Re: [tor-bugs] #29990 [Core Tor/Tor]: Handle zero monotonic time differences in the circuit padding code

["Tor Bug Tracker & Wiki" <blackhole@xxxxxxxxxxxxxx>]

#29990: Handle zero monotonic time differences in the circuit padding code
-------------------------------------------------+-------------------------
 Reporter:  teor                                 |          Owner:  (none)
     Type:  defect                               |         Status:  new
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.4.1.x-final
Component:  Core Tor/Tor                         |        Version:  Tor:
                                                 |  unspecified
 Severity:  Normal                               |     Resolution:
 Keywords:  wtf-pad, tor-relay, tor-cell,        |  Actual Points:
  padding, 041-must                              |
Parent ID:  #29500                               |         Points:  2
 Reviewer:                                       |        Sponsor:
                                                 |  Sponsor2
-------------------------------------------------+-------------------------

Comment (by teor):

 Replying to [comment:6 mikeperry]:
 > Aha, I think I found it! Our mocking forces us to call monotime_init()
 *before* we set the mocked time value. monotime_init() thus stores the
 first ratchet value at whatever the platform is at, and then we set fake
 mocked time to some later value.
 >
 > If monotime_init() gets a value from the host that is **greater** than
 what we choose to mock time at for our unittests, all subsequent
 monotime_abosolute() calls return zero.
 >
 > So, the right way to fix all of this mess is to either fix our mocking
 to allow us to call monotime_init() with a mocked time, or just always set
 our mocked time to some time after whatever monotime_init() stored.

 Oh wow. That's a tricky bug.

 > I am going to attempt the later, to see if it fixes whatever hackintosh
 VM is involved here (I don't have one of those).

 It's just a standard mac, running macOS as host, and macOS inside some
 virtual machine software. You can probably achieve the same thing using:
 * a script that rapidly changes your hardware or software clock, or
 * VMWare, VirtualBox, or qemu with clock skew between guest and host, and
 a VM setting that applies the host time to the guest (and an apparent VM
 bug which causes the time to switch rapidly between host and guest time).

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/29990#comment:7>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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