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[Launchpad Bug Tracker <1877454@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package openssh - 1:7.2p2-4ubuntu2.10

---------------
openssh (1:7.2p2-4ubuntu2.10) xenial; urgency=medium

  * Fix deadlock when AuthorizedKeysCommand produces a large output.
    (LP: #1877454)
    - d/p/authkeyscommand-deadlock-01.patch: Make sure to call fclose(2)
      and assign NULL to the file handler used to read
      "Authorized{Keys,Principal}Command" directives.
    - d/p/authkeyscommand-deadlock-02.patch: Consume entire output
      generated by the command pointed by
      "Authorized{Keys,Principal}Command" in order to avoid sending a
      SIGPIPE to the process.

openssh (1:7.2p2-4ubuntu2.9) xenial; urgency=medium

  * Apply upstream patch to stop using 2020 as a future date in regress
    tests. LP: #1859013

 -- Sergio Durigan Junior <sergio.durigan@xxxxxxxxxxxxx>  Wed, 13 May
2020 10:12:28 -0400

** Changed in: openssh (Ubuntu Xenial)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1877454

Title:
  openssh-server hangs with AuthorizedKeysCommand

Status in openssh package in Ubuntu:
  Fix Released
Status in openssh source package in Xenial:
  Fix Released

Bug description:
  [Impact]

  On a default Xenial install, when sshd is configured to obtain the
  list of allowed keys using AuthorizedKeysCommand (or the list of
  allowed certificate principals using AuthorizedPrincipalsCommand), and
  if the command pointed by Authorized{Keys,Principals}Command generates
  a lot of output, sshd will hang while reading this same output. In a
  nutshell, the problem happens when the subprocess generates enough
  data to fill the pipe's buffer; in this scenario, sshd will wait(2) on
  the subprocess, which will be blocked trying to write the rest of the
  output.

  [Test Case]

  In order to reproduce the bug, one can:

  $ lxc launch ubuntu-daily:xenial openssh-server-bug1877454
  $ lxc shell openssh-server-bug1877454
  # ssh-keygen
  (no need to choose a passphrase for the key, just hit ENTER on all prompts)
  # cat > authkeyscommand.sh << __EOF__
  #!/bin/bash

  cat /root/.ssh/id_rsa.pub
  echo
  head -c 1M < /dev/urandom
  __EOF__
  # chmod +x authkeyscommand.sh
  # cat >> /etc/ssh/sshd_config << __EOF__
  AuthorizedKeysCommand /root/authkeyscommand.sh
  AuthorizedKeysCommandUser root
  __EOF__
  # systemctl reload sshd.service
  # ssh root@127.0.0.1

  You will notice that ssh will stay there waiting for sshd's reply,
  which won't come.  The expected result would be for ssh to succeed.

  [Regression Potential]

  Since the affected code deals with executing a subprocess, reading its
  output through a pipe, and then relying on wait(2) to determine
  whether the subprocess exited correctly; and considering that this
  code is written in C without the help of features like RAII and with
  the use of goto statements, we are not able to disconsider the chances
  of making a mistake and forgetting to free a resource or to properly
  read/write from/to pipes, for example.  This is, after all, the reason
  the bug happened in the first place.

  Having said that, openssh contains extensive tests and the code is
  well organized and relatively easy to follow.  Upon close inspection,
  there doesn't seem to be an evident problem with the backported fixes.

  As usual when dealing with a somewhat older distribution, there is
  always the possibility of encountering problems because we will be
  recompiling openssh using the most recent versions of its build
  dependencies.

  [Original Description]

  Please consider applying this change to openssh-server distributed in Xenial (16.04).
  Without it, sshd can sporadically hang when making use of the `AuthorizedKeysCommand` option.

  https://github.com/openssh/openssh-
  portable/commit/ddd3d34e5c7979ca6f4a3a98a7d219a4ed3d98c2

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