commit 632688c797bb3968946343cafaaf51c882c59aed Author: Roger Dingledine arma@torproject.org Date: Wed Jan 27 23:48:57 2021 -0500

exit: Deny re-entry into the network

Exit relays now reject exit attempts to known relay addresses + ORPort and also to authorities on the ORPort and DirPort.

Closes #2667

Signed-off-by: David Goulet dgoulet@torproject.org --- changes/ticket2667 | 4 ++++ src/core/or/connection_edge.c | 24 ++++++++++++++++++++++++ 2 files changed, 28 insertions(+)

diff --git a/changes/ticket2667 b/changes/ticket2667 new file mode 100644 index 0000000000..cc42286ef9 --- /dev/null +++ b/changes/ticket2667 @@ -0,0 +1,4 @@ + o Major feature (exit): + - Re-entry into the network is now denied at the Exit level to all relays' + ORPort and authorities' ORPort+DirPort. This is to help mitigate a series + of attacks. See ticket for more information. Closes ticket 2667. diff --git a/src/core/or/connection_edge.c b/src/core/or/connection_edge.c index 859ad1c6fc..55e7841cc3 100644 --- a/src/core/or/connection_edge.c +++ b/src/core/or/connection_edge.c @@ -4263,6 +4263,30 @@ connection_exit_connect(edge_connection_t *edge_conn) return; }

+ /* Next, check for attempts to connect back into the Tor network. We don't + * want to allow these for the same reason we don't want to allow + * infinite-length circuits (see "A Practical Congestion Attack on Tor Using + * Long Paths", Usenix Security 2009). See also ticket 2667. + * + * The TORPROTOCOL reason is used instead of EXITPOLICY so client do NOT + * attempt to retry connecting onto another circuit that will also fail + * bringing considerable more load on the network if so. + * + * Since the address+port set here is a bloomfilter, in very rare cases, the + * check will create a false positive meaning that the destination could + * actually be legit and thus being denied exit. However, sending back a + * reason that makes the client retry results in much worst consequences in + * case of an attack so this is a small price to pay. */ + if (!connection_edge_is_rendezvous_stream(edge_conn) && + nodelist_reentry_probably_contains(&conn->addr, conn->port)) { + log_info(LD_EXIT, "%s tried to connect back to a known relay address. " + "Closing.", connection_describe(conn)); + connection_edge_end(edge_conn, END_STREAM_REASON_TORPROTOCOL); + circuit_detach_stream(circuit_get_by_edge_conn(edge_conn), edge_conn); + connection_free(conn); + return; + } + #ifdef HAVE_SYS_UN_H if (conn->socket_family != AF_UNIX) { #else