[Bug 1986521] Update Released

[Brian Murray]

The verification of the Stable Release Update for openssh has completed
successfully and the package is now being released to -updates.
Subsequently, the Ubuntu Stable Release Updates Team is being
unsubscribed and will not receive messages about this bug report.  In
the event that you encounter a regression using the package from
-updates please report a new bug using ubuntu-bug and tag the bug report
regression-update so we can easily find any regressions.

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https://bugs.launchpad.net/bugs/1986521

Title:
  ssh client spins if output fd closed

Status in portable OpenSSH:
  Unknown
Status in openssh package in Ubuntu:
  Fix Released
Status in openssh source package in Jammy:
  Fix Released

Bug description:
  [Impact]
  In certain edge cases where the terminal goes away while an ssh process is running, ssh can be left consuming 100% CPU.  This increases processing costs for cloud users and wastes energy.  While this is an uncommon error, googling indicates many people have run into it in several different ways.   It seems important to get this fixed in stable releases.

  This is a regression in jammy presumably due to change from select()
  to poll() (see OpennSSH 8.9 Release Announcement [1] ), fixed by
  upstream commit d6556de1db0822c76ba2745cf5c097d9472adf7c "upstream:
  fix poll() spin when a channel's output fd closes..." [2].

  1: https://lwn.net/Articles/885886/
  2. https://github.com/openssh/openssh-portable/commit/d6556de1db0822c76ba2745cf5c097d9472adf7c

  [Test Case]
  $ lxc launch ubuntu-daily:jammy ssh-cpu
  $ lxc shell ssh-cpu

  # passwd -d root
  # ssh-keygen -t rsa -N '' -f /root/.ssh/id_rsa

  # cat << EOF >>/etc/ssh/ssh_config
  StrictHostKeyChecking accept-new
  EOF

  # sed -ri 's/^PasswordAuthentication/#PasswordAuthentication/' /etc/ssh/sshd_config
  # cat << EOF >>/etc/ssh/sshd_config
  PermitRootLogin yes
  PubkeyAuthentication yes
  PermitEmptyPasswords yes
  PasswordAuthentication yes
  ChallengeResponseAuthentication no
  UsePAM no
  EOF

  # systemctl restart sshd

  # ssh localhost 2> >({exec 1>&2})

  You can shell into the container from a second terminal and use "htop"
  to verify that ssh is using 100% of one of the CPU cores:

  $ lxc shell ssh-cpu
  # htop

  This should show one CPU pegged at 100% due to the 'ssh localhost'
  process

  Next, return to the first terminal, exit out of the sub-ssh session and
  install the fix:

  # logout
  # add-apt-repository -yus ppa:bryce/openssh-sru-lp1986521
  # apt-get full-upgrade -y

  Now repeat the test in the first terminal window, while viewing htop in
  the second terminal:

  # ssh localhost 2> >({exec 1>&2})

  [Where Problems Could Occur]

  While the patch in question is well tested upstream, it has a
  relatively high line count and as such is difficult to assure
  correctness by visual code checking.  However, it's not clear that the
  line count could be significantly reduced without risking loss of
  correctness.  Thus this relies more on testing to assure robustness,
  than on code review.

  The code involves polling behavior, so issues to watch for would more
  likely involve process handling, i.e. problems with socket polling.

  Beyond that, the usual generic issues to watch for - build issues,
  dependency issues during build or on upgrade, and service restarting.

  [Original Report]
  The OpenSSH package 8.9p1 as shipped with U22.04 (8.9p1-3) suffers from the bug described at
  https://bugzilla.mindrot.org/show_bug.cgi?id=3411 and https://bugzilla.mindrot.org/show_bug.cgi?id=3405

  A command such as "xterm -e 'ssh -f remote.host sleep 60'" will pop up
  an xterm, ask for whatever authentication is needed, close the xterm,
  and leave the ssh client spinning consuming CPU time for 60 seconds
  before it exits. It should leave the ssh client idle for 60 seconds.
  Many uses of ssh to launch graphical applications will be caught by
  this bug.

  This is fixed in OpenSSH 9.0p1 as the first bugfix listed in its
  release notes at https://www.openssh.com/txt/release-9.0

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