[PATCH net-next v9 14/23] ovpn: implement peer lookup logic

Antonio Quartulli
2024-10-16T01:04:09Z

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>
---
drivers/net/ovpn/peer.c | 272 ++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 264 insertions(+), 8 deletions(-)

diff --git a/drivers/net/ovpn/peer.c b/drivers/net/ovpn/peer.c
index 73ef509faab9701192a45ffe78a46dbbbeab01c2..c7dc9032c2b55fd42befc1f3e7a0eca893a96576 100644
--- a/drivers/net/ovpn/peer.c
+++ b/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 "ovpnstruct.h"
#include "bind.h"
@@ -125,6 +126,94 @@ static bool ovpn_peer_skb_to_sockaddr(struct sk_buff *skb,
return true;
}

+/**
+ * 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;
+}
+
+#define ovpn_get_hash_head(_tbl, _key, _key_len) ({ \
+ typeof(_tbl) *__tbl = &(_tbl); \
+ (&(*__tbl)[jhash(_key, _key_len, 0) % HASH_SIZE(*__tbl)]); }) \
+
+/**
+ * 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_struct *ovpn,
+ __be32 addr)
+{
+ struct hlist_nulls_head *nhead;
+ struct hlist_nulls_node *ntmp;
+ struct ovpn_peer *tmp;
+
+ nhead = ovpn_get_hash_head(ovpn->peers->by_vpn_addr, &addr,
+ sizeof(addr));
+
+ hlist_nulls_for_each_entry_rcu(tmp, ntmp, nhead, hash_entry_addr4)
+ if (addr == tmp->vpn_addrs.ipv4.s_addr)
+ return tmp;
+
+ 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_struct *ovpn,
+ struct in6_addr *addr)
+{
+ struct hlist_nulls_head *nhead;
+ struct hlist_nulls_node *ntmp;
+ struct ovpn_peer *tmp;
+
+ nhead = ovpn_get_hash_head(ovpn->peers->by_vpn_addr, addr,
+ sizeof(*addr));
+
+ hlist_nulls_for_each_entry_rcu(tmp, ntmp, nhead, hash_entry_addr6)
+ if (ipv6_addr_equal(addr, &tmp->vpn_addrs.ipv6))
+ return tmp;
+
+ return NULL;
+}
+
/**
* ovpn_peer_transp_match - check if sockaddr and peer binding match
* @peer: the peer to get the binding from
@@ -202,14 +291,44 @@ ovpn_peer_get_by_transp_addr_p2p(struct ovpn_struct *ovpn,
struct ovpn_peer *ovpn_peer_get_by_transp_addr(struct ovpn_struct *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;
+ size_t sa_len;

if (unlikely(!ovpn_peer_skb_to_sockaddr(skb, &ss)))
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);
+
+ switch (ss.ss_family) {
+ case AF_INET:
+ sa_len = sizeof(struct sockaddr_in);
+ break;
+ case AF_INET6:
+ sa_len = sizeof(struct sockaddr_in6);
+ break;
+ default:
+ return NULL;
+ }
+
+ nhead = ovpn_get_hash_head(ovpn->peers->by_transp_addr, &ss, sa_len);
+
+ rcu_read_lock();
+ 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;
+ }
+ rcu_read_unlock();

return peer;
}
@@ -244,10 +363,27 @@ static struct ovpn_peer *ovpn_peer_get_by_id_p2p(struct ovpn_struct *ovpn,
*/
struct ovpn_peer *ovpn_peer_get_by_id(struct ovpn_struct *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;
}
@@ -269,6 +405,8 @@ struct ovpn_peer *ovpn_peer_get_by_dst(struct ovpn_struct *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
@@ -279,11 +417,109 @@ struct ovpn_peer *ovpn_peer_get_by_dst(struct ovpn_struct *ovpn,
if (unlikely(peer && !ovpn_peer_hold(peer)))
peer = NULL;
rcu_read_unlock();
+ return peer;
}

+ rcu_read_lock();
+ switch (skb_protocol_to_family(skb)) {
+ case AF_INET:
+ addr4 = ovpn_nexthop_from_skb4(skb);
+ peer = ovpn_peer_get_by_vpn_addr4(ovpn, addr4);
+ break;
+ case AF_INET6:
+ 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_struct *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", __func__,
+ &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_struct *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", __func__,
+ &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
@@ -296,6 +532,8 @@ bool ovpn_peer_check_by_src(struct ovpn_struct *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
@@ -304,15 +542,33 @@ bool ovpn_peer_check_by_src(struct ovpn_struct *ovpn, struct sk_buff *skb,
rcu_read_lock();
match = (peer == rcu_dereference(ovpn->peer));
rcu_read_unlock();
+ return match;
+ }
+
+ /* 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_to_family(skb)) {
+ case AF_INET:
+ 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 AF_INET6:
+ 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

--
2.45.2