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diff --git a/docs/HOME_DIRECTORY.md b/docs/HOME_DIRECTORY.md index f1b7faf..2efabae 100644 --- a/docs/HOME_DIRECTORY.md +++ b/docs/HOME_DIRECTORY.md @@ -8,24 +8,24 @@ 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 +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 +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. @@ -39,127 +39,119 @@ 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/$USER` (in our -example: `/home/lennart`), thus dropping the suffix while the home directory is -active. `systemd-homed` will automatically discover home directories named this -way in `/home/*.homedir` and synthesize NSS user records for them as they show -up. +`systemd-homed.service` by the user name, suffixed with `.homedir` +(example: `lennart.homedir` for a user `lennart`) but this is not enforced. +When the user is logged in, the directory is generally mounted to `/home/$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 +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 +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 +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. +* 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. +* 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.) +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/`, +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 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. +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 homedirectory 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. +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. +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. +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 |