Fix: mitigate as much as we can HS port scanning

Make hidden service port scanning harder by sending back REASON_DONE which
does not disclose that it was in fact an exit policy issue. After that, kill
the circuit immediately to avoid more bad requests on it.

This means that everytime an hidden service exit policy does match, the user
(malicious or not) needs to build a new circuit.

Fixes #13667.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>

changes/bug13667

+ o Major bugfixes:
+   - Make HS port scanning more difficult by sending back REASON_DONE if the
+     exit policy didn't match. Furthermore, immediately close the circuit to
+     avoid other connection attempts on it from the possible attacker trying
+     multiple ports on that same circuit.

src/or/connection_edge.c

@@ -2579,12 +2579,23 @@ connection_exit_begin_conn(cell_t *cell, circuit_t *circ)
     if (rend_service_set_connection_addr_port(n_stream, origin_circ) < 0) {
       log_info(LD_REND,"Didn't find rendezvous service (port %d)",
                n_stream->base_.port);
+      /* Send back reason DONE because we want to make hidden service port
+       * scanning harder thus instead of returning that the exit policy
+       * didn't match, which makes it obvious that the port is closed,
+       * return DONE and kill the circuit. That way, a user (malicious or
+       * not) needs one circuit per bad port unless it matches the policy of
+       * the hidden service. */
       relay_send_end_cell_from_edge(rh.stream_id, circ,
-                                    END_STREAM_REASON_EXITPOLICY,
+                                    END_STREAM_REASON_DONE,
                                     origin_circ->cpath->prev);
       connection_free(TO_CONN(n_stream));
       tor_free(address);
-      return 0;
+
+      /* Drop the circuit here since it might be someone deliberately
+       * scanning the hidden service ports. Note that this mitigates port
+       * scanning by adding more work on the attacker side to successfully
+       * scan but does not fully solve it. */
+      return END_CIRC_AT_ORIGIN;
     }
     assert_circuit_ok(circ);
     log_debug(LD_REND,"Finished assigning addr/port");