[Bug 1950787] Re: systemd-sysusers cannot mount /dev in privileged containers (to pass credentials)

[Stéphane Graber]

Closing the LXD task as there's not really anything we can do there.

The options here are pretty much:
 - Do nothing, if it's just privileged containers, it's usually not a big deal
 - Significantly rework apparmor mount handling logic and policies so this can be safely allowed
 - Ship unit overrides, either though lxd-agent-loader, through a systemd patch or a similar distro mechanism

Closing the LXD task as there currently isn't any change we can make to
our policies to safely allow this.

** Changed in: lxd (Ubuntu)
       Status: New => Invalid

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Title:
  systemd-sysusers cannot mount /dev in privileged containers (to pass
  credentials)

Status in lxd package in Ubuntu:
  Invalid
Status in systemd package in Ubuntu:
  New

Bug description:
  systemd-sysusers.service/systemd.exec fails to start in privileged containers, due to being unable to properly mount /dev for passing credentials, caused by the following config in the .service unit:
  ```
  # Optionally, pick up a root password and shell for the root user from a
  # credential passed to the service manager. This is useful for importing this
  # data from nspawn's --set-credential= switch.
  LoadCredential=passwd.hashed-password.root
  LoadCredential=passwd.plaintext-password.root
  LoadCredential=passwd.shell.root
  ```

  Reproducer:
  $ lxc profile set default security.privileged "true"
  $ lxc launch ubuntu-daily:jammy test
  $ lxc exec test bash
  # add-apt-repository ppa:ci-train-ppa-service/4704
  # apt install systemd # install systemd 249.5-2ubuntu1
  # systemctl restart systemd-sysusers
  # systemctl status systemd-sysusers
  # system --status=failed
  $ lxc profile set default security.privileged "false"

  A workaround is to disable it via:
  $ cat /etc/systemd/system/systemd-sysusers.service.d/override.conf:
  [Service]
  LoadCredential=

  Interesting logs:
  Nov 12 12:09:44 test systemd[1]: systemd-journald.service: Added fd 42 (n/a) to fd store.
  Nov 12 12:09:44 test systemd[431]: Mounting /dev (MS_REC|MS_SLAVE "")...
  Nov 12 12:09:44 test systemd[431]: Failed to mount n/a (type n/a) on /dev (MS_REC|MS_SLAVE ""): Permission denied
  Nov 12 12:09:44 test systemd[430]: (sd-mkdcreds) failed with exit status 1.
  Nov 12 12:09:44 test systemd[430]: systemd-sysusers.service: Failed to set up credentials: Protocol error
  Nov 12 12:09:44 test systemd[430]: systemd-sysusers.service: Failed at step CREDENTIALS spawning

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