[PATCH net-next v9 20/23] ovpn: kill key and notify userspace in case of IV exhaustion

[Antonio Quartulli]

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@xxxxxxxxxxx>
---
 drivers/net/ovpn/crypto.c  | 19 ++++++++++++++++
 drivers/net/ovpn/crypto.h  |  2 ++
 drivers/net/ovpn/io.c      | 18 ++++++++++++++-
 drivers/net/ovpn/netlink.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++
 drivers/net/ovpn/netlink.h |  2 ++
 5 files changed, 95 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ovpn/crypto.c b/drivers/net/ovpn/crypto.c
index f1f7510e2f735e367f96eb4982ba82c9af3c8bfc..56ae2f5e5c33e13933f26acd91c7786f7c02011a 100644
--- a/drivers/net/ovpn/crypto.c
+++ b/drivers/net/ovpn/crypto.c
@@ -55,6 +55,25 @@ 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 3b437d26b531c3034cca5343c755ef9c7ef57276..53a965ec0fad3402317c1ac580aa6faad579a09b 100644
--- a/drivers/net/ovpn/crypto.h
+++ b/drivers/net/ovpn/crypto.h
@@ -136,4 +136,6 @@ void ovpn_crypto_state_release(struct ovpn_crypto_state *cs);
 
 void ovpn_crypto_key_slots_swap(struct ovpn_crypto_state *cs);
 
+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 c857ac9015b75530cf3ae40f8db0d4db4c08e2fa..dc6479d5f3008c4e851bee8d4061e4e85bed40f2 100644
--- a/drivers/net/ovpn/io.c
+++ b/drivers/net/ovpn/io.c
@@ -225,6 +225,7 @@ void ovpn_recv(struct ovpn_peer *peer, struct sk_buff *skb)
 
 void ovpn_encrypt_post(void *data, int ret)
 {
+	struct ovpn_crypto_key_slot *ks = NULL;
 	struct ovpn_peer *peer = NULL;
 	struct sk_buff *skb = data;
 	unsigned int orig_len = 0;
@@ -237,15 +238,28 @@ void ovpn_encrypt_post(void *data, int ret)
 
 	/* crypto is done, cleanup skb CB and its members */
 	if (likely(ovpn_skb_cb(skb)->ctx)) {
+		ks = ovpn_skb_cb(skb)->ctx->ks;
 		peer = ovpn_skb_cb(skb)->ctx->peer;
 		orig_len = ovpn_skb_cb(skb)->ctx->orig_len;
 
-		ovpn_crypto_key_slot_put(ovpn_skb_cb(skb)->ctx->ks);
 		aead_request_free(ovpn_skb_cb(skb)->ctx->req);
 		kfree(ovpn_skb_cb(skb)->ctx);
 		ovpn_skb_cb(skb)->ctx = NULL;
 	}
 
+	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;
 
@@ -275,6 +289,8 @@ void ovpn_encrypt_post(void *data, int ret)
 		dev_core_stats_tx_dropped_inc(peer->ovpn->dev);
 	if (likely(peer))
 		ovpn_peer_put(peer);
+	if (likely(ks))
+		ovpn_crypto_key_slot_put(ks);
 	kfree_skb(skb);
 }
 
diff --git a/drivers/net/ovpn/netlink.c b/drivers/net/ovpn/netlink.c
index bb96645205b6704f7066b7de9a003e01e63c10f7..09c7df2b1c76c812a3bece6e63c9fac13c96c327 100644
--- a/drivers/net/ovpn/netlink.c
+++ b/drivers/net/ovpn/netlink.c
@@ -903,6 +903,61 @@ 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(100, 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, dev_net(peer->ovpn->dev), msg,
+				0, OVPN_NLGRP_PEERS, GFP_ATOMIC);
+
+	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