[Bug 2028282] Re: [SRU] SSH pubkey authetication fails when GSSAPI enabled

Tue Jan 7 09:28:12 UTC 2025

Autopkgtests are looking good for openssh 1:9.7p1-7ubuntu4.1 from
oracular-proposed. The intermittent failures from comment #13 have all
been resolved.

https://autopkgtest.ubuntu.com/results/autopkgtest-oracular/oracular/amd64/o/openssh/20241218_102717_96d50@/log.gz
https://autopkgtest.ubuntu.com/results/autopkgtest-oracular/oracular/arm64/o/openssh/20241217_173242_f3550@/log.gz
https://autopkgtest.ubuntu.com/results/autopkgtest-oracular/oracular/armhf/o/openssh/20241217_172946_d43fe@/log.gz
https://autopkgtest.ubuntu.com/results/autopkgtest-oracular/oracular/ppc64el/o/openssh/20241217_201054_4289f@/log.gz
https://autopkgtest.ubuntu.com/results/autopkgtest-oracular/oracular/s390x/o/openssh/20241217_203414_b4f03@/log.gz

All of them show 3 login attempts with the final one using privatekey
authentication:

"""
1140s ## Checking ssh logs to confirm gssapi-with-mic auth was used
1140s Dec 18 10:25:16 sshd-gssapi.example.fake sshd[1626]: Accepted gssapi-with-mic for testuser1525 from 127.0.0.1 port 34338 ssh2: testuser1525 at EXAMPLE.FAKE
1140s ## PASS test_gssapi_login
[...]
1140s ## Checking ssh logs to confirm gssapi-keyex auth was used
1140s Dec 18 10:25:16 sshd-gssapi.example.fake sshd[1679]: Accepted gssapi-keyex for testuser1525 from 127.0.0.1 port 34352 ssh2: testuser1525 at EXAMPLE.FAKE
1140s ## PASS test_gssapi_keyex_login
[...]
1140s ## Checking ssh logs to confirm publickey auth was used
1140s Dec 18 10:25:17 sshd-gssapi.example.fake sshd[1718]: Accepted publickey for testuser1525-2 from 127.0.0.1 port 34358 ssh2: ED25519 SHA256:LzS5GWVNXlP4zbwYWz2Dfd3CzLN7Z2S94q2Jpm66l7U
1140s ## PASS test_gssapi_keyex_pubkey_fallback
"""

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https://bugs.launchpad.net/bugs/2028282

Title:
  [SRU] SSH pubkey authetication fails when GSSAPI enabled

Status in openssh package in Ubuntu:
  Fix Released
Status in openssh source package in Jammy:
  Triaged
Status in openssh source package in Noble:
  Triaged
Status in openssh source package in Oracular:
  Fix Committed
Status in openssh source package in Plucky:
  Fix Released
Status in openssh package in Debian:
  Fix Released

Bug description:
  [ Impact ]

   * Login with publickey fails when openssh server is configured to use
  GSSAPI authentication, too. Error: "sign_and_send_pubkey: internal
  error: initial hostkey not recorded"

   * To trigger it, one needs to (a) perform a successful GSSAPI key
  exchange, (b) attempt public key authentication.

   * In addition, the client and the server must both have the hostbound
  authentication protocol extension enabled for the problem to manifest
  itself (On by default).

   * This is not a very common combination, but it can happen if one has
  Kerberos credentials for the correct realm but the wrong user, and a
  private key for the right user.

   * This SRU fixes this by adding an additional
  "ssh->kex->initial_hostkey != NULL" check in
  sshconnect2.c:sign_and_send_pubkey(), as suggested by upstream in
  https://bugzilla.mindrot.org/show_bug.cgi?id=3406 (comment 2).

  [ Test Plan ]

   The reproducer was codified in autopkgtests, thanks to Colin Watson!

   * Make sure to have the latest debian/tests/ssh-gssapi test case
  (included as of 1:9.9p1-2, and shipped as part of this SRU),
  especially the delta described in
  https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/2028282/+attachment/5845545/+files/dep8-verifier.diff

   * Execute the "ssh-gssapi" dep8 test:
  $ autopkgtest -U openssh --apt-pocket=proposed=src:openssh --test-name=ssh-gssapi -- lxd autopkgtest/ubuntu/oracular/amd64

   * Confirm the log contains 3 login attempts, with the final one using the "publickey" authentication method ("Accepted publickey for testuser..."):
  """
  ## Checking ssh logs to confirm publickey auth was used
  Dec 14 22:44:16 sshd-gssapi.example.fake sshd-session[2213]: Accepted publickey for testuser2020-2 from 127.0.0.1 port 43364 ssh2: ED25519 SHA256:7vF3468XCZOawompwDThLsGsnPoUaP5Ki/3KaQLq/2M
  ## PASS test_gssapi_keyex_pubkey_fallback
  """

  [ Test Plan 2 ]

   * In addition to the codified test for this specific issue, we want
  to confirm normal password and publickey login are still working as
  expected.

   * Enable "PasswordAuthentication yes" in /etc/ssh/sshd_config &
  restart ssh.service

   * Login using password, confirm success

   * Copy public key over to system-under-test

   * Enable "PubkeyAuthentication yes" in /etc/ssh/sshd_config & restart

   * Login using private key, confirm success

  [ Where problems could occur ]

   * This SRU tweaks the authentication logic of OpenSSH, therefore it's
  a high-impact change. If something goes wrong, it could lock people
  out of their remote machines.

   * The change has been deployed to Debian testing and Ubuntu Plucky
  since October 2024, without major issues raised.

   * I've added "[ Test Plan 2 ]" to confirm normal publickey & password
  login is still working as expected

  [ Other Info ]

   * Fixed as of 1:9.9p1-2 (e.g. in Plucky)

   * Rejected upstream, due to being a bug in the Debian delta:
     https://bugzilla.mindrot.org/show_bug.cgi?id=3406

   * Fixed in Debian by Colin Watson:
     https://salsa.debian.org/ssh-team/openssh/-/commit/7d291bb

  === original bug report ===
  Since the upgrade from Ubuntu 20.04 to 22.04 the SSH login via a SSH pubkey to our servers fails, while password and kerberos are still working.

  $ssh user at server
  sign_and_send_pubkey: internal error: initial hostkey not recorded

  This seem related to the bugreport at openssh:
  https://bugzilla.mindrot.org/show_bug.cgi?id=3406

  ProblemType: Bug
  DistroRelease: Ubuntu 22.04
  Package: openssh-server 1:8.9p1-3ubuntu0.1
  ProcVersionSignature: Ubuntu 5.15.0-76.83-generic 5.15.99
  Uname: Linux 5.15.0-76-generic x86_64
  ApportVersion: 2.20.11-0ubuntu82.5
  Architecture: amd64
  CasperMD5CheckResult: pass
  CloudArchitecture: x86_64
  CloudID: none
  CloudName: none
  CloudPlatform: none
  CloudSubPlatform: config
  Date: Thu Jul 20 17:25:01 2023
  InstallationDate: Installed on 2020-08-24 (1060 days ago)
  InstallationMedia: Ubuntu-Server 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
  SourcePackage: openssh
  UpgradeStatus: Upgraded to jammy on 2023-07-20 (0 days ago)

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