** Description changed:
[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 [email protected]
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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https://bugs.launchpad.net/bugs/1877454
Title:
openssh-server hangs with AuthorizedKeysCommand
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