[Bug 2011458] Re: ssh fails to rebind when it is killed with -HUP

Mon May 1 09:54:03 UTC 2023

** Patch added: "openssh_9.0p1-1ubuntu8.1.debdiff"
   https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/2011458/+attachment/5670029/+files/openssh_9.0p1-1ubuntu8.1.debdiff

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Title:
  ssh fails to rebind when it is killed with -HUP

Status in openssh package in Ubuntu:
  In Progress
Status in openssh source package in Kinetic:
  In Progress

Bug description:
  [Impact]

  The sshd re-execution logic is generally broken with systemd socket activation, which means that (1) sshd fails when it is told to re-exec
  via SIGHUP (e.g. systemctl reload ssh), and (2) sshd fails when started in debug mode.

  [Test Case]

  (1) Test systemctl reload ssh:

  * On a machine with openssh-server installed, make a connection to
  localhost to activate ssh.service (the connection does not need to be
  complete, so you can just say "no" at the host key verification
  stage):

  $ ssh localhost
  [...]

  * Send SIGHUP to sshd by calling systemctl reload ssh:

  $ systemctl reload ssh

  * Check the service state:

  $ systemctl status ssh
  × ssh.service - OpenBSD Secure Shell server
       Loaded: loaded (/lib/systemd/system/ssh.service; disabled; preset: enabled)
      Drop-In: /etc/systemd/system/ssh.service.d
               └─00-socket.conf
       Active: failed (Result: exit-code) since Mon 2023-04-17 20:43:27 UTC; 4s ago
     Duration: 2min 44.132s
  TriggeredBy: ● ssh.socket
         Docs: man:sshd(8)
               man:sshd_config(5)
      Process: 1112 ExecStart=/usr/sbin/sshd -D $SSHD_OPTS (code=exited, status=255/EXCEPTION)
      Process: 1152 ExecReload=/usr/sbin/sshd -t (code=exited, status=0/SUCCESS)
      Process: 1153 ExecReload=/bin/kill -HUP $MAINPID (code=exited, status=0/SUCCESS)
     Main PID: 1112 (code=exited, status=255/EXCEPTION)
          CPU: 79ms

  Apr 17 20:40:43 lunar systemd[1]: Started ssh.service - OpenBSD Secure Shell server.
  Apr 17 20:41:06 lunar sshd[1113]: Connection closed by 127.0.0.1 port 54666 [preauth]
  Apr 17 20:43:27 lunar systemd[1]: Reloading ssh.service - OpenBSD Secure Shell server...
  Apr 17 20:43:27 lunar sshd[1112]: Received SIGHUP; restarting.
  Apr 17 20:43:27 lunar systemd[1]: Reloaded ssh.service - OpenBSD Secure Shell server.
  Apr 17 20:43:27 lunar sshd[1112]: error: Bind to port 22 on 0.0.0.0 failed: Address already in use.
  Apr 17 20:43:27 lunar sshd[1112]: error: Bind to port 22 on :: failed: Address already in use.
  Apr 17 20:43:27 lunar sshd[1112]: fatal: Cannot bind any address.
  Apr 17 20:43:27 lunar systemd[1]: ssh.service: Main process exited, code=exited, status=255/EXCEPTION
  Apr 17 20:43:27 lunar systemd[1]: ssh.service: Failed with result 'exit-code'.

  * On an affected machine, the service will fail as shown above.

  (2) Test debug mode:

  * On a machine with openssh-server installed, edit /etc/default/ssh to
  configure debug mode for sshd:

  $ cat /etc/default/ssh 
  # Default settings for openssh-server. This file is sourced by /bin/sh from
  # /etc/init.d/ssh.

  # Options to pass to sshd
  SSHD_OPTS=-ddd

  * Attempt to make a connection to localhost:

  $ ssh localhost
  kex_exchange_identification: read: Connection reset by peer
  Connection reset by 127.0.0.1 port 22

  * On an affected machine, the attempt will fail as shown above, and
  the service will be in a failed state:

  $ systemctl status ssh
  × ssh.service - OpenBSD Secure Shell server
       Loaded: loaded (/lib/systemd/system/ssh.service; disabled; preset: enabled)
      Drop-In: /etc/systemd/system/ssh.service.d
               └─00-socket.conf
       Active: failed (Result: exit-code) since Mon 2023-04-17 20:46:34 UTC; 2min 27s ago
     Duration: 5ms
  TriggeredBy: ● ssh.socket
         Docs: man:sshd(8)
               man:sshd_config(5)
      Process: 1166 ExecStartPre=/usr/sbin/sshd -t (code=exited, status=0/SUCCESS)
      Process: 1167 ExecStart=/usr/sbin/sshd -D $SSHD_OPTS (code=exited, status=255/EXCEPTION)
     Main PID: 1167 (code=exited, status=255/EXCEPTION)
          CPU: 40ms

  Apr 17 20:46:34 lunar sshd[1167]: Server listening on :: port 22.
  Apr 17 20:46:34 lunar sshd[1167]: debug3: fd 4 is not O_NONBLOCK
  Apr 17 20:46:34 lunar sshd[1167]: debug1: Server will not fork when running in debugging mode.
  Apr 17 20:46:34 lunar sshd[1167]: debug3: send_rexec_state: entering fd = 7 config len 3456
  Apr 17 20:46:34 lunar sshd[1167]: debug3: ssh_msg_send: type 0
  Apr 17 20:46:34 lunar sshd[1167]: debug3: send_rexec_state: done
  Apr 17 20:46:34 lunar sshd[1167]: debug1: rexec start in 4 out 4 newsock 4 pipe -1 sock 7
  Apr 17 20:46:34 lunar systemd[1]: Started ssh.service - OpenBSD Secure Shell server.
  Apr 17 20:46:34 lunar systemd[1]: ssh.service: Main process exited, code=exited, status=255/EXCEPTION
  Apr 17 20:46:34 lunar systemd[1]: ssh.service: Failed with result 'exit-code'.

  [Where problems could occur]

  The fix expands Ubuntu's patch for systemd socket activation to try
  and make sure that any fds passed from systemd are not closed across
  re-executions of sshd. If we saw a problem, it would most likely be an
  attempt to operate on a closed fd, or the wrong fd, as a result of an
  edge case in one of the re-execution paths.

  [Original Description]

  In kinetic and lunar gce images we are facing an issue when ssh is being killed with -HUP
  SSH is failing to rebind port 22. It is not failing in other previous systems.

  It can be reproduced by running

  # pkill -o -HUP sshd || true
  # journalctl -n 20
  Mar 13 14:58:52 mar131454-025105 sshd[1371]: Received SIGHUP; restarting.
  Mar 13 14:58:52 mar131454-025105 sshd[1371]: error: Bind to port 22 on 0.0.0.0 failed: Address already in use.
  Mar 13 14:58:52 mar131454-025105 sshd[1371]: error: Bind to port 22 on :: failed: Address already in use.
  Mar 13 14:58:52 mar131454-025105 sshd[1371]: fatal: Cannot bind any address.
  Mar 13 14:58:52 mar131454-025105 systemd[1]: ssh.service: Main process exited, code=exited, status=255/EXCEPTION
  Mar 13 14:58:52 mar131454-025105 systemd[1]: ssh.service: Failed with result 'exit-code'.

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