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

[Sergio Durigan Junior]

On Thursday, June 25 2020, Łukasz Zemczak wrote:

> Thank you for the verification Sergio! Could you please note down which
> package version has been used for verification and what kind of
> verification has been performed?

Hi Łukasz,

The package version I tested is 1:7.2p2-4ubuntu2.10.  I performed the
steps listed in the "Test Case" section of the SRU above.  Everything
worked as expected:

$ ssh root at 127.0.0.1
Warning: Permanently added '127.0.0.1' (ECDSA) to the list of known hosts.
Welcome to Ubuntu 16.04.6 LTS (GNU/Linux 5.4.0-31-generic x86_64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage


The programs included with the Ubuntu system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Ubuntu comes with ABSOLUTELY NO WARRANTY, to the extent permitted by
applicable law.

$

Thanks!

-- 
Sergio
GPG key ID: E92F D0B3 6B14 F1F4 D8E0  EB2F 106D A1C8 C3CB BF14

-- 
You received this bug notification because you are a member of Ubuntu
Foundations Bugs, which is subscribed to openssh in Ubuntu.
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

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/1877454/+subscriptions