systemd-homed and JSON User/Group Record Support in Desktop Environments

Starting with version 245, systemd supports a new subsystem systemd-homed.service for managing regular (“human”) users and their home directories. Along with it a new concept userdb got merged that brings rich, extensible JSON user/group records, extending the classic UNIX/glibc NSS struct passwd/struct group structures. Both additions are added in a fully backwards compatible way, accessible through getpwnam()/getgrnam()/… (i.e. libc NSS) and PAM as usual, meaning that for basic support no changes in the upper layers of the stack (in particular desktop environments, such as GNOME or KDE) have to be made. However, for better support a number of changes to desktop environments are recommended. A few areas where that applies are discussed below.

Before reading on, please read up on the basic concepts, specifically:

Support for Suspending Home Directory Access during System Suspend

One key feature of systemd-homed managed encrypted home directories is the ability that access to them can be suspended automatically during system sleep, removing any cryptographic key material from memory while doing so. This is important in a world where most laptop users seldom shut down their computers but most of the time just suspend them instead. Previously, the encryption keys for the home directories remained in memory during system suspend, so that sufficiently equipped attackers could read them from there and gain full access to the device. By removing the key material from memory before suspend, and re-requesting it on resume this attack vector can be closed down effectively.

Supporting this mechanism requires support in the desktop environment, since the encryption keys (i.e. the user’s login password) need to be reacquired on system resume, from a lock screen or similar. This lock screen must run in system context, and cannot run in the user’s own context, since otherwise it might end up accessing the home directory of the user even though access to it is temporarily suspended and thus will hang if attempted.

It is suggested that desktop environments that implement lock screens run them from system context, for example by switching back to the display manager, and only revert back to the session after re-authentication via this system lock screen (re-authentication in this case refers to passing the user’s login credentials to the usual PAM authentication hooks). Or in other words, when going into system suspend it is recommended that GNOME Shell switches back to the GNOME Display Manager login screen which now should double as screen lock, and only switches back to the shell’s UI after the user re-authenticated there.

Note that this change in behavior is a good idea in any case, and does not create any dependencies on systemd-homed or systemd-specific APIs. It’s simply a change of behavior regarding use of existing APIs, not a suggested hook-up to a any new API.

A display manager which supports this kind of out-of-context screen lock operation needs to inform systemd-homed about this so that systemd-homed knows that it is safe to suspend the user’s home directory on suspend. This is done via the suspend= argument to the pam_systemd_home PAM module. A display manager should hence change its PAM stack configuration to set this parameter to on. systemd-homed will not suspend home directories if there’s at least one active session of the user that does not support suspending, as communicated via this parameter.

User Management UIs

The rich user/group records userdb and systemd-homed support carry various fields of relevance to UIs that manage the local user database or parts thereof. In particular, most of the metadata accounts-daemon (also see below) supports is directly available in these JSON records. Hence it makes sense for any user management UI to expose them directly.

systemd-homed exposes APIs to add, remove and make changes to local users via D-Bus, with full polkit hook-up. On the command line this is exposed via the homectl command. A graphical UI that exposes similar functionality would be very useful, exposing the various new account settings, and in particular providing a stream-lined UI for enrolling new-style authentication tokens such as PKCS#11/YubiKey-style devices. (Ideally, if the user plugs in an uninitialized YubiKey during operation it might be nice if the Desktop would automatically ask if a key pair shall be written to it and the local account be bound to it, systemd-homed provides enough YubiKey/PKCS#11 support to make this a reality today; except that it will not take care of token initialization).

A strong point of systemd-homed is per-user resource management. In particular disk space assignments are something that most likely should be exposed in a user management UI. Various metadata fields are supplied allowing exposure of disk space assignment “slider” UI. Note however that the file system back-ends of systemd-homed.service have different feature sets. Specifically, only btrfs has online file system shrinking support, ext4 only offline file system shrinking support, and xfs no shrinking support at all (all three file systems support online file system growing however). This means if the LUKS back-end is used, disk space assignment cannot be instant for logged in users, unless btrfs is used.

Note that only systemd-homed provides an API for modifying/creating/deleting users. The generic userdb subsystem (which might have other back-ends, besides systemd-homed, for example LDAP or Windows) exclusively provides a read-only interface. (This is unlikely to change, as the other back-ends might have very different concepts of adding or modifying users, i.e. might not even have any local concept for that at all). This means any user management UI that intends to change (and not just view) user accounts should talk directly to systemd-homed to make use of its features; there’s no abstraction available to support other back-ends under the same API.

Unfortunately there’s currently no documentation for the systemd-homed D-Bus API. Consider using the homectl sources as guidelines for implementing a user management UI. The JSON user/records are well documented however, see above, and the D-Bus API provides limited introspection.

Relationship to accounts-daemon

For a long time accounts-daemon has been included in Linux distributions providing richer user accounts. The functionality of this daemon overlaps in many areas with the functionality of systemd-homed or userdb, but there are systematic differences, which means that systemd-homed cannot replace accounts-daemon fully. Most importantly: accounts-daemon provides “side-car” metadata for any type of user account, while systemd-homed only provides additional metadata for the users it defines itself. In other words: accounts-daemon will augment foreign accounts; systemd-homed cannot be used to augment users defined elsewhere, for example in LDAP or as classic /etc/passwd records.

This probably means that for the time being, a user management UI (or other UI) that wants to support rich user records with compatibility with the status quo ante should probably talk to both systemd-homed and accounts-daemon at the same time, and ignore accounts-daemon’s records if systemd-homed defines them. While I (Lennart) personally believe in the long run systemd-homed is the way to go for rich user records, any UI that wants to manage and support rich records for classic records has to support accounts-daemon in parallel for the time being.

In the short term, it might make sense to also expose the userdb provided records via accounts-daemon, so that clients of the latter can consume them without changes. However, I think in the long run accounts-daemon should probably be removed from the general stack, hence this sounds like a temporary solution only.

In case you wonder, there’s no automatic mechanism for converting existing users registered in /etc/passwd or LDAP to users managed by systemd-homed. There’s documentation for doing this manually though, see Converting Existing Users to systemd-homed managed Users.

Future Additions

JSON user/group records are extensible, hence we can easily add any additional fields desktop environments require. For example pattern-based authentication is likely very useful on touch-based devices, and the user records should hence learn them natively. Fields for other authentication mechanisms, such as fingerprint authentication should be provided as well, eventually.

It is planned to extend the userdb Varlink API to support look-ups by partial user name and real name (GECOS) data, so that log-in screens can optionally implement simple complete-as-you-type login screens.

It is planned to extend the systemd-homed D-Bus API to instantly inform clients about hardware associated with a specific user being plugged in, to which login screens can listen in order to initiate authentication. Specifically, any YubiKey-like security token plugged in that is associated with a local user record should initiate authentication for that user, making typing in of the username unnecessary.