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