		return;
+	}
+
+	peer->tcp.out_msg.skb = skb;
+	peer->tcp.out_msg.len = skb->len;
+	peer->tcp.out_msg.offset = 0;
+	ovpn_tcp_send_sock(peer, sk);
+}
+
+void ovpn_tcp_send_skb(struct ovpn_peer *peer, struct socket *sock,
+		       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)) {
+		if (skb_queue_len(&peer->tcp.out_queue) >=
+		    READ_ONCE(net_hotdata.max_backlog)) {
+			dev_core_stats_tx_dropped_inc(peer->ovpn->dev);
+			kfree_skb(skb);
+			goto unlock;
+		}
+		__skb_queue_tail(&peer->tcp.out_queue, skb);
+	} else {
+		ovpn_tcp_send_sock_skb(peer, sock->sk, skb);
+	}
+unlock:
+	spin_unlock(&sock->sk->sk_lock.slock);
+}
+
+static void ovpn_tcp_release(struct sock *sk)
+{
+	struct sk_buff_head queue;
+	struct ovpn_socket *sock;
+	struct ovpn_peer *peer;
+	struct sk_buff *skb;
+
+	rcu_read_lock();
+	sock = rcu_dereference_sk_user_data(sk);
+	if (!sock) {
+		rcu_read_unlock();
+		goto release;
+	}
+
+	peer = sock->peer;
+
+	/* during initialization this function is called before
+	 * assigning sock->peer
+	 */
+	if (unlikely(!peer || !ovpn_peer_hold(peer))) {
+		rcu_read_unlock();
+		goto release;
+	}
+	rcu_read_unlock();
+
+	__skb_queue_head_init(&queue);
+	skb_queue_splice_init(&peer->tcp.out_queue, &queue);
+
+	while ((skb = __skb_dequeue(&queue)))
+		ovpn_tcp_send_sock_skb(peer, sk, skb);
+
+	ovpn_peer_put(peer);
+release:
+	tcp_release_cb(sk);
+}
+
+static int ovpn_tcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
+{
+	struct ovpn_socket *sock;
+	int ret, linear = PAGE_SIZE;
+	struct ovpn_peer *peer;
+	struct sk_buff *skb;
+
+	lock_sock(sk);
+	rcu_read_lock();
+	sock = rcu_dereference_sk_user_data(sk);
+	if (unlikely(!sock || !sock->peer || !ovpn_peer_hold(sock->peer))) {
+		rcu_read_unlock();
+		release_sock(sk);
+		return -EIO;
+	}
+	rcu_read_unlock();
+	peer = sock->peer;
+
+	if (msg->msg_flags & ~MSG_DONTWAIT) {
+		ret = -EOPNOTSUPP;
+		goto peer_free;
+	}
+
+	if (peer->tcp.out_msg.skb) {
+		ret = -EAGAIN;
+		goto peer_free;
+	}
+
+	if (size < linear)
+		linear = size;
+
+	skb = sock_alloc_send_pskb(sk, linear, size - linear,
+				   msg->msg_flags & MSG_DONTWAIT, &ret, 0);
+	if (!skb) {
+		net_err_ratelimited("%s: skb alloc failed: %d\n",
+				    netdev_name(peer->ovpn->dev), ret);
+		goto peer_free;
+	}
+
+	skb_put(skb, linear);
+	skb->len = size;
+	skb->data_len = size - linear;
+
+	ret = skb_copy_datagram_from_iter(skb, 0, &msg->msg_iter, size);
+	if (ret) {
+		kfree_skb(skb);
+		net_err_ratelimited("%s: skb copy from iter failed: %d\n",
+				    netdev_name(peer->ovpn->dev), ret);
+		goto peer_free;
+	}
+
+	ovpn_tcp_send_sock_skb(peer, sk, skb);
+	ret = size;
+peer_free:
+	release_sock(sk);
+	ovpn_peer_put(peer);
+	return ret;
+}
+
+static void ovpn_tcp_data_ready(struct sock *sk)
+{
+	struct ovpn_socket *sock;
+
+	trace_sk_data_ready(sk);
+
+	rcu_read_lock();
+	sock = rcu_dereference_sk_user_data(sk);
+	if (likely(sock && sock->peer))
+		strp_data_ready(&sock->peer->tcp.strp);
+	rcu_read_unlock();
+}
+
+static void ovpn_tcp_write_space(struct sock *sk)
+{
+	struct ovpn_socket *sock;
+
+	rcu_read_lock();
+	sock = rcu_dereference_sk_user_data(sk);
+	if (likely(sock && sock->peer)) {
+		schedule_work(&sock->tcp_tx_work);
+		sock->peer->tcp.sk_cb.sk_write_space(sk);
+	}
+	rcu_read_unlock();
+}
+
+static void ovpn_tcp_build_protos(struct proto *new_prot,
+				  struct proto_ops *new_ops,
+				  const struct proto *orig_prot,
+				  const struct proto_ops *orig_ops);
+
+/* Set TCP encapsulation callbacks */
+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)
+		return -EBUSY;
+
+	/* only a fully connected socket is expected. Connection should be
+	 * handled in userspace
+	 */
+	if (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);
+		return -EINVAL;
+	}
+
+	ret = strp_init(&peer->tcp.strp, sock->sk, &cb);
+	if (ret < 0) {
+		DEBUG_NET_WARN_ON_ONCE(1);
+		return ret;
+	}
+
+	__sk_dst_reset(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;
+
+	/* 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;
+
+	if (sock->sk->sk_family == AF_INET) {
+		sock->sk->sk_prot = &ovpn_tcp_prot;
+		sock->sk->sk_socket->ops = &ovpn_tcp_ops;
+	} else {
+		sock->sk->sk_prot = &ovpn_tcp6_prot;
+		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;
+
+	/* enqueue the RX worker */
+	strp_check_rcv(&peer->tcp.strp);
+
+	return 0;
+}
+
+static void ovpn_tcp_close(struct sock *sk, long timeout)
+{
+	struct ovpn_socket *sock;
+	struct ovpn_peer *peer;
+
+	rcu_read_lock();
+	sock = rcu_dereference_sk_user_data(sk);
+	if (!sock || !sock->peer || !ovpn_peer_hold(sock->peer)) {
+		rcu_read_unlock();
+		return;
+	}
+	peer = sock->peer;
+	rcu_read_unlock();
+
+	ovpn_peer_del(sock->peer, OVPN_DEL_PEER_REASON_TRANSPORT_DISCONNECT);
+	ovpn_peer_put(peer);
+	peer->tcp.sk_cb.prot->close(sk, timeout);
+}
+
+static __poll_t ovpn_tcp_poll(struct file *file, struct socket *sock,
+			      poll_table *wait)
+{
+	__poll_t mask = datagram_poll(file, sock, wait);
+	struct ovpn_socket *ovpn_sock;
+
+	rcu_read_lock();
+	ovpn_sock = rcu_dereference_sk_user_data(sock->sk);
+	if (ovpn_sock && ovpn_sock->peer &&
+	    !skb_queue_empty(&ovpn_sock->peer->tcp.user_queue))
+		mask |= EPOLLIN | EPOLLRDNORM;
+	rcu_read_unlock();
+
+	return mask;
+}
+
+static void ovpn_tcp_build_protos(struct proto *new_prot,
+				  struct proto_ops *new_ops,
+				  const struct proto *orig_prot,
+				  const struct proto_ops *orig_ops)
+{
+	memcpy(new_prot, orig_prot, sizeof(*new_prot));
+	memcpy(new_ops, orig_ops, sizeof(*new_ops));
+	new_prot->recvmsg = ovpn_tcp_recvmsg;
+	new_prot->sendmsg = ovpn_tcp_sendmsg;
+	new_prot->close = ovpn_tcp_close;
+	new_prot->release_cb = ovpn_tcp_release;
+	new_ops->poll = ovpn_tcp_poll;
+}
+
+/* Initialize TCP static objects */
+void __init ovpn_tcp_init(void)
+{
+	ovpn_tcp_build_protos(&ovpn_tcp_prot, &ovpn_tcp_ops, &tcp_prot,
+			      &inet_stream_ops);
+
+#if IS_ENABLED(CONFIG_IPV6)
+	ovpn_tcp_build_protos(&ovpn_tcp6_prot, &ovpn_tcp6_ops, &tcpv6_prot,
+			      &inet6_stream_ops);
+#endif
+}
diff --git a/drivers/net/ovpn/tcp.h b/drivers/net/ovpn/tcp.h
new file mode 100644
index 0000000000000000000000000000000000000000..10aefa834cf358f39f4fc250063d6ef13e0353b0
--- /dev/null
+++ b/drivers/net/ovpn/tcp.h
@@ -0,0 +1,36 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*  OpenVPN data channel offload
+ *
+ *  Copyright (C) 2019-2025 OpenVPN, Inc.
+ *
+ *  Author:	Antonio Quartulli <antonio@...nvpn.net>
+ */
+
+#ifndef _NET_OVPN_TCP_H_
+#define _NET_OVPN_TCP_H_
+
+#include <linux/net.h>
+#include <linux/skbuff.h>
+#include <linux/types.h>
+
+#include "peer.h"
+#include "skb.h"
+#include "socket.h"
+
+void __init ovpn_tcp_init(void);
+
+int ovpn_tcp_socket_attach(struct ovpn_socket *ovpn_sock,
+			   struct ovpn_peer *peer);
+void ovpn_tcp_socket_detach(struct ovpn_socket *ovpn_sock);
+void ovpn_tcp_socket_wait_finish(struct ovpn_socket *sock);
+
+/* Prepare skb and enqueue it for sending to peer.
+ *
+ * Preparation consist in prepending the skb payload with its size.
+ * 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_tx_work(struct work_struct *work);
+
+#endif /* _NET_OVPN_TCP_H_ */
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 9a3cf51eab787859ec82432ee6eb9f94e709b292..567161ae11fa43ab3e03d28c53b98670e9b1ce2f 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1173,7 +1173,7 @@ void tcp_release_cb(struct sock *sk)
 	if ((flags & TCPF_ACK_DEFERRED) && inet_csk_ack_scheduled(sk))
 		tcp_send_ack(sk);
 }
-EXPORT_IPV6_MOD(tcp_release_cb);
+EXPORT_SYMBOL(tcp_release_cb);
 
 void __init tcp_tasklet_init(void)
 {
diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
index f60ec8b0f8ea40b2d635d802a3bc4f9b9d844417..3e812187e125cec7deac88413b85a35dd5b22a2d 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -715,6 +715,7 @@ const struct proto_ops inet6_stream_ops = {
 #endif
 	.set_rcvlowat	   = tcp_set_rcvlowat,
 };
+EXPORT_SYMBOL_GPL(inet6_stream_ops);
 
 const struct proto_ops inet6_dgram_ops = {
 	.family		   = PF_INET6,

-- 
2.45.3

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