SYSTEMD.IMAGE-POLICY(7)

NAME
systemd.image-policy - Disk Image Dissection Policy

DESCRIPTION

In systemd, whenever a disk image (DDI) implementing the UAPI.2 Discoverable Partitions Specification[1] is activated, a policy may be specified controlling which partitions to mount and what kind of cryptographic protection to require, and which type of filesystem is allowed. Such a disk image dissection policy is a string that contains per-partition-type rules, separated by colons (":"). The individual rules consist of a partition identifier, an equal sign ("="), and one or more flags which may be set per partition. If multiple flags are specified per partition they are separated by a plus sign ("+").

The partition identifiers currently defined are: root, usr, home, srv, esp, xbootldr, swap, root-verity, root-verity-sig, usr-verity, usr-verity-sig, tmp, var. These identifiers match the relevant partition types in the Discoverable Partitions Specification, but are agnostic to CPU architectures. If the partition identifier is left empty it defines the default policy for partitions defined in the Discoverable Partitions Specification for which no policy flags are explicitly listed in the policy string.

The following partition policy flags are defined that dictate the existence/absence, the use, and the protection level of partitions:

    • unprotected for partitions that shall exist and be used, but shall come without cryptographic protection, lacking both Verity authentication and LUKS encryption.

    • verity for partitions that shall exist and be used, with Verity authentication. (Note: if a DDI image carries a data partition, along with a Verity partition and a signature partition for it, and only the verity flag is set (signed is not), then the image will be set up with Verity, but the signature data will not be used. Or in other words: any DDI with a set of partitions that qualify for signature also implicitly qualifies for verity, and in fact also unprotected).

    • signed for partitions that shall exist and be used, with Verity authentication, which are also accompanied by a PKCS#7 signature of the Verity root hash.

    • encrypted for partitions which shall exist and be used and are encrypted with LUKS.

    • unused for partitions that shall exist but shall not be used.

    • absent for partitions that shall not exist on the image.

The following filesystem policy flags are defined that dictate which filesystem types are allowed for which partition:

Table 1. Filesystems types supported by the policy
Filesystem flag
"btrfs"
"erofs"
"ext4"
"f2fs"
"squashfs"
"vfat"
"xfs"

By setting a combination of the flags above, alternatives can be declared. For example the combination "unused+absent" means: the partition may exist (in which case it shall not be used) or may be absent. The combination of "unprotected+verity+signed+encrypted+unused+absent" may be specified via the special shortcut "open", and indicates that the partition may exist or may be absent, but if it exists is used, regardless of the protection level.

As special rule: if none of the flags above are set for a listed partition identifier, the default policy of open is implied, i.e. setting none of these flags listed above means effectively all flags listed above will be set.

The following partition policy flags are defined that dictate the state of specific GPT partition flags:

    • read-only-off, read-only-on to require that the partitions have the read-only partition flag off or on.

    • growfs-off, growfs-on to require that the partitions have the growfs partition flag off or on.

If both read-only-off and read-only-on are set for a partition, then the state of the read-only flag on the partition is not dictated by the policy. Setting neither flag is equivalent to setting both, i.e. setting neither of these two flags means effectively both will be set. A similar logic applies to growfs-off/growfs-on.

If partitions are not listed within an image policy string, the default policy flags are applied (configurable via an empty partition identifier, see above). If no default policy flags are configured in the policy string, it is implied to be "absent+unused", except for the Verity partition and their signature partitions where the policy is automatically derived from minimal protection level of the data partition they protect, as encoded in the policy.

SPECIAL POLICIES

The special image policy string "*" is short for "use everything", i.e. is equivalent to:

    =verity+signed+encrypted+unprotected+unused+absent

The special image policy string "-" is short for "use nothing", i.e. is equivalent to:

    =unused+absent

The special image policy string "~" is short for "everything must be absent", i.e. is equivalent to:

    =absent

USE

Most systemd components that support operating with disk images support a --image-policy= command line option to specify the image policy to use, and default to relatively open policies (typically the "*" policy, as described above), under the assumption that trust in disk images is established before the images are passed to the program in question.

For the host image itself systemd-gpt-auto-generator(8) is responsible for processing the GPT partition table and making use of the included discoverable partitions. It accepts an image policy via the kernel command line option systemd.image_policy=.

Note that image policies do not dictate how the components will mount and use disk images — they only dictate which parts to avoid and which protection level and arrangement to require while mounting/using them. For example, systemd-sysext(8) only cares for the /usr/ and /opt/ trees inside a disk image, and thus ignores any /home/ partitions (and similar) in all cases, which might be included in the image, regardless whether the configured image policy would allow access to it or not. Similar, systemd-nspawn(1) is not going to make use of any discovered swap device, regardless of whether the policy would allow that or not.

Use the image-policy command of the systemd-analyze(1) tool to analyze image policy strings, and determine what a specific policy string means for a specific partition.

EXAMPLES

The following image policy string dictates one read-only Verity-enabled /usr/ partition must exist, plus encrypted root and swap partitions. All other partitions are ignored:

    usr=verity+read-only-on:root=encrypted:swap=encrypted

The following image policy string dictates an encrypted, writable root file system, and optional /srv/ file system that must be encrypted if it exists and no swap partition may exist:

    root=encrypted+read-only-off:srv=encrypted+absent:swap=absent

The following image policy string dictates a single root partition that may be encrypted, but does not have to be, and ignores swap partitions, and uses all other partitions if they are available, possibly with encryption.

    root=unprotected+encrypted:swap=absent+unused:=unprotected+encrypted+absent

The following image policy string dictates a single root partition that can only be erofs or squashfs, and ignores swap partitions, and uses all other partitions if they are available, possibly with encryption.

    root=erofs+squashfs:swap=absent+unused:=unprotected+encrypted+absent

SEE ALSO

systemd(1), systemd-dissect(1), systemd-gpt-auto-generator(8), systemd-sysext(8), systemd-analyze(1)

NOTES

1.  UAPI.2 Discoverable Partitions Specification
    https://uapi-group.org/specifications/specs/discoverable_partitions_specification