[tor-bugs] #33650 [Core Tor/Tor]: Verify that intro2 cell extensions actually work

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#33650: Verify that intro2 cell extensions actually work
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 Reporter:  arma                                 |          Owner:
                                                 |  mikeperry
     Type:  task                                 |         Status:
                                                 |  accepted
 Priority:  Medium                               |      Milestone:
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tor-dos tor-dos-2020 anonymous-      |  Actual Points:
  credentials research                           |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by mikeperry):

 Notes for the INTRO1->INTRO2 codepaths:
  - Intro1 is sent from `hs_circ_send_introduce1()`, and built in
 `hs_cell_build_introduce1()`
  - There are actually two extension areas: one is in the unencrypted
 section, set in `hs_cell_build_introduce1()`, and one is in the encrypted
 section, set in `introduce1_set_encrypted()`
  - The Intropoint processes the INTRO1 cell in
 `hs_intro_received_introduce1()`
  - v3 INTRO1 cells go into `handle_introduce1()` and the cell's
 unencrypted fields are parsed and validated
  - Requested rate limits are checked in `hs_dos_can_send_intro2()`
  - The cell is sent as an INTRO2 exactly as is towards the service
  - The service receives it in `hs_service_receive_introduce2()`
  - The service checks the IP in `service_handle_introduce2()`, and if
 valid, parses the cell in `hs_circ_handle_introduce2()` (via
 `hs_cell_parse_introduce2()` and ultimately the intro1 parsing functions)
  - If parsing was successful, a rend circuit is launched from
 `hs_circ_handle_introduce2()`

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/33650#comment:5>
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