[Bug 1927078] Re: Don't allow useradd to use fully numeric names

[Łukasz Zemczak]

Put a mention about it in the release notes for .3. Will do the same for
impish.

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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 Released
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 at focal:/home/ubuntu# useradd -m 0
  root at focal:/home/ubuntu# id 0
  uid=1005(0) gid=1005(0) groups=1005(0)

  ..

  0 at 192.168.122.6'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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