ovpn: implement basic RX path (UDP)

Packets received over the socket are forwarded to the user device.

Implementation is UDP only. TCP will be added by a later patch.

Note: no decryption/decapsulation exists yet, packets are forwarded as
they arrive without much processing.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Link: https://patch.msgid.link/20250415-b4-ovpn-v26-8-577f6097b964@openvpn.net
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: ordex

drivers/net/ovpn/io.c

@@ -9,15 +9,77 @@
 
 #include <linux/netdevice.h>
 #include <linux/skbuff.h>
+#include <net/gro_cells.h>
 #include <net/gso.h>
 
-#include "io.h"
 #include "ovpnpriv.h"
 #include "peer.h"
+#include "io.h"
+#include "netlink.h"
+#include "proto.h"
 #include "udp.h"
 #include "skb.h"
 #include "socket.h"
 
+/* Called after decrypt to write the IP packet to the device.
+ * This method is expected to manage/free the skb.
+ */
+static void ovpn_netdev_write(struct ovpn_peer *peer, struct sk_buff *skb)
+{
+	unsigned int pkt_len;
+	int ret;
+
+	/* we can't guarantee the packet wasn't corrupted before entering the
+	 * VPN, therefore we give other layers a chance to check that
+	 */
+	skb->ip_summed = CHECKSUM_NONE;
+
+	/* skb hash for transport packet no longer valid after decapsulation */
+	skb_clear_hash(skb);
+
+	/* post-decrypt scrub -- prepare to inject encapsulated packet onto the
+	 * interface, based on __skb_tunnel_rx() in dst.h
+	 */
+	skb->dev = peer->ovpn->dev;
+	skb_set_queue_mapping(skb, 0);
+	skb_scrub_packet(skb, true);
+
+	skb_reset_network_header(skb);
+	skb_reset_transport_header(skb);
+	skb_reset_inner_headers(skb);
+
+	/* cause packet to be "received" by the interface */
+	pkt_len = skb->len;
+	ret = gro_cells_receive(&peer->ovpn->gro_cells, skb);
+	if (likely(ret == NET_RX_SUCCESS))
+		/* update RX stats with the size of decrypted packet */
+		dev_dstats_rx_add(peer->ovpn->dev, pkt_len);
+}
+
+static void ovpn_decrypt_post(struct sk_buff *skb, int ret)
+{
+	struct ovpn_peer *peer = ovpn_skb_cb(skb)->peer;
+
+	if (unlikely(ret < 0))
+		goto drop;
+
+	ovpn_netdev_write(peer, skb);
+	/* skb is passed to upper layer - don't free it */
+	skb = NULL;
+drop:
+	if (unlikely(skb))
+		dev_dstats_rx_dropped(peer->ovpn->dev);
+	ovpn_peer_put(peer);
+	kfree_skb(skb);
+}
+
+/* RX path entry point: decrypt packet and forward it to the device */
+void ovpn_recv(struct ovpn_peer *peer, struct sk_buff *skb)
+{
+	ovpn_skb_cb(skb)->peer = peer;
+	ovpn_decrypt_post(skb, 0);
+}
+
 static void ovpn_encrypt_post(struct sk_buff *skb, int ret)
 {
 	struct ovpn_peer *peer = ovpn_skb_cb(skb)->peer;

drivers/net/ovpn/io.h

@@ -21,4 +21,6 @@
 
 netdev_tx_t ovpn_net_xmit(struct sk_buff *skb, struct net_device *dev);
 
+void ovpn_recv(struct ovpn_peer *peer, struct sk_buff *skb);
+
 #endif /* _NET_OVPN_OVPN_H_ */

drivers/net/ovpn/main.c

@@ -11,6 +11,7 @@
 #include <linux/module.h>
 #include <linux/netdevice.h>
 #include <linux/inetdevice.h>
+#include <net/gro_cells.h>
 #include <net/ip.h>
 #include <net/rtnetlink.h>
 #include <uapi/linux/if_arp.h>
@@ -21,8 +22,25 @@
 #include "io.h"
 #include "peer.h"
 #include "proto.h"
+#include "udp.h"
+
+static int ovpn_net_init(struct net_device *dev)
+{
+	struct ovpn_priv *ovpn = netdev_priv(dev);
+
+	return gro_cells_init(&ovpn->gro_cells, dev);
+}
+
+static void ovpn_net_uninit(struct net_device *dev)
+{
+	struct ovpn_priv *ovpn = netdev_priv(dev);
+
+	gro_cells_destroy(&ovpn->gro_cells);
+}
 
 static const struct net_device_ops ovpn_netdev_ops = {
+	.ndo_init		= ovpn_net_init,
+	.ndo_uninit		= ovpn_net_uninit,
 	.ndo_start_xmit		= ovpn_net_xmit,
 };
 

drivers/net/ovpn/ovpnpriv.h

@@ -10,6 +10,7 @@
 #ifndef _NET_OVPN_OVPNSTRUCT_H_
 #define _NET_OVPN_OVPNSTRUCT_H_
 
+#include <net/gro_cells.h>
 #include <uapi/linux/if_link.h>
 #include <uapi/linux/ovpn.h>
 
@@ -19,12 +20,14 @@
  * @mode: device operation mode (i.e. p2p, mp, ..)
  * @lock: protect this object
  * @peer: in P2P mode, this is the only remote peer
+ * @gro_cells: pointer to the Generic Receive Offload cell
  */
 struct ovpn_priv {
 	struct net_device *dev;
 	enum ovpn_mode mode;
 	spinlock_t lock; /* protect writing to the ovpn_priv object */
 	struct ovpn_peer __rcu *peer;
+	struct gro_cells gro_cells;
 };
 
 #endif /* _NET_OVPN_OVPNSTRUCT_H_ */

drivers/net/ovpn/proto.h

@@ -10,6 +10,11 @@
 #ifndef _NET_OVPN_PROTO_H_
 #define _NET_OVPN_PROTO_H_
 
+#include "main.h"
+
+#include <linux/bitfield.h>
+#include <linux/skbuff.h>
+
 /* When the OpenVPN protocol is ran in AEAD mode, use
  * the OpenVPN packet ID as the AEAD nonce:
  *
@@ -34,5 +39,48 @@
 #define OVPN_NONCE_WIRE_SIZE (OVPN_NONCE_SIZE - OVPN_NONCE_TAIL_SIZE)
 
 #define OVPN_OPCODE_SIZE		4 /* DATA_V2 opcode size */
+#define OVPN_OPCODE_KEYID_MASK		0x07000000
+#define OVPN_OPCODE_PKTTYPE_MASK	0xF8000000
+#define OVPN_OPCODE_PEERID_MASK		0x00FFFFFF
+
+/* packet opcodes of interest to us */
+#define OVPN_DATA_V1			6 /* data channel v1 packet */
+#define OVPN_DATA_V2			9 /* data channel v2 packet */
+
+#define OVPN_PEER_ID_UNDEF		0x00FFFFFF
+
+/**
+ * ovpn_opcode_from_skb - extract OP code from skb at specified offset
+ * @skb: the packet to extract the OP code from
+ * @offset: the offset in the data buffer where the OP code is located
+ *
+ * Note: this function assumes that the skb head was pulled enough
+ * to access the first 4 bytes.
+ *
+ * Return: the OP code
+ */
+static inline u8 ovpn_opcode_from_skb(const struct sk_buff *skb, u16 offset)
+{
+	u32 opcode = be32_to_cpu(*(__be32 *)(skb->data + offset));
+
+	return FIELD_GET(OVPN_OPCODE_PKTTYPE_MASK, opcode);
+}
+
+/**
+ * ovpn_peer_id_from_skb - extract peer ID from skb at specified offset
+ * @skb: the packet to extract the OP code from
+ * @offset: the offset in the data buffer where the OP code is located
+ *
+ * Note: this function assumes that the skb head was pulled enough
+ * to access the first 4 bytes.
+ *
+ * Return: the peer ID
+ */
+static inline u32 ovpn_peer_id_from_skb(const struct sk_buff *skb, u16 offset)
+{
+	u32 opcode = be32_to_cpu(*(__be32 *)(skb->data + offset));
+
+	return FIELD_GET(OVPN_OPCODE_PEERID_MASK, opcode);
+}
 
-#endif /* _NET_OVPN_PROTO_H_ */
+#endif /* _NET_OVPN_OVPNPROTO_H_ */

drivers/net/ovpn/socket.c

@@ -23,8 +23,10 @@ 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);
 }
@@ -179,7 +181,6 @@ struct ovpn_socket *ovpn_socket_new(struct socket *sock, struct ovpn_peer *peer)
 		goto sock_release;
 	}
 
-	ovpn_sock->ovpn = peer->ovpn;
 	ovpn_sock->sock = sock;
 	kref_init(&ovpn_sock->refcount);
 
@@ -190,6 +191,15 @@ struct ovpn_socket *ovpn_socket_new(struct socket *sock, struct ovpn_peer *peer)
 		goto sock_release;
 	}
 
+	if (sock->sk->sk_protocol == IPPROTO_UDP) {
+		/* in UDP we only link the ovpn instance since the socket is
+		 * shared among multiple peers
+		 */
+		ovpn_sock->ovpn = peer->ovpn;
+		netdev_hold(peer->ovpn->dev, &ovpn_sock->dev_tracker,
+			    GFP_KERNEL);
+	}
+
 	rcu_assign_sk_user_data(sock->sk, ovpn_sock);
 sock_release:
 	release_sock(sock->sk);

drivers/net/ovpn/socket.h

@@ -20,12 +20,19 @@ struct ovpn_peer;
 /**
  * struct ovpn_socket - a kernel socket referenced in the ovpn code
  * @ovpn: ovpn instance owning this socket (UDP only)
+ * @dev_tracker: reference tracker for associated dev (UDP only)
  * @sock: the low level sock object
  * @refcount: amount of contexts currently referencing this object
  * @rcu: member used to schedule RCU destructor callback
  */
 struct ovpn_socket {
-	struct ovpn_priv *ovpn;
+	union {
+		struct {
+			struct ovpn_priv *ovpn;
+			netdevice_tracker dev_tracker;
+		};
+	};
+
 	struct socket *sock;
 	struct kref refcount;
 	struct rcu_head rcu;