Provided by: systemd_255.4-1ubuntu8.11_amd64 
NAME
systemd-stub, sd-stub, linuxx64.efi.stub, linuxia32.efi.stub, linuxaa64.efi.stub - A simple UEFI kernel
boot stub
SYNOPSIS
/usr/lib/systemd/boot/efi/linuxx64.efi.stub
/usr/lib/systemd/boot/efi/linuxia32.efi.stub
/usr/lib/systemd/boot/efi/linuxaa64.efi.stub
ESP/.../foo.efi.extra.d/*.addon.efi
ESP/.../foo.efi.extra.d/*.cred
ESP/.../foo.efi.extra.d/*.raw
ESP/loader/addons/*.addon.efi
ESP/loader/credentials/*.cred
DESCRIPTION
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.
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 (usually called "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:
• A ".linux" section with the ELF Linux kernel image.
• An ".osrel" section with OS release information, i.e. the contents of the os-release(5) file of the
OS the kernel belongs to.
• A ".cmdline" section with the kernel command line to pass to the invoked kernel.
• An ".initrd" section with the initrd.
• A ".splash" section with an image (in the Windows .BMP format) to show on screen before invoking the
kernel.
• A ".dtb" section with a compiled binary DeviceTree.
• A ".uname" section with the kernel version information, i.e. the output of uname -r for the kernel
included in the ".linux" section.
• An ".sbat" section with SBAT[1] revocation metadata.
• A ".pcrsig" section with a set of cryptographic signatures for the expected TPM2 PCR values after the
kernel has been booted, in JSON format. This is useful for implementing TPM2 policies that bind disk
encryption and similar to kernels that are signed by a specific key.
• A ".pcrpkey" section with a public key in the PEM format matching the signature data in the the
".pcrsig" section.
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
don't 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).
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.
The contents of eight of these nine 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.
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.
For further details about the UKI concept, see the UKI specification[2].
COMPANION FILES
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:
• For a kernel binary called foo.efi, it will look for files with the .cred suffix in a directory named
foo.efi.extra.d/ next to it. If the kernel binary uses a counter for the purpose of Automatic Boot
Assessment[3], this counter will be ignored. For example, foo+3-0.efi will look in directory
foo.efi.extra.d/. A cpio archive is generated from all files found that way, placing them in the
/.extra/credentials/ directory of the initrd file hierarchy. The main initrd may then access them in
this directory. This is supposed to be used to store auxiliary, encrypted, authenticated credentials
for use with LoadCredentialEncrypted= in the UEFI System Partition. See systemd.exec(5) and systemd-
creds(1) for details on encrypted credentials. The generated cpio archive is measured into TPM PCR 12
(if a TPM is present).
• Similarly, files foo.efi.extra.d/*.raw are packed up in a cpio archive and placed in the
/.extra/sysext/ directory in the initrd file hierarchy. This is supposed to be used to pass
additional system extension images to the initrd. See systemd-sysext(8) for details on system
extension images. The generated cpio archive containing these system extension images is measured
into TPM PCR 13 (if a TPM is present).
• Similarly, files foo.efi.extra.d/*.addon.efi are loaded and verified as PE binaries, and a ".cmdline"
section is parsed from them. Addons are supposed to be used to pass additional kernel command line
parameters or Devicetree blobs, regardless of the kernel image being booted, for example to allow
platform vendors to ship platform-specific configuration.
In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a
a ".uname" section, the addon will be rejected if they do not match exactly. It is recommended to
always add a ".sbat" section to all signed addons, so that they may be revoked with a SBAT policy
update, without requiring blocklisting via DBX/MOKX. The ukify(1) tool will add a SBAT policy by
default if none is passed when building addons. For more information on SBAT see Shim
documentation[1].
Addon files are sorted, loaded, and measured into TPM PCR 12 (if a TPM is present) and appended to
the kernel command line. UKI command line options are listed first, then options from addons in
/loader/addons/*.addon.efi, and finally UKI-specific addons. Device tree blobs are loaded and
measured following the same algorithm. Addons are always loaded in the same order based on the
filename, so that, given the same set of addons, the same set of measurements can be expected in
PCR12. However, note that the filename is not protected by the PE signature, and as such an attacker
with write access to the ESP could potentially rename these files to change the order in which they
are loaded, in a way that could alter the functionality of the kernel, as some options might be
order-dependent. If you sign such addons, you should pay attention to the PCR12 values and make use
of an attestation service so that improper use of your signed addons can be detected and dealt with
using one of the aforementioned revocation mechanisms.
• Files /loader/credentials/*.cred are packed up in a cpio archive and placed in the
/.extra/global_credentials/ directory of the initrd file hierarchy. This is supposed to be used to
pass additional credentials to the initrd, regardless of the kernel being booted. The generated cpio
archive is measured into TPM PCR 12 (if a TPM is present).
• Additionally, files /loader/addons/*.addon.efi are loaded and verified as PE binaries, and ".cmdline"
and/or ".dtb" sections are parsed from them. This is supposed to be used to pass additional command
line parameters or Devicetree blobs to the kernel, regardless of the kernel being booted.
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.
TPM PCR NOTES
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).
Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means every
type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials 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:
Table 1. OS Resource PCR Summary
┌───────────────────────────────────────┬─────────────────┐
│ OS Resource │ Measurement PCR │
├───────────────────────────────────────┼─────────────────┤
│ systemd-stub code (the entry point of │ 4 │
│ the unified PE binary) │ │
├───────────────────────────────────────┼─────────────────┤
│ Core kernel code (embedded in unified │ 4 + 11 │
│ PE binary) │ │
├───────────────────────────────────────┼─────────────────┤
│ OS release information (embedded in │ 4 + 11 │
│ the unified PE binary) │ │
├───────────────────────────────────────┼─────────────────┤
│ Main initrd (embedded in unified PE │ 4 + 9 + 11 │
│ binary) │ │
├───────────────────────────────────────┼─────────────────┤
│ Default kernel command line (embedded │ 4 + 11 │
│ in unified PE binary) │ │
├───────────────────────────────────────┼─────────────────┤
│ Overridden kernel command line │ 12 │
├───────────────────────────────────────┼─────────────────┤
│ Boot splash (embedded in the unified │ 4 + 11 │
│ PE binary) │ │
├───────────────────────────────────────┼─────────────────┤
│ TPM2 PCR signature JSON (embedded in │ 4 + 9 │
│ unified PE binary, synthesized into │ │
│ initrd) │ │
├───────────────────────────────────────┼─────────────────┤
│ TPM2 PCR PEM public key (embedded in │ 4 + 9 + 11 │
│ unified PE binary, synthesized into │ │
│ initrd) │ │
├───────────────────────────────────────┼─────────────────┤
│ Credentials (synthesized initrd from │ 9 + 12 │
│ companion files) │ │
├───────────────────────────────────────┼─────────────────┤
│ System Extensions (synthesized initrd │ 9 + 13 │
│ from companion files) │ │
└───────────────────────────────────────┴─────────────────┘
EFI VARIABLES
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:
LoaderDevicePartUUID
Contains the partition UUID of the EFI System Partition the EFI image was run from. systemd-gpt-
auto-generator(8) uses this information to automatically find the disk booted from, in order to
discover various other partitions on the same disk automatically.
Added in version 250.
LoaderFirmwareInfo, LoaderFirmwareType
Brief firmware information. Use bootctl(1) to view this data.
Added in version 250.
LoaderImageIdentifier
The path of EFI executable, relative to the EFI System Partition's root directory. Use bootctl(1) to
view this data.
Added in version 250.
StubInfo
Brief stub information. Use bootctl(1) to view this data.
Added in version 250.
StubPcrKernelImage
The PCR register index the kernel image, initrd image, boot splash, devicetree database, and the
embedded command line are measured into, formatted as decimal ASCII string (e.g. "11"). This
variable is set if a measurement was successfully completed, and remains unset otherwise.
Added in version 252.
StubPcrKernelParameters
The PCR register index the kernel command line and credentials are measured into, formatted as
decimal ASCII string (e.g. "12"). This variable is set if a measurement was successfully completed,
and remains unset otherwise.
Added in version 252.
StubPcrInitRDSysExts
The PCR register index the systemd extensions for the initrd, which are picked up from the file
system the kernel image is located on. Formatted as decimal ASCII string (e.g. "13"). This variable
is set if a measurement was successfully completed, and remains unset otherwise.
Added in version 252.
Note that some of the variables above may also be set by the boot loader. The stub will only set them if
they aren't set already. Some of these variables are defined by the Boot Loader Interface[4].
INITRD RESOURCES
The following resources are passed as initrd cpio archives to the booted kernel, and thus make up the
initial file system hierarchy in the initrd execution environment:
/
The main initrd from the ".initrd" PE section of the unified kernel image.
Added in version 252.
/.extra/credentials/*.cred
Credential files (suffix ".cred") that are placed next to the unified kernel image (as described
above) are copied into the /.extra/credentials/ directory in the initrd execution environment.
Added in version 252.
/.extra/global_credentials/*.cred
Similarly, credential files in the /loader/credentials/ directory in the file system the unified
kernel image is placed in are copied into the /.extra/global_credentials/ directory in the initrd
execution environment.
Added in version 252.
/.extra/sysext/*.raw
System extension image files (suffix ".raw") that are placed next to the unified kernel image (as
described above) are copied into the /.extra/sysext/ directory in the initrd execution environment.
Added in version 252.
/.extra/tpm2-pcr-signature.json
The TPM2 PCR signature JSON object included in the ".pcrsig" PE section of the unified kernel image
is copied into the /.extra/tpm2-pcr-signature.json file in the initrd execution environment.
Added in version 252.
/.extra/tpm2-pcr-pkey.pem
The PEM public key included in the ".pcrpkey" PE section of the unified kernel image is copied into
the /.extra/tpm2-pcr-public-key.pem file in the initrd execution environment.
Added in version 252.
Note that all these files are located in the "tmpfs" file system the kernel sets up for the initrd file
hierarchy and are thus lost when the system transitions from the initrd execution environment into the
host file system. If these resources shall be kept around over this transition they need to be copied to
a place that survives the transition first, for example via a suitable tmpfiles.d(5) line. By default,
this is done for the TPM2 PCR signature and public key files.
SMBIOS TYPE 11 STRINGS
systemd-stub can be configured using SMBIOS Type 11 strings. Applicable strings consist of a name,
followed by "=", followed by the value. systemd-stub will search the table for a string with a specific
name, and if found, use its value. The following strings are read:
io.systemd.stub.kernel-cmdline-extra
If set, the value of this string is added to the list of kernel command line arguments that are
measured in PCR12 and passed to the kernel.
Added in version 254.
ASSEMBLING KERNEL IMAGES
In order to assemble a bootable Unified Kernel Image from various components as described above, use
ukify(1).
SEE ALSO
systemd-boot(7), systemd.exec(5), systemd-creds(1), systemd-sysext(8), Boot Loader Specification[5], Boot
Loader Interface[4], ukify(1), systemd-measure(1), TPM2 PCR Measurements Made by systemd[6]
NOTES
1. SBAT
https://github.com/rhboot/shim/blob/main/SBAT.md
2. UKI specification
https://uapi-group.org/specifications/specs/unified_kernel_image/
3. Automatic Boot Assessment
https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT
4. Boot Loader Interface
https://systemd.io/BOOT_LOADER_INTERFACE
5. Boot Loader Specification
https://uapi-group.org/specifications/specs/boot_loader_specification
6. TPM2 PCR Measurements Made by systemd
https://systemd.io/TPM2_PCR_MEASUREMENTS
systemd 255 SYSTEMD-STUB(7)