[tor-bugs] #9819 [Tor]: Circuits through entry guards aren't distinguished on whether they originated locally

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#9819: Circuits through entry guards aren't distinguished on whether they
originated locally
--------------------+------------------------------------
 Reporter:  andrea  |          Owner:
     Type:  defect  |         Status:  new
 Priority:  normal  |      Milestone:  Tor: 0.2.5.x-final
Component:  Tor     |        Version:  Tor: 0.2.4.17-rc
 Keywords:          |  Actual Points:
Parent ID:          |         Points:
--------------------+------------------------------------
 When a node is both a relay and a client, and another node extends a
 circuit through it to one of its entry guards, that circuit isn't
 distinguished from a circuit which originated locally.  If
 entry_guard_register_connect_status() decides we should retry a different
 entry guard, the circuit will be killed in channel_do_open_actions().
 This potentially could leak information about entry guards in
 circumstances which appear to be hard to exploit.

 IRC log:

 {{{
 07:42 < athena> skruffy: what makes you believe 'channel_do_open_actions()
 can
                 leak of used guards if relay used as client' ?
 07:43 < skruffy> if entry_guard_register_connect_status() failed it will
 close circuits
 07:49 < athena> sooo - in other words, you think if an attacker can build
 a
                 circuit through a node N which is both a relay and a
 client to an
                 another node E, and then arrange for
                 entry_guard_register_connect_status() to fail, the
 attacker can
                 test a hypothesis about whether E is an entry guard for N?
 07:52 < skruffy> something like that
 07:53 < athena> okay; how would the attacker do that?
 07:54 < athena> you may have something there; it seems fishy that killing
 pending
                 circuits on an entry guard we don't want to use doesn't
                 distinguish between locally originating circuits and
 circuits from
                 another relay - but i think if so it probably was there
 pre-
                 channels and i'm not convinced it's exploitable yet
 07:59 < skruffy> yes it's pre-chans.
 08:02 < athena> hmm, it looks like the only circumstance in which
                 entry_guard_register_connect_status() can fail is if this
 is a
                 first connection to a new entry guard *and* an old entry
 guard
                 which was offline has just come back
 08:03 < athena> it doesn't seem that practical to exploit - for an
 attacker to try
                 to selectively manipulate connectivity to the old entry
 guards to
                 induce those conditions requires already knowing them
 08:04 < nickm> It couldn't hurt to treat this as a bug that we should fix
 sooner
                 or later, though.
 08:04 < athena> i think agree conceptually that locally originating
 circuits and
                 relay circuits should be distinguished and failing
                 entry_guard_register_connect_status() should only kill the
 local
                 ones, though
 }}}

 Proposed fix:

 Modify circuit_n_chan_done() to take another possible parameter that
 notifies or_circuits of success but tells all other pending circuits to
 give up, and pass this from channel_do_open_actions() in case of
 entry_guard_register_connect_status() failing.

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/9819>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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