[tor-bugs] #25501 [Core Tor/Tor]: Ensure WTF-Pad padding comes from the expected hop (was: Control-flow issues solved for WTF-pad)

[Tor Bug Tracker & Wiki]

#25501: Ensure WTF-Pad padding comes from the expected hop
-------------------------------------------------+-------------------------
 Reporter:  dgoulet                              |          Owner:
                                                 |  mikeperry
     Type:  task                                 |         Status:
                                                 |  assigned
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.3.5.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  control-flow, tor-circuit, 035       |  Actual Points:
  -roadmap-master                                |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
                                                 |  Sponsor2
-------------------------------------------------+-------------------------
Changes (by mikeperry):

 * owner:  dgoulet => mikeperry


Old description:

> Roadmap master ticket for this sponsored task.
>
> See child tickets for specific tasks.

New description:

 When first looking at the WTF-Pad design for integration into Tor, we were
 concerned that there may be flow control issues with padding causing our
 SENDME windows to empty prematurely. It turns out that RELAY_DROP does not
 count towards these windows though, so no updates are needed there.

 However, we should add an additional check to ensure that RELAY_DROP cells
 come from the expected hop (middle). This check is easy to do -- just
 inspect the layer_hint after the cell is recognized and see where it came
 from. In this way, we can prevent a malicious Exit node or RP from
 injecting end-to-end side channel cells, while still allowing padding.

--

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25501#comment:3>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online