[PATCH net-next v18 22/25] ovpn: kill key and notify userspace in case of IV exhaustion

Antonio Quartulli
2025-01-13T09:31:34Z

IV wrap-around is cryptographically dangerous for a number of ciphers,
therefore kill the key and inform userspace (via netlink) should the
IV space go exhausted.

Userspace has two ways of deciding when the key has to be renewed before
exhausting the IV space:
1) time based approach:
after X seconds/minutes userspace generates a new key and sends it
to the kernel. This is based on guestimate and normally default
timer value works well.

2) packet count based approach:
after X packets/bytes userspace generates a new key and sends it to
the kernel. Userspace keeps track of the amount of traffic by
periodically polling GET_PEER and fetching the VPN/LINK stats.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
---
drivers/net/ovpn/crypto.c | 19 ++++++++++++++++
drivers/net/ovpn/crypto.h | 2 ++
drivers/net/ovpn/io.c | 13 +++++++++++
drivers/net/ovpn/netlink.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++
drivers/net/ovpn/netlink.h | 2 ++
5 files changed, 91 insertions(+)

diff --git a/drivers/net/ovpn/crypto.c b/drivers/net/ovpn/crypto.c
index 6fccd73c6cf7d2566d1b819cb6d5d7b2ea98e81d..47a627822e95e3a1079a710c66037ec74173e653 100644
--- a/drivers/net/ovpn/crypto.c
+++ b/drivers/net/ovpn/crypto.c
@@ -54,6 +54,25 @@ void ovpn_crypto_state_release(struct ovpn_crypto_state *cs)
}
}

+/* removes the key matching the specified id from the crypto context */
+void ovpn_crypto_kill_key(struct ovpn_crypto_state *cs, u8 key_id)
+{
+ struct ovpn_crypto_key_slot *ks = NULL;
+
+ spin_lock_bh(&cs->lock);
+ if (rcu_access_pointer(cs->slots[0])->key_id == key_id) {
+ ks = rcu_replace_pointer(cs->slots[0], NULL,
+ lockdep_is_held(&cs->lock));
+ } else if (rcu_access_pointer(cs->slots[1])->key_id == key_id) {
+ ks = rcu_replace_pointer(cs->slots[1], NULL,
+ lockdep_is_held(&cs->lock));
+ }
+ spin_unlock_bh(&cs->lock);
+
+ if (ks)
+ ovpn_crypto_key_slot_put(ks);
+}
+
/* Reset the ovpn_crypto_state object in a way that is atomic
* to RCU readers.
*/
diff --git a/drivers/net/ovpn/crypto.h b/drivers/net/ovpn/crypto.h
index 87addc7bf07c02c3c23da7e6d1f86249d1d867c6..30df70525bbf106a70da758532f1bd6ef1a02369 100644
--- a/drivers/net/ovpn/crypto.h
+++ b/drivers/net/ovpn/crypto.h
@@ -140,4 +140,6 @@ int ovpn_crypto_config_get(struct ovpn_crypto_state *cs,
enum ovpn_key_slot slot,
struct ovpn_key_config *keyconf);

+void ovpn_crypto_kill_key(struct ovpn_crypto_state *cs, u8 key_id);
+
#endif /* _NET_OVPN_OVPNCRYPTO_H_ */
diff --git a/drivers/net/ovpn/io.c b/drivers/net/ovpn/io.c
index 8162b12f7a36b897d685c70f1befd87d774826a1..8b9b98f9e962f17464e9d436b23c130d9ca0d764 100644
--- a/drivers/net/ovpn/io.c
+++ b/drivers/net/ovpn/io.c
@@ -244,6 +244,19 @@ void ovpn_encrypt_post(void *data, int ret)
if (likely(ovpn_skb_cb(skb)->req))
aead_request_free(ovpn_skb_cb(skb)->req);

+ if (unlikely(ret == -ERANGE)) {
+ /* we ran out of IVs and we must kill the key as it can't be
+ * use anymore
+ */
+ netdev_warn(peer->ovpn->dev,
+ "killing key %u for peer %u\n", ks->key_id,
+ peer->id);
+ ovpn_crypto_kill_key(&peer->crypto, ks->key_id);
+ /* let userspace know so that a new key must be negotiated */
+ ovpn_nl_key_swap_notify(peer, ks->key_id);
+ goto err;
+ }
+
if (unlikely(ret < 0))
goto err;

diff --git a/drivers/net/ovpn/netlink.c b/drivers/net/ovpn/netlink.c
index 4d46eb6d6f14bba8c3a8335d08922bb8f6d3149f..6f82fa0b42077ae87b72b3ffe2ed99a1e72dbb3c 100644
--- a/drivers/net/ovpn/netlink.c
+++ b/drivers/net/ovpn/netlink.c
@@ -1047,6 +1047,61 @@ int ovpn_nl_key_del_doit(struct sk_buff *skb, struct genl_info *info)
return 0;
}

+/**
+ * ovpn_nl_key_swap_notify - notify userspace peer's key must be renewed
+ * @peer: the peer whose key needs to be renewed
+ * @key_id: the ID of the key that needs to be renewed
+ *
+ * Return: 0 on success or a negative error code otherwise
+ */
+int ovpn_nl_key_swap_notify(struct ovpn_peer *peer, u8 key_id)
+{
+ struct nlattr *k_attr;
+ struct sk_buff *msg;
+ int ret = -EMSGSIZE;
+ void *hdr;
+
+ netdev_info(peer->ovpn->dev, "peer with id %u must rekey - primary key unusable.\n",
+ peer->id);
+
+ msg = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_ATOMIC);
+ if (!msg)
+ return -ENOMEM;
+
+ hdr = genlmsg_put(msg, 0, 0, &ovpn_nl_family, 0, OVPN_CMD_KEY_SWAP_NTF);
+ if (!hdr) {
+ ret = -ENOBUFS;
+ goto err_free_msg;
+ }
+
+ if (nla_put_u32(msg, OVPN_A_IFINDEX, peer->ovpn->dev->ifindex))
+ goto err_cancel_msg;
+
+ k_attr = nla_nest_start(msg, OVPN_A_KEYCONF);
+ if (!k_attr)
+ goto err_cancel_msg;
+
+ if (nla_put_u32(msg, OVPN_A_KEYCONF_PEER_ID, peer->id))
+ goto err_cancel_msg;
+
+ if (nla_put_u16(msg, OVPN_A_KEYCONF_KEY_ID, key_id))
+ goto err_cancel_msg;
+
+ nla_nest_end(msg, k_attr);
+ genlmsg_end(msg, hdr);
+
+ genlmsg_multicast_netns(&ovpn_nl_family, sock_net(peer->sock->sock->sk),
+ msg, 0, OVPN_NLGRP_PEERS, GFP_KERNEL);
+
+ return 0;
+
+err_cancel_msg:
+ genlmsg_cancel(msg, hdr);
+err_free_msg:
+ nlmsg_free(msg);
+ return ret;
+}
+
/**
* ovpn_nl_register - perform any needed registration in the NL subsustem
*
diff --git a/drivers/net/ovpn/netlink.h b/drivers/net/ovpn/netlink.h
index 9e87cf11d1e9813b7a75ddf3705ab7d5fabe899f..33390b13c8904d40b629662005a9eb92ff617c3b 100644
--- a/drivers/net/ovpn/netlink.h
+++ b/drivers/net/ovpn/netlink.h
@@ -12,4 +12,6 @@
int ovpn_nl_register(void);
void ovpn_nl_unregister(void);

+int ovpn_nl_key_swap_notify(struct ovpn_peer *peer, u8 key_id);
+
#endif /* _NET_OVPN_NETLINK_H_ */

--
2.45.2