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

[Stéphane Graber]

Privileged containers have a much stricter apparmor policy applied than unprivileged containers.
That's because unprivileged containers primarily rely on the user namespace to prevent breakout and taking over of the host whereas privileged containers rely entirely on apparmor.

As apparmor isn't particularly good at dealing with mounts, especially
with mount namespaces, there is no safe way for us to allow this
operation in privileged containers.

As you point out above, we've recently started using a systemd generator
to dynamically generate unit overrides based on the environment, letting
us disable specific features that interfere with container security.


This is used in all of the community images, so in this case you could try it by using "images:ubuntu/jammy" instead of "ubuntu-daily:jammy". We've been considering getting the generator into the lxd-agent-loader package which is included in all Ubuntu images though so far we've found it to be too volatile for that (we were updating it up to twice a week for a while...).

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https://bugs.launchpad.net/bugs/1950787

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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