systemd-stub(7)

.SH "NAME" systemd-stub, sd-stub, linuxx64.efi.stub, linuxia32.efi.stub, linuxaa64.efi.stub - A simple UEFI kernel boot stub

.SH "SYNOPSIS"

.PP

.SH "DESCRIPTION"

.PP systemd-stub (stored in per-architecture files linuxx64.efi.stub, linuxia32.efi.stub, linuxaa64.efi.stub on disk) is a simple UEFI boot stub. An UEFI boot stub is attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux kernel is executable as regular UEFI binary, and is able to do various preparations before switching the system into the Linux world.

.PP The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary itself. This allows combining various resources inside a single PE binary image (a "Unified Kernel Image" or "UKI" for short), which may then be signed via UEFI SecureBoot as a whole, covering all individual resources at once. Specifically it may include the following PE sections:

.PP In a basic UKI, the sections listed above appear at most once, with the exception of .dtbauto and .efifw sections. In a multi-profile UKI, multiple sets of these sections are present in a single file and form "profiles", one of which can be selected at boot. For this, the PE section .profile is defined to be used as the separator between sets of sections. The .profile section itself may contain meta-information about the section, and follows a similar structure as the contents of the .osrel section. For further details about multi-profile UKIs, see below.
.PP If UEFI SecureBoot is enabled and the .cmdline section is present in the executed image, any attempts to override the kernel command line by passing one as invocation parameters to the EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI SecureBoot, or do not include a kernel command line PE section in the kernel image file. If a command line is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is present).
.PP If a DeviceTree is embedded in the .dtb section, it replaces an existing DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the EFI_DT_FIXUP_PROTOCOL for hardware specific fixups to the DeviceTree.

.PP The contents of 11 of these 12 sections are measured into TPM PCR 11. It is otherwise not used and thus the result can be pre-calculated without too much effort. The .pcrsig section is not included in this PCR measurement, since it is supposed to contain signatures for the output of the measurement operation, and thus cannot also be input to it. If an UKI contains multiple profiles, only the PE sections of the selected profile (and those of the base profile, except if overridden) are measured.

.PP If non-zero, the selected numeric profile is measured into PCR 12.

.PP When .pcrsig and/or .pcrpkey sections are present in a unified kernel image their contents are passed to the booted kernel in an synthetic initrd cpio archive that places them in the /.extra/tpm2-pcr-signature.json and /.extra/tpm2-pcr-public-key.pem files. Typically, a tmpfiles.d(5) line then ensures they are copied into /run/systemd/tpm2-pcr-signature.json and /run/systemd/tpm2-pcr-public-key.pem where they remain accessible even after the system transitions out of the initrd environment into the host file system. Tools such systemd-cryptsetup@.service(8), systemd-cryptenroll(1) and systemd-creds(1) will automatically use files present under these paths to unlock protected resources (encrypted storage or credentials) or bind encryption to booted kernels.

.PP For further details about the UKI concept, see the UAPI.5 UKI specification[3].

.SH "COMPANION FILES"

.PP The systemd-stub UEFI boot stub automatically collects three types of auxiliary companion files optionally placed in drop-in directories on the same partition as the EFI binary, dynamically generates cpio initrd archives from them, and passes them to the kernel. Specifically:

.PP These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed by systemd-creds encrypt -T (see systemd-creds(1) for details); in case of the system extension images by using signed Verity images.

.PP Note that earlier components of the boot process might register additional initrds, and thus additional "companion" resources such as system extensions, configuration extensions and credentials for consumption by the kernel and OS eventually booted. For example, systemd-boot(7) does this for resources configured in UAPI.1 Type #1 extra lines. systemd-stub will combine any resources provided that way with the companion file resources it acquires itself.

.SH "MULTI-PROFILE UKIS"

.PP In many contexts it is useful to allow invocation of a single UKI in multiple different modes (or "profiles") without compromising the cryptographic integrity, measurements and so on of the boot process. For example, a single UKI might provide three distinct profiles: a regular boot one, one that invokes a "factory reset" operation, and one that boots into a storage target mode (as in systemd-storagetm.service(8)). Each profile would then use the same .linux and .initrd sections, but would have a separate .cmdline section. For example the latter two profiles would extend the regular kernel command line with systemd.unit=factory-reset.target or rd.systemd.unit=storagetm.target.

.PP A single UKI may support multiple profiles by means of the special .profile PE section. This section acts as separator between the PE sections of the individual profiles. .profile PE sections hence may appear multiple times in a single UKI, and the other PE sections listed above may appear multiple times too, if .profile are used, but only once before the first .profile section, once between each subsequent pair, and once after the last appearance of .profile. The sections listed before the first .profile are considered the "base" profile of the UKI. Each .profile section then introduces a new profile, which are numbered starting from zero. The PE sections following each .profile are specific to that profile. When booting into a specific profile, the base section's profiles are used in combination with the specific profile's sections: if the same section is defined in both, the per-profile section overrides the base profile's version, otherwise the base profile sections are used.

.PP A UKI that contains no .profile is consider equivalent to one that just contains a single .profile, as having only a single profile @0.

.PP Here's a simple example for a multi-profile UKI's sections, inspired by the setup suggested above:

.sp
Table 1. Multi-Profile UKI Example

.PP The section list above would define three profiles. The first four sections make up the base profile. A .profile section then introduces profile @0. It does not override any sections (or add any) from the base section, hence it is immediately followed by another .profile section that then introduces section @1. This profile overrides the kernel command line. Finally, the last two sections define section @2, again overriding the command line. (Note that in this example the first .cmdline could also moved behind the first .profile with equivalent effect. To keep things nicely extensible, it is probably a good idea to keep the generic command line in the base section instead of profile 0, in case later added profiles might want to reuse it.)

.PP The profile to boot may be controlled via the UKI's own command line: if the first argument starts with @, followed by a positive integer number in decimal, it selects the profile to boot into. If the first argument is not specified like that, the UKI will automatically boot into profile 0.

.PP A .profile section may contain meta-information about the profile. It follows a similar format as .osrel (i.e. an environment-variable-assignment-block-like list of newline separated strings). Currently two fields are defined: ID= is supposed to carry a short identifying string that identifies the profile (e.g. ID=factory-reset). TITLE= should contain a human-readable string that may appear in the boot menu entry for this profile (e.g. TITLE='Factory Reset this Device').

.SH "TPM PCR NOTES"

.PP Note that when a unified kernel using systemd-stub is invoked the firmware will measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the core kernel, the embedded initrd and kernel command line (see above for a full list), including all UKI profiles.

.PP Also note that when systemd-stub measures a PE section, it will measure the amount of bytes that the section takes up in memory (VirtualSize) and not the amount of bytes that the section takes up on disk (SizeOfRawData). This means that if the size in memory is larger than the size on disk, systemd-stub will end up measuring extra zeroes. To avoid this from happening, it is recommended to make sure that the size in memory of each section that is measured by systemd-stub is always smaller than or equal to the size on disk. ukify automatically makes sure this is the case when building UKIs or addons.

.PP Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means every type of initrd (of the selected UKI profile) will possibly be measured two or three times: the initrds embedded in the kernel image will be measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials (and the one synthesized from configuration extensions) will be measured to both PCR 9 and PCR 12; the initrd synthesized from system extensions will be measured to both PCR 4 and PCR 9. Let's summarize the OS resources and the PCRs they are measured to:

.sp
Table 2. OS Resource PCR Summary

.SH "EFI VARIABLES"

.PP The following EFI variables are defined, set and read by systemd-stub, under the vendor UUID 4a67b082-0a4c-41cf-b6c7-440b29bb8c4f, for communication between the boot stub and the OS:

.PP LoaderDevicePartUUID