Adding upstream version 255.4.upstream/255.4

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+---
+title: Home Directories
+category: Users, Groups and Home Directories
+layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
+---
+
+# Home Directories
+
+[`systemd-homed.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-homed.service.html)
+manages home directories of regular ("human") users. Each directory it manages
+encapsulates both the data store and the user record of the user, so that it
+comprehensively describes the user account, and is thus naturally portable
+between systems without any further, external metadata. This document describes
+the format used by these home directories, in the context of the storage
+mechanism used.
+
+## General Structure
+
+Inside of the home directory a file `~/.identity` contains the JSON formatted
+user record of the user. It follows the format defined in
+[`JSON User Records`](USER_RECORD). It is recommended to bring the
+record into 'normalized' form (i.e. all objects should contain their fields
+sorted alphabetically by their key) before storing it there, though this is not
+required nor enforced. Since the user record is cryptographically signed, the
+user cannot make modifications to the file on their own (at least not without
+corrupting it, or knowing the private key used for signing the record). Note
+that user records are stored here without their `binding`, `status` and
+`secret` sections, i.e. only with the sections included in the signature plus
+the signature section itself.
+
+## Storage Mechanism: Plain Directory/`btrfs` Subvolume
+
+If the plain directory or `btrfs` subvolume storage mechanism of
+`systemd-homed` is used (i.e. `--storage=directory` or `--storage=subvolume` on
+the
+[`homectl(1)`](https://www.freedesktop.org/software/systemd/man/homectl.html)
+command line) the home directory requires no special setup besides including
+the user record in the `~/.identity` file.
+
+It is recommended to name home directories managed this way by
+`systemd-homed.service` by the user name, suffixed with `.homedir` (example:
+`lennart.homedir` for a user `lennart`) but this is not enforced. When the user
+is logged in, the directory is generally mounted to `/home/$USER` (in our
+example: `/home/lennart`), thus dropping the suffix while the home directory is
+active. `systemd-homed` will automatically discover home directories named this
+way in `/home/*.homedir` and synthesize NSS user records for them as they show
+up.
+
+## Storage Mechanism: `fscrypt` Directories
+
+This storage mechanism is mostly identical to the plain directory storage
+mechanism, except that the home directory is encrypted using `fscrypt`. (Use
+`--storage=fscrypt` on the `homectl` command line.) Key management is
+implemented via extended attributes on the directory itself: for each password
+an extended attribute `trusted.fscrypt_slot0`, `trusted.fscrypt_slot1`,
+`trusted.fscrypt_slot2`, … is maintained. Its value contains a colon-separated
+pair of Base64 encoded data fields. The first field contains a salt value, the
+second field the encrypted volume key. The latter is encrypted using AES256 in
+counter mode, using a key derived from the password via PBKDF2-HMAC-SHA512,
+together with the salt value. The construction is similar to what LUKS does for
+`dm-crypt` encrypted volumes. Note that extended attributes are not encrypted
+by `fscrypt` and hence are suitable for carrying the key slots. Moreover, by
+using extended attributes, the slots are directly attached to the directory and
+an independent sidecar key database is not required.
+
+## Storage Mechanism: `cifs` Home Directories
+
+In this storage mechanism, the home directory is mounted from a CIFS server and
+service at login, configured inside the user record. (Use `--storage=cifs` on
+the `homectl` command line.) The local password of the user is used to log into
+the CIFS service. The directory share needs to contain the user record in
+`~/.identity` as well. Note that this means that the user record needs to be
+registered locally before it can be mounted for the first time, since CIFS
+domain and server information needs to be known *before* the mount. Note that
+for all other storage mechanisms it is entirely sufficient if the directories
+or storage artifacts are placed at the right locations — all information to
+activate them can be derived automatically from their mere availability.
+
+## Storage Mechanism: `luks` Home Directories
+
+This is the most advanced and most secure storage mechanism and consists of a
+Linux file system inside a LUKS2 volume inside a loopback file (or on removable
+media). (Use `--storage=luks` on the `homectl` command line.)  Specifically:
+
+* The image contains a GPT partition table. For now it should only contain a
+  single partition, and that partition must have the type UUID
+  `773f91ef-66d4-49b5-bd83-d683bf40ad16`. Its partition label must be the
+  user name.
+
+* This partition must contain a LUKS2 volume, whose label must be the user
+  name. The LUKS2 volume must contain a LUKS2 token field of type
+  `systemd-homed`. The JSON data of this token must have a `record` field,
+  containing a string with base64-encoded data. This data is the JSON user
+  record, in the same serialization as in `~/.identity`, though encrypted. The
+  JSON data of this token must also have an `iv` field, which contains a
+  base64-encoded binary initialization vector for the encryption. The
+  encryption used is the same as the LUKS2 volume itself uses, unlocked by the
+  same volume key, but based on its own IV.
+
+* Inside of this LUKS2 volume must be a Linux file system, one of `ext4`,
+  `btrfs` and `xfs`. The file system label must be the user name.
+
+* This file system should contain a single directory named after the user. This
+  directory will become the home directory of the user when activated. It
+  contains a second copy of the user record in the `~/.identity` file, like in
+  the other storage mechanisms.
+
+The image file should reside in a directory `/home/` on the system,
+named after the user, suffixed with `.home`. When activated, the container home
+directory is mounted to the same path, though with the `.home` suffix dropped —
+unless a different mount point is defined in the user record. (e.g.: the
+loopback file `/home/waldo.home` is mounted to `/home/waldo` while activated.)
+When the image is stored on removable media (such as a USB stick), the image
+file can be directly `dd`'ed onto it; the format is unchanged. The GPT envelope
+should ensure the image is properly recognizable as a home directory both when
+used in a loopback file and on a removable USB stick. (Note that when mounting
+a home directory from a USB stick, it too defaults to a directory in `/home/`,
+named after the username, with no further suffix.)
+
+Rationale for the GPT partition table envelope: this way the image is nicely
+discoverable and recognizable already by partition managers as a home
+directory. Moreover, when copied onto a USB stick the GPT envelope makes sure
+the stick is properly recognizable as a portable home directory
+medium. (Moreover, it allows embedding additional partitions later on, for
+example on a multi-purpose USB stick that contains both a home
+directory and a generic storage volume.)
+
+Rationale for including the encrypted user record in the LUKS2 header:
+Linux kernel file system implementations are generally not robust towards
+maliciously formatted file systems; there's a good chance that file system
+images can be used as attack vectors, exploiting the kernel. Thus it is
+necessary to validate the home directory image *before* mounting it and
+establishing a minimal level of trust. Since the user record data is
+cryptographically signed and user records not signed with a recognized private
+key are not accepted, a minimal level of trust between the system and the home
+directory image is established.
+
+Rationale for storing the home directory one level below to root directory of
+the contained file system: this way special directories such as `lost+found/`
+do not show up in the user's home directory.
+
+## Algorithm
+
+Regardless of the storage mechanism used, an activated home directory
+necessarily involves a mount point to be established. In case of the
+directory-based storage mechanisms (`directory`, `subvolume` and `fscrypt`)
+this is a bind mount. In case of `cifs` this is a CIFS network mount, and in
+case of the LUKS2 backend a regular block device mount of the file system
+contained in the LUKS2 image. By requiring a mount for all cases (even for
+those that already are a directory), a clear logic is defined to distinguish
+active and inactive home directories, so that the directories become
+inaccessible under their regular path the instant they are
+deactivated. Moreover, the `nosuid`, `nodev` and `noexec` flags configured in
+the user record are applied when the bind mount is established.
+
+During activation, the user records retained on the host, the user record
+stored in the LUKS2 header (in case of the LUKS2 storage mechanism) and the
+user record stored inside the home directory in `~/.identity` are
+compared. Activation is only permitted if they match the same user and are
+signed by a recognized key. When the three instances differ in `lastChangeUSec`
+field, the newest record wins, and is propagated to the other two locations.
+
+During activation, the file system checker (`fsck`) appropriate for the
+selected file system is automatically invoked, ensuring the file system is in a
+healthy state before it is mounted.
+
+If the UID assigned to a user does not match the owner of the home directory in
+the file system, the home directory is automatically and recursively `chown()`ed
+to the correct UID.
+
+Depending on the `luksDiscard` setting of the user record, either the backing
+loopback file is `fallocate()`ed during activation, or the mounted file system
+is `FITRIM`ed after mounting, to ensure the setting is correctly enforced.
+
+When deactivating a home directory, the file system or block device is trimmed
+or extended as configured in the `luksOfflineDiscard` setting of the user
+record.