[tor-bugs] #32040 [Core Tor/Tor]: HS intro padding machine reactivates after receiving INTRODUCE_ACK

Tor Bug Tracker & Wiki blackhole at torproject.org
Fri Oct 11 10:49:52 UTC 2019


#32040: HS intro padding machine reactivates after receiving INTRODUCE_ACK
------------------------------+--------------------------------
     Reporter:  asn           |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.4.3.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  wtf-pad padding
Actual Points:                |  Parent ID:
       Points:  1             |   Reviewer:
      Sponsor:  Sponsor2      |
------------------------------+--------------------------------
 While reseaching wtf-pad I noticed that the intro machines don't work
 together and they display a weird shutdown/reactivation pattern.

 In particular, what happens is that after INTRODUCE1 is sent, the machine
 starts up and sends all its padding, and then shuts down. Then when
 INTRODUCE_ACK arrives, it reactivates (probably because INTRODUCE_ACK is
 part of the accepted purposes and the machine is shutdown/freed at that
 time) and sends again padding, then again shuts down...

 This does not work as intended and causes extra cells to fly in with a
 pattern that probably does not help them blend in.

 Here are some Tor logs (with padanalysis incoming/outgoing cells logs):
 {{{
 Oct 11 13:25:54.000 [warn] outgoing-cell: EXTEND2 0
 Oct 11 13:25:54.000 [warn] incoming-cell: EXTENDED2 0 66
 Oct 11 13:25:54.000 [warn] outgoing-cell: EXTEND2 0
 Oct 11 13:25:54.000 [warn] incoming-cell: EXTENDED2 0 66
 Oct 11 13:25:57.000 [warn] outgoing-cell: EXTEND 0
 Oct 11 13:25:58.000 [warn] incoming-cell: EXTENDED 0 148
 Oct 11 13:25:58.000 [warn] outgoing-cell: INTRODUCE1 4
 Oct 11 13:25:57.000 [info] circpad_negotiate_padding(): Negotiating
 padding on circuit 8 (10), command 2
 Oct 11 13:25:58.000 [warn] outgoing-cell: PADDING_NEGOTIATE 4
 Oct 11 13:25:57.000 [info] circpad_machine_spec_transition(): Circuit 8
 circpad machine 0 transitioning from 0 to 1
 Oct 11 13:25:58.000 [warn] incoming-cell: PADDING_NEGOTIATED 4 4
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: PADDING_NEGOTIATED 4 4
 Oct 11 13:25:58.000 [info] circpad_handle_padding_negotiated(): Received
 STOP command on PADDING_NEGOTIATED for circuit 5
 Oct 11 13:25:58.000 [info] circpad_circuit_machineinfo_free_idx(): Freeing
 padding info idx 0 on circuit 5 (7)
 Oct 11 13:25:58.000 [warn] incoming-cell: INTRODUCE_ACK 4 0
 Oct 11 13:25:58.000 [info] rend_client_introduction_acked(): Received ack.
 Telling rend circ...
 Oct 11 13:25:58.000 [info] circpad_setup_machine_on_circ(): Registering
 machine client_ip_circ to origin circ 5 (8)
 Oct 11 13:25:58.000 [info] circpad_node_supports_padding(): Checking
 padding: supported
 Oct 11 13:25:58.000 [info] circpad_negotiate_padding(): Negotiating
 padding on circuit 5 (8), command 2
 Oct 11 13:25:58.000 [warn] outgoing-cell:  8 PADDING_NEGOTIATE 4
 Oct 11 13:25:58.000 [info] circpad_machine_spec_transition(): Circuit 5
 circpad machine 0 transitioning from 0 to 1
 Oct 11 13:25:58.000 [info] circpad_marked_circuit_for_padding(): Circuit 5
 is not marked for close because of a pending padding machine in index 0.
 Oct 11 13:25:58.000 [warn] incoming-cell: PADDING_NEGOTIATED 4 4
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: DROP 4 0
 Oct 11 13:25:58.000 [warn] incoming-cell: PADDING_NEGOTIATED 4 4
 Oct 11 13:25:58.000 [info] circpad_handle_padding_negotiated(): Received
 STOP command on PADDING_NEGOTIATED for circuit 5
 Oct 11 13:25:58.000 [info] circpad_circuit_machineinfo_free_idx(): Freeing
 padding info idx 0 on circuit 5 (15)
 ...
 Oct 11 13:36:11.000 [info] circuit_mark_for_close_(): Circuit 4130340667
 (id: 5) marked for close at src/core/or/circuituse.c:1509 (orig reason: 9,
 new reason: 0)
 }}}

 I'm not sure what the fix should be here. It might be that we need to
 remove INTRODUCE_ACK from the list of accepted purposes, but if we do that
 then if INTRODUCE_ACK arrives earlier we will stop padding after. Hmm.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/32040>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online


More information about the tor-bugs mailing list