[Bug 1877454] Re: openssh-server hangs with AuthorizedKeysCommand

[Brian Murray]

Hello Jeremy, or anyone else affected,

Accepted openssh into xenial-proposed. The package will build now and be
available at
https://launchpad.net/ubuntu/+source/openssh/1:7.2p2-4ubuntu2.10 in a
few hours, and then in the -proposed repository.

Please help us by testing this new package.  See
https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how
to enable and use -proposed.  Your feedback will aid us getting this
update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug,
mentioning the version of the package you tested, what testing has been
performed on the package and change the tag from verification-needed-
xenial to verification-done-xenial. If it does not fix the bug for you,
please add a comment stating that, and change the tag to verification-
failed-xenial. In either case, without details of your testing we will
not be able to proceed.

Further information regarding the verification process can be found at
https://wiki.ubuntu.com/QATeam/PerformingSRUVerification .  Thank you in
advance for helping!

N.B. The updated package will be released to -updates after the bug(s)
fixed by this package have been verified and the package has been in
-proposed for a minimum of 7 days.

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

** Tags added: verification-needed verification-needed-xenial

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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 Committed

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 at 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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