2020-04-09 17:02:57 +00:00
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---
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title: systemd-homed and JSON User/Group Record Support in Desktop Environments
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2020-04-11 16:03:24 +00:00
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category: Users, Groups and Home Directories
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2020-04-09 17:02:57 +00:00
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layout: default
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---
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# `systemd-homed` and JSON User/Group Record Support in Desktop Environments
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Starting with version 245, systemd supports a new subsystem
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[`systemd-homed.service`](https://www.freedesktop.org/software/systemd/man/systemd-homed.service.html)
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for managing regular ("human") users and their home directories. Along with it
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a new concept `userdb` got merged that brings rich, extensible JSON user/group
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records, extending the classic UNIX/glibc NSS `struct passwd`/`struct group`
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structures. Both additions are added in a fully backwards compatible way,
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accessible through `getpwnam()`/`getgrnam()`/… (i.e. libc NSS) and PAM as
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usual, meaning that for basic support no changes in the upper layers of the
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stack (in particular desktop environments, such as GNOME or KDE) have to be
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made. However, for better support a number of changes to desktop environments
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are recommended. A few areas where that applies are discussed below.
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Before reading on, please read up on the basic concepts, specifically:
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* [Home Directories](https://systemd.io/HOME_DIRECTORY)
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* [JSON User Records](https://systemd.io/USER_RECORD)
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* [JSON Group Records](https://systemd.io/GROUP_RECORD)
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* [User/Group Record Lookup API via Varlink](https://systemd.io/USER_GROUP_API)
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## Support for Suspending Home Directory Access during System Suspend
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One key feature of `systemd-homed` managed encrypted home directories is the
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ability that access to them can be suspended automatically during system sleep,
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removing any cryptographic key material from memory while doing so. This is
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important in a world where most laptop users seldom shut down their computers
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but most of the time just suspend them instead. Previously, the encryption keys
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for the home directories remained in memory during system suspend, so that
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sufficiently equipped attackers could read them from there and gain full access
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to the device. By removing the key material from memory before suspend, and
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re-requesting it on resume this attack vector can be closed down effectively.
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Supporting this mechanism requires support in the desktop environment, since
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the encryption keys (i.e. the user's login password) need to be reacquired on
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system resume, from a lock screen or similar. This lock screen must run in
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system context, and cannot run in the user's own context, since otherwise it
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might end up accessing the home directory of the user even though access to it
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is temporarily suspended and thus will hang if attempted.
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It is suggested that desktop environments that implement lock screens run them
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from system context, for example by switching back to the display manager, and
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only revert back to the session after re-authentication via this system lock
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screen (re-authentication in this case refers to passing the user's login
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credentials to the usual PAM authentication hooks). Or in other words, when
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going into system suspend it is recommended that GNOME Shell switches back to
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the GNOME Display Manager login screen which now should double as screen lock,
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and only switches back to the shell's UI after the user re-authenticated there.
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Note that this change in behavior is a good idea in any case, and does not
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create any dependencies on `systemd-homed` or systemd-specific APIs. It's
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simply a change of behavior regarding use of existing APIs, not a suggested
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hook-up to a any new API.
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A display manager which supports this kind of out-of-context screen lock
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operation needs to inform systemd-homed about this so that systemd-homed knows
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that it is safe to suspend the user's home directory on suspend. This is done
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via the `suspend=` argument to the
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[`pam_systemd_home`](https://www.freedesktop.org/software/systemd/man/pam_systemd_home.html)
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PAM module. A display manager should hence change its PAM stack configuration
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to set this parameter to on. `systemd-homed` will not suspend home directories
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if there's at least one active session of the user that does not support
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suspending, as communicated via this parameter.
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## User Management UIs
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The rich user/group records `userdb` and `systemd-homed` support carry various
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fields of relevance to UIs that manage the local user database or parts
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thereof. In particular, most of the metadata `accounts-daemon` (also see below)
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supports is directly available in these JSON records. Hence it makes sense for
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any user management UI to expose them directly.
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`systemd-homed` exposes APIs to add, remove and make changes to local users via
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2020-04-18 15:47:51 +00:00
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D-Bus, with full [polkit](https://www.freedesktop.org/software/polkit/docs/latest/)
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hook-up. On the command line this is exposed via the
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2020-04-09 17:02:57 +00:00
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`homectl` command. A graphical UI that exposes similar functionality would be
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very useful, exposing the various new account settings, and in particular
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providing a stream-lined UI for enrolling new-style authentication tokens such
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as PKCS#11/YubiKey-style devices. (Ideally, if the user plugs in an
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uninitialized YubiKey during operation it might be nice if the Desktop would
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automatically ask if a key pair shall be written to it and the local account be
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bound to it, `systemd-homed` provides enough YubiKey/PKCS#11 support to make
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this a reality today; except that it will not take care of token
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initialization).
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A strong point of `systemd-homed` is per-user resource management. In
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particular disk space assignments are something that most likely should be
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exposed in a user management UI. Various metadata fields are supplied allowing
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exposure of disk space assignment "slider" UI. Note however that the file system
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back-ends of `systemd-homed.service` have different feature sets. Specifically,
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only btrfs has online file system shrinking support, ext4 only offline file
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system shrinking support, and xfs no shrinking support at all (all three file
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systems support online file system growing however). This means if the LUKS
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back-end is used, disk space assignment cannot be instant for logged in users,
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unless btrfs is used.
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Note that only `systemd-homed` provides an API for modifying/creating/deleting
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users. The generic `userdb` subsystem (which might have other back-ends, besides
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`systemd-homed`, for example LDAP or Windows) exclusively provides a read-only
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interface. (This is unlikely to change, as the other back-ends might have very
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different concepts of adding or modifying users, i.e. might not even have any
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local concept for that at all). This means any user management UI that intends
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to change (and not just view) user accounts should talk directly to
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`systemd-homed` to make use of its features; there's no abstraction available
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to support other back-ends under the same API.
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Unfortunately there's currently no documentation for the `systemd-homed` D-Bus
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API. Consider using the `homectl` sources as guidelines for implementing a user
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management UI. The JSON user/records are well documented however, see above,
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and the D-Bus API provides limited introspection.
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## Relationship to `accounts-daemon`
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For a long time `accounts-daemon` has been included in Linux distributions
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providing richer user accounts. The functionality of this daemon overlaps in
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many areas with the functionality of `systemd-homed` or `userdb`, but there are
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systematic differences, which means that `systemd-homed` cannot replace
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`accounts-daemon` fully. Most importantly: `accounts-daemon` provides
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"side-car" metadata for *any* type of user account, while `systemd-homed` only
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provides additional metadata for the users it defines itself. In other words:
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`accounts-daemon` will augment foreign accounts; `systemd-homed` cannot be used
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to augment users defined elsewhere, for example in LDAP or as classic
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`/etc/passwd` records.
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This probably means that for the time being, a user management UI (or other UI)
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that wants to support rich user records with compatibility with the status quo
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ante should probably talk to both `systemd-homed` and `accounts-daemon` at the
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same time, and ignore `accounts-daemon`'s records if `systemd-homed` defines
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them. While I (Lennart) personally believe in the long run `systemd-homed` is
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the way to go for rich user records, any UI that wants to manage and support
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rich records for classic records has to support `accounts-daemon` in parallel
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for the time being.
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In the short term, it might make sense to also expose the `userdb` provided
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records via `accounts-daemon`, so that clients of the latter can consume them
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without changes. However, I think in the long run `accounts-daemon` should
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probably be removed from the general stack, hence this sounds like a temporary
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solution only.
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In case you wonder, there's no automatic mechanism for converting existing
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users registered in `/etc/passwd` or LDAP to users managed by
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`systemd-homed`. There's documentation for doing this manually though, see
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[Converting Existing Users to systemd-homed managed
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Users](https://systemd.io/CONVERTING_TO_HOMED).
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## Future Additions
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JSON user/group records are extensible, hence we can easily add any additional
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fields desktop environments require. For example pattern-based authentication
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is likely very useful on touch-based devices, and the user records should hence
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learn them natively. Fields for other authentication mechanisms, such as
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fingerprint authentication should be provided as well, eventually.
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It is planned to extend the `userdb` Varlink API to support look-ups by partial
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user name and real name (GECOS) data, so that log-in screens can optionally
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implement simple complete-as-you-type login screens.
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It is planned to extend the `systemd-homed` D-Bus API to instantly inform clients
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about hardware associated with a specific user being plugged in, to which login
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screens can listen in order to initiate authentication. Specifically, any
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YubiKey-like security token plugged in that is associated with a local user
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record should initiate authentication for that user, making typing in of the
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username unnecessary.
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