I think it would be good to write a release note entry about this change
for Ubuntu 21.10 and possibly updating the release notes for Ubuntu
20.04.3 (if these changes make it there in time).
** Also affects: ubuntu-release-notes
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1927078
Title:
Don't allow useradd to use fully numeric names
Status in Release Notes for Ubuntu:
New
Status in shadow package in Ubuntu:
Fix Released
Status in shadow source package in Focal:
Fix Committed
Status in shadow source package in Groovy:
Won't Fix
Status in shadow source package in Hirsute:
Fix Committed
Status in shadow source package in Impish:
Fix Released
Bug description:
[Impact]
* If a fully numeric username is created, it will cause
problems with systemd. One example is that the user with
this type of name can log in, but loginctl will not create
a session for them.
* This can also cause users to be unable to log in to a gdm
environment
[Test Case]
* `useradd 123` (this command should succeed)
* `userdel 123` to clean up the user that was just added
* Install `shadow` from -proposed
* `useradd 123` should now fail
[Regression Potential]
* If there were a logic error in the fix, it is possible
that valid usernames would now be disallowed.
* Many test cases have been added to ensure this is not
the case, and --badnames would still provide a work-around
* [racb] Users may have scripts that are currently using numeric usernames
and these scripts will break as a consequence of this deliberate change in
stable Ubuntu releases. However, based on the discussion in the bug, we think
this is preferable to leaving such users with unstable behaviour such as
systemd's behaviour described.
[Original Description]
Fully numeric names support in Ubuntu is inconsistent in Focal onwards
because systemd does not like them[1] but are still allowed by default
by useradd, leaving the session behavior in hands of the running
applications. Two examples:
1. After creating a user named "0", the user can log in via ssh or
console but loginctl won't create a session for it:
root@focal:/home/ubuntu# useradd -m 0
root@focal:/home/ubuntu# id 0
uid=1005(0) gid=1005(0) groups=1005(0)
..
[email protected]'s password:
Welcome to Ubuntu 20.04.2 LTS (GNU/Linux 5.8.0-48-generic x86_64)
Last login: Thu Apr 8 16:17:06 2021 from 192.168.122.1
$ loginctl
No sessions.
$ w
16:20:09 up 4 min, 1 user, load average: 0.03, 0.14, 0.08
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
0 pts/0 192.168.122.1 16:17 0.00s 0.00s 0.00s w
And pam-systemd shows the following message:
Apr 08 16:17:06 focal sshd[1584]: pam_unix(sshd:session): session opened for
user 0 by (uid=0)
Apr 08 16:17:06 focal sshd[1584]: pam_systemd(sshd:session): pam-systemd
initializing
Apr 08 16:17:06 focal sshd[1584]: pam_systemd(sshd:session): Failed to get
user record: Invalid argument
2. With that same username, every successful authentication in gdm
will loop back to gdm again instead of starting gnome, making the user
unable to login.
Making useradd fail (unless --badnames is set) when a fully numeric
name is used will make the default OS behavior consistent.
[Other info]
- Upstream does not support fully numeric usernames
- useradd has a --badnames parameter that would still allow the use of these
type of names
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