[tor-bugs] #22460 [Core Tor/Tor]: Received a bad CERTS cell: Link certificate does not match TLS certificate

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#22460: Received a bad CERTS cell: Link certificate does not match TLS certificate
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 Reporter:  teor                                 |          Owner:
     Type:  defect                               |         Status:
                                                 |  needs_review
 Priority:  High                                 |      Milestone:  Tor:
                                                 |  0.3.1.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Major                                |     Resolution:
 Keywords:  tor-relay certs handshake ed25519    |  Actual Points:
  needs-analysis 030-backport                    |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by nickm):

 Replying to [comment:26 nickm]:
 > > I bet that the "The link certificate didn't match the TLS public key"
 thing is similar, but I'm not sure.
 >
 > It's similar, I think.  I believe this bug happens when we send an x.509
 cert as part of a TLS handshake, then rotate our TLS context, then send
 our certs cell from connection_or_send_certs_cell().  This timing for this
 rotation means that the link certificate in our certs cell won't match the
 one from the TLS handshake.

 I'm trying to think of a good bugfix for this one that doesn't have a race
 condition.  The problem with this case is that it means that my fix above
 is incomplete -- we need to update the signing->link certificate on TLS
 rotation, yes, but we should still serve the old signing->link certificate
 on all connections that existed before the TLS context rotated.

 I think the answer may be to cache the signing->link certificate at the
 time that the connection is created?  It's not perfectly elegant, but it
 would work.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22460#comment:27>
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