config-ssh module doesn't respect Match conditions in sshd_config

This bug was originally filed in Launchpad as LP: #1915772

Launchpad details

affected_projects = []
assignee = None
assignee_name = None
date_closed = None
date_created = 2021-02-16T04:32:44.605773+00:00
date_fix_committed = None
date_fix_released = None
id = 1915772
importance = low
is_complete = False
lp_url = https://bugs.launchpad.net/cloud-init/+bug/1915772
milestone = None
owner = manoli-yiannakakis-roche
owner_name = Emmanuel Yiannakakis
private = False
status = triaged
submitter = manoli-yiannakakis-roche
submitter_name = Emmanuel Yiannakakis
tags = []
duplicates = []

Launchpad user Emmanuel Yiannakakis(manoli-yiannakakis-roche) wrote on 2021-02-16T04:32:44.605773+00:00

Summary Per https://www.freebsd.org/cgi/man.cgi?sshd_config(5)

Match Introduces a conditional block. If all of the criteria on the Match line are satisfied, the keywords on the following lines override those set in the global section of the config file, until either anotherMatch line or the end of the file. If a keyword appears in multiple Match blocks that are satisfied, only the first instance of the keyword is applied.

Say I have a Match setup for a group to use a special location of an AuthorizedKeysFile, basically to move this out of the homedir these restricted users are jailed in.

Match Group my-special-group AuthorizedKeysFile /etc/ssh/authorized_keys/%u Relevant Code: https://github.com/canonical/cloud-init/blob/09193e5141ca45b822617399047204abd701047e/cloudinit/ssh_util.py#L274 and ultimately lies in the implementation at https://github.com/canonical/cloud-init/blob/09193e5141ca45b822617399047204abd701047e/cloudinit/ssh_util.py#L344

the way parse_ssh_config_map parses the file, the last AuthorizedKeysFile entry wins. I suggest just stop reading the file if you get to a Match stanza (either here or in parse_ssh_config_lines). If you get really fancy, you could see if the username you're looking up is under an explicit Match User ec2-user stanza. But as it is now, it's an all-or-nothing where my AuthorizedKeysFile wins.

Process Setup an sshd_config utilizing a Match option, like

Match Group my-special-group AuthorizedKeysFile /etc/ssh/authorized_keys/%u and then have cloud-init do it's ssh configuration

Current and expected result Current: the last AuthorizedKeysFile statement wins, regardless if it's at the global level or underneath a Match Expected: cloud-init only respects the globally defined AuthorizedKeysFile, or falls back to the standard homedir location

Screenshot n/a

canonical / cloud-init

config-ssh module doesn't respect Match conditions in sshd_config #3842