if_ovpn: use epoch to free peers

Avoid a possible use-after-free in the rx path.
ovpn_decrypt_rx_cb() calls ovpn_finish_rx() which releases the lock,
but continues to use the peer.
Ensure that the peer cannot be freed until we're sure all potential
users have stopped using it (i.e. have left net_epoch).

Reported by:	Kevin Day <kevin@your.org>
MFC after:	1 week
Sponsored by:	Rubicon Communications, LLC ("Netgate")

sys/net/if_ovpn.c

@@ -161,6 +161,7 @@ struct ovpn_kpeer {
 	struct callout		 ping_rcv;
 
 	counter_u64_t		 counters[OVPN_PEER_COUNTER_SIZE];
+	struct epoch_context	 epoch_ctx;
 };
 
 struct ovpn_counters {
@@ -568,6 +569,15 @@ ovpn_notify_float(struct ovpn_softc *sc, uint32_t peerid,
 	return (0);
 }
 
+static void
+_ovpn_free_peer(struct epoch_context *ctx) {
+	struct ovpn_kpeer *peer = __containerof(ctx, struct ovpn_kpeer,
+	    epoch_ctx);
+
+	uma_zfree_pcpu(pcpu_zone_4, peer->last_active);
+	free(peer, M_OVPN);
+}
+
 static void
 ovpn_peer_release_ref(struct ovpn_kpeer *peer, bool locked)
 {
@@ -606,8 +616,8 @@ ovpn_peer_release_ref(struct ovpn_kpeer *peer, bool locked)
 
 	callout_stop(&peer->ping_send);
 	callout_stop(&peer->ping_rcv);
-	uma_zfree_pcpu(pcpu_zone_4, peer->last_active);
-	free(peer, M_OVPN);
+
+	NET_EPOCH_CALL(_ovpn_free_peer, &peer->epoch_ctx);
 
 	if (! locked)
 		OVPN_WUNLOCK(sc);