Provided by: systemd-boot_256.5-2ubuntu4_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/.../foo.efi.extra.d/*.sysext.raw
ESP/.../foo.efi.extra.d/*.confext.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 ".ucode" section with an initrd containing microcode, to be handed to the kernel
before any other initrd. This initrd must not be compressed.
• 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 ".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/*.sysext.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/*.confext.raw are packed up in a cpio archive and
placed in the /.extra/confext/ directory in the initrd file hierarchy. This is
supposed to be used to pass additional configuration extension images to the initrd.
See systemd-confext(8) for details on configuration extension images. The generated
cpio archive containing these system extension images is measured into TPM PCR 12 (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 both the
addon and the UKI contain 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 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:
Table 1. OS Resource PCR Summary
┌─────────────────────────────────┬─────────────────┐
│OS Resource │ Measurement PCR │
├─────────────────────────────────┼─────────────────┤
│systemd-stub code (the entry │ 4 │
│point of the unified PE binary) │ │
├─────────────────────────────────┼─────────────────┤
│Core kernel code (embedded in │ 4 + 11 │
│unified PE binary) │ │
├─────────────────────────────────┼─────────────────┤
│OS release information (embedded │ 4 + 11 │
│in the unified PE binary) │ │
├─────────────────────────────────┼─────────────────┤
│Main initrd (embedded in unified │ 4 + 9 + 11 │
│PE binary) │ │
├─────────────────────────────────┼─────────────────┤
│Microcode initrd (embedded in │ 4 + 9 + 11 │
│unified PE binary) │ │
├─────────────────────────────────┼─────────────────┤
│Default kernel command line │ 4 + 11 │
│(embedded in unified PE binary) │ │
├─────────────────────────────────┼─────────────────┤
│Overridden kernel command line │ 12 │
├─────────────────────────────────┼─────────────────┤
│Boot splash (embedded in the │ 4 + 11 │
│unified PE binary) │ │
├─────────────────────────────────┼─────────────────┤
│TPM2 PCR signature JSON │ 4 + 9 │
│(embedded in unified PE binary, │ │
│synthesized into initrd) │ │
├─────────────────────────────────┼─────────────────┤
│TPM2 PCR PEM public key │ 4 + 9 + 11 │
│(embedded in unified PE binary, │ │
│synthesized into initrd) │ │
├─────────────────────────────────┼─────────────────┤
│Credentials (synthesized initrd │ 9 + 12 │
│from companion files) │ │
├─────────────────────────────────┼─────────────────┤
│System Extensions (synthesized │ 9 + 13 │
│initrd from companion files) │ │
├─────────────────────────────────┼─────────────────┤
│Configuration Extensions │ 9 + 12 │
│(synthesized initrd 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 system 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.
StubPcrInitRDConfExts
The PCR register index the configuration 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. "12"). This variable is set if a measurement was successfully completed,
and remains unset otherwise.
Added in version 255.
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/*.sysext.raw
System extension image files (suffix ".sysext.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/confext/*.confext.raw
Configuration extension image files (suffix ".confext.raw") that are placed next to
the unified kernel image (as described above) are copied into the /.extra/confext/
directory in the initrd execution environment.
Added in version 255.
/.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. Unless systemd-stub detects it is running
inside a confidential computing environment, 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