commit ceedcfe9f249746e92d5a4fe9f0589b6691b4d84 Author: Taylor Yu catalyst@torproject.org Date: Tue Mar 28 17:35:11 2017 -0400

Refactor and comment new_route_len()

Add a new helper function route_len_for_purpose(), which explicitly lists all of the known circuit purposes for a circuit with a chosen exit node (unlike previously, where the default route length for a chosen exit was DEFAULT_ROUTE_LEN + 1 except for two purposes). Add a non-fatal assertion for unhandled purposes that conservatively returns DEFAULT_ROUTE_LEN + 1.

Add copious comments documenting which circuits need an extra hop and why.

Thanks to nickm and dgoulet for providing background information. --- src/or/circuitbuild.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 88 insertions(+), 7 deletions(-)

diff --git a/src/or/circuitbuild.c b/src/or/circuitbuild.c index 5277e10..f8b3609 100644 --- a/src/or/circuitbuild.c +++ b/src/or/circuitbuild.c @@ -1545,8 +1545,93 @@ onionskin_answer(or_circuit_t *circ, return 0; }

-/** Choose a length for a circuit of purpose <b>purpose</b>: three + the - * number of endpoints that would give something away about our destination. +/** Helper for new_route_len(). Choose a circuit length for purpose + * <b>purpose</b>: DEFAULT_ROUTE_LEN (+ 1 if someone else chose the + * exit). If someone else chose the exit, they could be colluding + * with the exit, so add a randomly selected node to preserve + * anonymity. + * + * Here, "exit node" sometimes means an OR acting as an internal + * endpoint, rather than as a relay to an external endpoint. This + * means there need to be at least DEFAULT_ROUTE_LEN routers between + * us and the internal endpoint to preserve the same anonymity + * properties that we would get when connecting to an external + * endpoint. These internal endpoints can include: + * + * - Connections to a directory of hidden services + * (CIRCUIT_PURPOSE_C_GENERAL) + * + * - A client connecting to an introduction point, which the hidden + * service picked (CIRCUIT_PURPOSE_C_INTRODUCING, via + * circuit_get_open_circ_or_launch() which rewrites it from + * CIRCUIT_PURPOSE_C_INTRODUCE_ACK_WAIT) + * + * - A hidden service connecting to a rendezvous point, which the + * client picked (CIRCUIT_PURPOSE_S_CONNECT_REND, via + * rend_service_receive_introduction() and + * rend_service_relaunch_rendezvous) + * + * There are currently two situations where we picked the exit node + * ourselves, making DEFAULT_ROUTE_LEN a safe circuit length: + * + * - We are a hidden service connecting to an introduction point + * (CIRCUIT_PURPOSE_S_ESTABLISH_INTRO, via + * rend_service_launch_establish_intro()) + * + * - We are a router testing its own reachabiity + * (CIRCUIT_PURPOSE_TESTING, via consider_testing_reachability()) + * + * onion_pick_cpath_exit() bypasses us (by not calling + * new_route_len()) in the one-hop tunnel case, so we don't need to + * handle that. + */ +static int +route_len_for_purpose(uint8_t purpose, extend_info_t *exit_ei) +{ + int routelen = DEFAULT_ROUTE_LEN; + int known_purpose = 0; + + if (!exit_ei) + return routelen; + + switch (purpose) { + /* These two purposes connect to a router that we chose, so + * DEFAULT_ROUTE_LEN is safe. */ + case CIRCUIT_PURPOSE_S_ESTABLISH_INTRO: + /* hidden service connecting to introduction point */ + case CIRCUIT_PURPOSE_TESTING: + /* router reachability testing */ + known_purpose = 1; + break; + + /* These three purposes connect to a router that someone else + * might have chosen, so add an extra hop to protect anonymity. */ + case CIRCUIT_PURPOSE_C_GENERAL: + /* connecting to hidden service directory */ + case CIRCUIT_PURPOSE_C_INTRODUCING: + /* client connecting to introduction point */ + case CIRCUIT_PURPOSE_S_CONNECT_REND: + /* hidden service connecting to rendezvous point */ + known_purpose = 1; + routelen++; + break; + + default: + /* Got a purpose not listed above along with a chosen exit. + * Increase the circuit length by one anyway for safety. */ + routelen++; + break; + } + + if (BUG(exit_ei && !known_purpose)) { + log_warn(LD_BUG, "Unhandled purpose %d with a chosen exit; " + "assuming routelen %d.", purpose, routelen); + } + return routelen; +} + +/** Choose a length for a circuit of purpose <b>purpose</b> and check + * if enough routers are available. * * If the routerlist <b>nodes</b> doesn't have enough routers * to handle the desired path length, return -1. @@ -1559,11 +1644,7 @@ new_route_len(uint8_t purpose, extend_info_t *exit_ei, smartlist_t *nodes)

tor_assert(nodes);

- routelen = DEFAULT_ROUTE_LEN; - if (exit_ei && - purpose != CIRCUIT_PURPOSE_TESTING && - purpose != CIRCUIT_PURPOSE_S_ESTABLISH_INTRO) - routelen++; + routelen = route_len_for_purpose(purpose, exit_ei);

num_acceptable_routers = count_acceptable_nodes(nodes);