ovpn: implement peer lookup logic

In a multi-peer scenario there are a number of situations when a
specific peer needs to be looked up.

We may want to lookup a peer by:
1. its ID
2. its VPN destination IP
3. its transport IP/port couple

For each of the above, there is a specific routing table referencing all
peers for fast look up.

Case 2. is a bit special in the sense that an outgoing packet may not be
sent to the peer VPN IP directly, but rather to a network behind it. For
this reason we first perform a nexthop lookup in the system routing
table and then we use the retrieved nexthop as peer search key.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Link: https://patch.msgid.link/20250415-b4-ovpn-v26-15-577f6097b964@openvpn.net
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/ovpn/peer.c

@@ -10,6 +10,7 @@
 #include <linux/skbuff.h>
 #include <linux/list.h>
 #include <linux/hashtable.h>
+#include <net/ip6_route.h>
 
 #include "ovpnpriv.h"
 #include "bind.h"
@@ -150,6 +151,121 @@ static int ovpn_peer_skb_to_sockaddr(struct sk_buff *skb,
 	return -1;
 }
 
+/**
+ * ovpn_nexthop_from_skb4 - retrieve IPv4 nexthop for outgoing skb
+ * @skb: the outgoing packet
+ *
+ * Return: the IPv4 of the nexthop
+ */
+static __be32 ovpn_nexthop_from_skb4(struct sk_buff *skb)
+{
+	const struct rtable *rt = skb_rtable(skb);
+
+	if (rt && rt->rt_uses_gateway)
+		return rt->rt_gw4;
+
+	return ip_hdr(skb)->daddr;
+}
+
+/**
+ * ovpn_nexthop_from_skb6 - retrieve IPv6 nexthop for outgoing skb
+ * @skb: the outgoing packet
+ *
+ * Return: the IPv6 of the nexthop
+ */
+static struct in6_addr ovpn_nexthop_from_skb6(struct sk_buff *skb)
+{
+	const struct rt6_info *rt = skb_rt6_info(skb);
+
+	if (!rt || !(rt->rt6i_flags & RTF_GATEWAY))
+		return ipv6_hdr(skb)->daddr;
+
+	return rt->rt6i_gateway;
+}
+
+/* variable name __tbl2 needs to be different from __tbl1
+ * in the macro below to avoid confusing clang
+ */
+#define ovpn_get_hash_slot(_tbl, _key, _key_len) ({	\
+	typeof(_tbl) *__tbl2 = &(_tbl);			\
+	jhash(_key, _key_len, 0) % HASH_SIZE(*__tbl2);	\
+})
+
+#define ovpn_get_hash_head(_tbl, _key, _key_len) ({		\
+	typeof(_tbl) *__tbl1 = &(_tbl);				\
+	&(*__tbl1)[ovpn_get_hash_slot(*__tbl1, _key, _key_len)];\
+})
+
+/**
+ * ovpn_peer_get_by_vpn_addr4 - retrieve peer by its VPN IPv4 address
+ * @ovpn: the openvpn instance to search
+ * @addr: VPN IPv4 to use as search key
+ *
+ * Refcounter is not increased for the returned peer.
+ *
+ * Return: the peer if found or NULL otherwise
+ */
+static struct ovpn_peer *ovpn_peer_get_by_vpn_addr4(struct ovpn_priv *ovpn,
+						    __be32 addr)
+{
+	struct hlist_nulls_head *nhead;
+	struct hlist_nulls_node *ntmp;
+	struct ovpn_peer *tmp;
+	unsigned int slot;
+
+begin:
+	slot = ovpn_get_hash_slot(ovpn->peers->by_vpn_addr4, &addr,
+				  sizeof(addr));
+	nhead = &ovpn->peers->by_vpn_addr4[slot];
+
+	hlist_nulls_for_each_entry_rcu(tmp, ntmp, nhead, hash_entry_addr4)
+		if (addr == tmp->vpn_addrs.ipv4.s_addr)
+			return tmp;
+
+	/* item may have moved during lookup - check nulls and restart
+	 * if that's the case
+	 */
+	if (get_nulls_value(ntmp) != slot)
+		goto begin;
+
+	return NULL;
+}
+
+/**
+ * ovpn_peer_get_by_vpn_addr6 - retrieve peer by its VPN IPv6 address
+ * @ovpn: the openvpn instance to search
+ * @addr: VPN IPv6 to use as search key
+ *
+ * Refcounter is not increased for the returned peer.
+ *
+ * Return: the peer if found or NULL otherwise
+ */
+static struct ovpn_peer *ovpn_peer_get_by_vpn_addr6(struct ovpn_priv *ovpn,
+						    struct in6_addr *addr)
+{
+	struct hlist_nulls_head *nhead;
+	struct hlist_nulls_node *ntmp;
+	struct ovpn_peer *tmp;
+	unsigned int slot;
+
+begin:
+	slot = ovpn_get_hash_slot(ovpn->peers->by_vpn_addr6, addr,
+				  sizeof(*addr));
+	nhead = &ovpn->peers->by_vpn_addr6[slot];
+
+	hlist_nulls_for_each_entry_rcu(tmp, ntmp, nhead, hash_entry_addr6)
+		if (ipv6_addr_equal(addr, &tmp->vpn_addrs.ipv6))
+			return tmp;
+
+	/* item may have moved during lookup - check nulls and restart
+	 * if that's the case
+	 */
+	if (get_nulls_value(ntmp) != slot)
+		goto begin;
+
+	return NULL;
+}
+
 /**
  * ovpn_peer_transp_match - check if sockaddr and peer binding match
  * @peer: the peer to get the binding from
@@ -227,14 +343,43 @@ ovpn_peer_get_by_transp_addr_p2p(struct ovpn_priv *ovpn,
 struct ovpn_peer *ovpn_peer_get_by_transp_addr(struct ovpn_priv *ovpn,
 					       struct sk_buff *skb)
 {
-	struct ovpn_peer *peer = NULL;
+	struct ovpn_peer *tmp, *peer = NULL;
 	struct sockaddr_storage ss = { 0 };
+	struct hlist_nulls_head *nhead;
+	struct hlist_nulls_node *ntmp;
+	unsigned int slot;
+	ssize_t sa_len;
 
-	if (unlikely(!ovpn_peer_skb_to_sockaddr(skb, &ss)))
+	sa_len = ovpn_peer_skb_to_sockaddr(skb, &ss);
+	if (unlikely(sa_len < 0))
 		return NULL;
 
 	if (ovpn->mode == OVPN_MODE_P2P)
-		peer = ovpn_peer_get_by_transp_addr_p2p(ovpn, &ss);
+		return ovpn_peer_get_by_transp_addr_p2p(ovpn, &ss);
+
+	rcu_read_lock();
+begin:
+	slot = ovpn_get_hash_slot(ovpn->peers->by_transp_addr, &ss, sa_len);
+	nhead = &ovpn->peers->by_transp_addr[slot];
+
+	hlist_nulls_for_each_entry_rcu(tmp, ntmp, nhead,
+				       hash_entry_transp_addr) {
+		if (!ovpn_peer_transp_match(tmp, &ss))
+			continue;
+
+		if (!ovpn_peer_hold(tmp))
+			continue;
+
+		peer = tmp;
+		break;
+	}
+
+	/* item may have moved during lookup - check nulls and restart
+	 * if that's the case
+	 */
+	if (!peer && get_nulls_value(ntmp) != slot)
+		goto begin;
+	rcu_read_unlock();
 
 	return peer;
 }
@@ -269,10 +414,27 @@ static struct ovpn_peer *ovpn_peer_get_by_id_p2p(struct ovpn_priv *ovpn,
  */
 struct ovpn_peer *ovpn_peer_get_by_id(struct ovpn_priv *ovpn, u32 peer_id)
 {
-	struct ovpn_peer *peer = NULL;
+	struct ovpn_peer *tmp, *peer = NULL;
+	struct hlist_head *head;
 
 	if (ovpn->mode == OVPN_MODE_P2P)
-		peer = ovpn_peer_get_by_id_p2p(ovpn, peer_id);
+		return ovpn_peer_get_by_id_p2p(ovpn, peer_id);
+
+	head = ovpn_get_hash_head(ovpn->peers->by_id, &peer_id,
+				  sizeof(peer_id));
+
+	rcu_read_lock();
+	hlist_for_each_entry_rcu(tmp, head, hash_entry_id) {
+		if (tmp->id != peer_id)
+			continue;
+
+		if (!ovpn_peer_hold(tmp))
+			continue;
+
+		peer = tmp;
+		break;
+	}
+	rcu_read_unlock();
 
 	return peer;
 }
@@ -330,6 +492,8 @@ struct ovpn_peer *ovpn_peer_get_by_dst(struct ovpn_priv *ovpn,
 				       struct sk_buff *skb)
 {
 	struct ovpn_peer *peer = NULL;
+	struct in6_addr addr6;
+	__be32 addr4;
 
 	/* in P2P mode, no matter the destination, packets are always sent to
 	 * the single peer listening on the other side
@@ -340,11 +504,109 @@ struct ovpn_peer *ovpn_peer_get_by_dst(struct ovpn_priv *ovpn,
 		if (unlikely(peer && !ovpn_peer_hold(peer)))
 			peer = NULL;
 		rcu_read_unlock();
+		return peer;
 	}
 
+	rcu_read_lock();
+	switch (skb->protocol) {
+	case htons(ETH_P_IP):
+		addr4 = ovpn_nexthop_from_skb4(skb);
+		peer = ovpn_peer_get_by_vpn_addr4(ovpn, addr4);
+		break;
+	case htons(ETH_P_IPV6):
+		addr6 = ovpn_nexthop_from_skb6(skb);
+		peer = ovpn_peer_get_by_vpn_addr6(ovpn, &addr6);
+		break;
+	}
+
+	if (unlikely(peer && !ovpn_peer_hold(peer)))
+		peer = NULL;
+	rcu_read_unlock();
+
 	return peer;
 }
 
+/**
+ * ovpn_nexthop_from_rt4 - look up the IPv4 nexthop for the given destination
+ * @ovpn: the private data representing the current VPN session
+ * @dest: the destination to be looked up
+ *
+ * Looks up in the IPv4 system routing table the IP of the nexthop to be used
+ * to reach the destination passed as argument. If no nexthop can be found, the
+ * destination itself is returned as it probably has to be used as nexthop.
+ *
+ * Return: the IP of the next hop if found or dest itself otherwise
+ */
+static __be32 ovpn_nexthop_from_rt4(struct ovpn_priv *ovpn, __be32 dest)
+{
+	struct rtable *rt;
+	struct flowi4 fl = {
+		.daddr = dest
+	};
+
+	rt = ip_route_output_flow(dev_net(ovpn->dev), &fl, NULL);
+	if (IS_ERR(rt)) {
+		net_dbg_ratelimited("%s: no route to host %pI4\n",
+				    netdev_name(ovpn->dev), &dest);
+		/* if we end up here this packet is probably going to be
+		 * thrown away later
+		 */
+		return dest;
+	}
+
+	if (!rt->rt_uses_gateway)
+		goto out;
+
+	dest = rt->rt_gw4;
+out:
+	ip_rt_put(rt);
+	return dest;
+}
+
+/**
+ * ovpn_nexthop_from_rt6 - look up the IPv6 nexthop for the given destination
+ * @ovpn: the private data representing the current VPN session
+ * @dest: the destination to be looked up
+ *
+ * Looks up in the IPv6 system routing table the IP of the nexthop to be used
+ * to reach the destination passed as argument. If no nexthop can be found, the
+ * destination itself is returned as it probably has to be used as nexthop.
+ *
+ * Return: the IP of the next hop if found or dest itself otherwise
+ */
+static struct in6_addr ovpn_nexthop_from_rt6(struct ovpn_priv *ovpn,
+					     struct in6_addr dest)
+{
+#if IS_ENABLED(CONFIG_IPV6)
+	struct dst_entry *entry;
+	struct rt6_info *rt;
+	struct flowi6 fl = {
+		.daddr = dest,
+	};
+
+	entry = ipv6_stub->ipv6_dst_lookup_flow(dev_net(ovpn->dev), NULL, &fl,
+						NULL);
+	if (IS_ERR(entry)) {
+		net_dbg_ratelimited("%s: no route to host %pI6c\n",
+				    netdev_name(ovpn->dev), &dest);
+		/* if we end up here this packet is probably going to be
+		 * thrown away later
+		 */
+		return dest;
+	}
+
+	rt = dst_rt6_info(entry);
+
+	if (!(rt->rt6i_flags & RTF_GATEWAY))
+		goto out;
+
+	dest = rt->rt6i_gateway;
+out:
+	dst_release((struct dst_entry *)rt);
+#endif
+	return dest;
+}
+
 /**
  * ovpn_peer_check_by_src - check that skb source is routed via peer
  * @ovpn: the openvpn instance to search
@@ -357,21 +619,40 @@ bool ovpn_peer_check_by_src(struct ovpn_priv *ovpn, struct sk_buff *skb,
 			    struct ovpn_peer *peer)
 {
 	bool match = false;
+	struct in6_addr addr6;
+	__be32 addr4;
 
 	if (ovpn->mode == OVPN_MODE_P2P) {
 		/* in P2P mode, no matter the destination, packets are always
 		 * sent to the single peer listening on the other side
 		 */
-		match = (peer == rcu_access_pointer(ovpn->peer));
+		return peer == rcu_access_pointer(ovpn->peer);
+	}
+
+	/* This function performs a reverse path check, therefore we now
+	 * lookup the nexthop we would use if we wanted to route a packet
+	 * to the source IP. If the nexthop matches the sender we know the
+	 * latter is valid and we allow the packet to come in
+	 */
+
+	switch (skb->protocol) {
+	case htons(ETH_P_IP):
+		addr4 = ovpn_nexthop_from_rt4(ovpn, ip_hdr(skb)->saddr);
+		rcu_read_lock();
+		match = (peer == ovpn_peer_get_by_vpn_addr4(ovpn, addr4));
+		rcu_read_unlock();
+		break;
+	case htons(ETH_P_IPV6):
+		addr6 = ovpn_nexthop_from_rt6(ovpn, ipv6_hdr(skb)->saddr);
+		rcu_read_lock();
+		match = (peer == ovpn_peer_get_by_vpn_addr6(ovpn, &addr6));
+		rcu_read_unlock();
+		break;
 	}
 
 	return match;
 }
 
-#define ovpn_get_hash_head(_tbl, _key, _key_len) ({		\
-	typeof(_tbl) *__tbl = &(_tbl);				\
-	(&(*__tbl)[jhash(_key, _key_len, 0) % HASH_SIZE(*__tbl)]); }) \
-
 /**
  * ovpn_peer_add_mp - add peer to related tables in a MP instance
  * @ovpn: the instance to add the peer to