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

[Emmanuel Yiannakakis]

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

[Paride Legovini (paride)]

Hello Emmanual and thanks for your bug report. You are right: the current implementation doesn't allow for the more fine-grained setup you outlined using Match stanzas. I'm marking this report as Triaged as I think the issue is well understood; feel free to change the status back to New if you think that further discussion is needed. Thanks!

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3842