[tor-bugs] #23603 [Core Tor/Tor]: hs: Cleanup race between circuit close and free with the HS circuitmap

Tor Bug Tracker & Wiki blackhole at torproject.org
Mon Sep 25 17:39:22 UTC 2017

#23603: hs: Cleanup race between circuit close and free with the HS circuitmap
 Reporter:  dgoulet       |          Owner:  (none)
     Type:  defect        |         Status:  new
 Priority:  High          |      Milestone:  Tor: 0.3.2.x-final
Component:  Core Tor/Tor  |        Version:
 Severity:  Normal        |     Resolution:
 Keywords:  tor-hs        |  Actual Points:
Parent ID:                |         Points:
 Reviewer:                |        Sponsor:

Comment (by dgoulet):

 Ok, those latest logs were enlightening and 99.9% certainty now :)

 The offending function is `hs_service_intro_circ_has_closed()` called in

 Same scenario has above but just simpler for the sake of the example:

 1. mark for close intro circuits
 2. launch new ones to same intro points.
 3. `about_to_free()` the circuits in step (1).
  --> This calls `has_closed()` which removes the intro point object if it
 has been retried too many times.
 4. New circuit opens, boom no intro point object and we get the warning of
 `Unknown auth key...`.

 From asn's log, which suspend/wakeup many times his tor, we happen to try
 intro points many times (successfully) and in their 18h-24h lifetime, it
 was done more than 3 times which triggered this condition in the has
 closed function:

   if (ip->circuit_retries > MAX_INTRO_POINT_CIRCUIT_RETRIES) {
     remember_failing_intro_point(ip, desc, approx_time());
     service_intro_point_remove(service, ip);

 This can only happen if the service launch the new circuits *before* the
 free() of the previous circuits happens *and* the circuit retry counter
 has reached > 3.

 Couple of possible fixes here. First, we should NOT clean anything in that
 function. We should ONLY do that in `cleanup_intro_points()` which would
 have cleanup everything properly (circuit included). For this, we need to
 move the "remember failing intro point" logic in there in case we've tried
 too many times.

 Second, this `circuit_retries` counter should probably not be incremented
 when we just go out of hibernation and `tor` is assuming at that point
 that all circuits are not working. We either do a callback from
 hibernation into the HS subsystem to cleanup those counters or another
 possible option would be to reset to 0 the counter on a successful intro
 point establishment.

Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23603#comment:6>
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