kernel-command-line

title: kernel-command-line(7)

summary: Kernel command line parameters

text:
# kernel-command-line(7)

## NAME
kernel-command-line - Kernel command line parameters

## SYNOPSIS
/proc/cmdline

## DESCRIPTION
The kernel, the programs running in the initrd and in the host system may be configured at boot via kernel command line arguments. In addition, various systemd tools look at the EFI variable "SystemdOptions" (if available). Both sources are combined, but the kernel command line has higher priority. Please note that the EFI variable is only used by systemd tools, and is ignored by the kernel and other user space tools, so it is not a replacement for the kernel command line.

For command line parameters understood by the kernel, please see kernel-parameters.html[1] and bootparam(7).

For command line parameters understood by the initrd, see the documentation of the specific initrd implementation of your installation.

## CORE OS COMMAND LINE ARGUMENTS
systemd.unit=, rd.systemd.unit=, systemd.dump_core, systemd.crash_chvt, systemd.crash_shell, systemd.crash_action=, systemd.confirm_spawn, systemd.service_watchdogs, systemd.show_status, systemd.status_unit_format=, systemd.log_target=, systemd.log_level=, systemd.log_location=, systemd.log_color, systemd.log_ratelimit_kmsg, systemd.default_standard_output=, systemd.default_standard_error=, systemd.setenv=, systemd.machine_id=, systemd.set_credential=, systemd.set_credential_binary=, systemd.import_credentials=, systemd.reload_limit_interval_sec=, systemd.reload_limit_burst=

Parameters understood by the system and service manager to control system behavior. For details, see systemd(1). Added in version 186.

systemd.mask=, systemd.wants=, systemd.debug_shell, systemd.default_debug_tty=

Additional parameters understood by systemd-debug-generator(8), to mask or start specific units at boot, or invoke a debug shell on tty9. Added in version 215.

systemd.run=, systemd.run_success_action=, systemd.run_failure_action=

Additional parameters understood by systemd-run-generator(8), to run a command line specified on the kernel command line as system service after booting up. Added in version 240.

systemd.early_core_pattern=

During early boot, the generation of core dump files is disabled until a core dump handler (if any) takes over. This parameter allows specifying an absolute path where core dump files should be stored until a handler is installed. The path should be absolute and may contain specifiers, see core(5) for details. Added in version 240.

systemd.restore_state=

This parameter is understood by several system tools to control whether or not they should restore system state from the previous boot. For details, see systemd-backlight@.service(8) and systemd-rfkill.service(8). Added in version 209.

systemd.ssh_auto=, systemd.ssh_listen=

These parameters are interpreted by systemd-ssh-generator(8) and may be used to control SSH sockets the system shall be reachable on. Added in version 256.

systemd.volatile=

This parameter controls whether the system shall boot up in volatile mode. Takes a boolean argument, or the special values "state" or "overlay". If false (the default), normal boot mode is selected, the root directory and /var/ are mounted as specified on the kernel command line or /etc/fstab, or otherwise configured. If true, full state-less boot mode is selected. In this case, the root directory is mounted as volatile memory file system ("tmpfs"), and only /usr/ is mounted from the file system configured as root device, in read-only mode. This enables fully state-less boots were the vendor-supplied OS is used as shipped, with only default configuration and no stored state in effect, as /etc/ and /var/ (as well as all other resources shipped in the root file system) are reset at boot and lost on shutdown. If this setting is set to "state" the root file system is mounted read-only, however /var/ is mounted as a volatile memory file system ("tmpfs"), so that the system boots up with the normal configuration applied, but all state reset at boot and lost at shutdown. If this setting is set to "overlay" the root file system is set up as "overlayfs" mount combining the read-only root directory with a writable "tmpfs", so that no modifications are made to disk, but the file system may be modified nonetheless with all changes being lost at reboot. For details, see systemd-volatile-root.service(8) and systemd-fstab-generator(8). Added in version 233.

quiet

Parameter understood by both the kernel and the system and service manager to control console log verbosity. For details, see systemd(1). Added in version 186.

debug

Parameter understood by both the kernel and the system and service manager to control console log verbosity. For details, see systemd(1). Added in version 205.

-b, rd.emergency, emergency, rd.rescue, rescue, single, s, S, 1, 2, 3, 4, 5

Parameters understood by the system and service manager, as compatibility and convenience options. For details, see systemd(1). Added in version 186.

locale.LANG=, locale.LANGUAGE=, locale.LC_CTYPE=, locale.LC_NUMERIC=, locale.LC_TIME=, locale.LC_COLLATE=, locale.LC_MONETARY=, locale.LC_MESSAGES=, locale.LC_PAPER=, locale.LC_NAME=, locale.LC_ADDRESS=, locale.LC_TELEPHONE=, locale.LC_MEASUREMENT=, locale.LC_IDENTIFICATION=

Parameters understood by the system and service manager to control locale and language settings. For details, see systemd(1). Added in version 186.

fsck.mode=, fsck.repair=

Parameters understood by the file system checker services. For details, see systemd-fsck@.service(8). Added in version 186.

quotacheck.mode=

Parameter understood by the file quota checker service. For details, see systemd-quotacheck.service(8). Added in version 186.

systemd.journald.forward_to_syslog=, systemd.journald.forward_to_kmsg=, systemd.journald.forward_to_console=, systemd.journald.forward_to_wall=

Parameters understood by the journal service. For details, see systemd-journald.service(8). Added in version 186.

vconsole.keymap=, vconsole.keymap_toggle=, vconsole.font=, vconsole.font_map=, vconsole.font_unimap=

Parameters understood by the virtual console setup logic. For details, see vconsole.conf(5). Added in version 186.

udev.log_level=, rd.udev.log_level=, udev.children_max=, rd.udev.children_max=, udev.exec_delay=, rd.udev.exec_delay=, udev.event_timeout=, rd.udev.event_timeout=, udev.timeout_signal=, rd.udev.timeout_signal=, udev.blockdev_read_only, rd.udev.blockdev_read_only, net.ifnames=, net.naming_scheme=

Parameters understood by the device event managing daemon. For details, see systemd-udevd.service(8). Added in version 186.

plymouth.enable=

May be used to disable the Plymouth boot splash. For details, see plymouth(8). Added in version 186.

luks=, rd.luks=, luks.crypttab=, rd.luks.crypttab=, luks.name=, rd.luks.name=, luks.uuid=, rd.luks.uuid=, luks.options=, rd.luks.options=, luks.key=, rd.luks.key=

Configures the LUKS full-disk encryption logic at boot. For details, see systemd-cryptsetup-generator(8). Added in version 186.

fstab=, rd.fstab=

Configures the /etc/fstab logic at boot. For details, see systemd-fstab-generator(8). Added in version 186.

root=, rootfstype=, rootflags=, ro, rw

Configures the root file system and its file system type and mount options, as well as whether it shall be mounted read-only or read-write initially. For details, see systemd-fstab-generator(8). If root= is not set (or set to "gpt-auto") the automatic root partition discovery implemented by systemd-gpt-auto-generator(8) will be in effect. In this case, rootfstype=, rootflags=, ro, rw will be interpreted by systemd-gpt-auto-generator.

Added in version 215.

mount.usr=, mount.usrfstype=, mount.usrflags=

Configures the /usr file system (if required) and its file system type and mount options. For details, see systemd-fstab-generator(8). Added in version 235.

veritytab=, rd.veritytab=, roothash=, systemd.verity=, rd.systemd.verity=, systemd.verity_root_data=, systemd.verity_root_hash=, systemd.verity_root_options=, usrhash=, systemd.verity_usr_data=, systemd.verity_usr_hash=, systemd.verity_usr_options=

Configures the integrity protection root hash for the root and /usr file systems, and other related parameters. For details, see systemd-veritysetup-generator(8). Added in version 233.

systemd.getty_auto=

Configures whether the serial-getty@.service will run. For details, see systemd-getty-generator(8). Added in version 250.

systemd.gpt_auto=, rd.systemd.gpt_auto=

Configures whether GPT-based partition auto-discovery shall be attempted. For details, see systemd-gpt-auto-generator(8). Added in version 215.

systemd.image_policy=

When GPT-based partition auto-discovery is used, configures the image dissection policy string to apply, as per systemd.image-policy(7). For details see systemd-gpt-auto-generator(8). Added in version 254.

systemd.default_timeout_start_sec=

Overrides the default start job timeout DefaultTimeoutStartSec= at boot. For details, see systemd-system.conf(5). Added in version 230.

systemd.default_device_timeout_sec=

Overrides the default device timeout DefaultDeviceTimeoutSec= at boot. For details, see systemd-system.conf(5). Added in version 254.

systemd.watchdog_device=

Overrides the watchdog device path WatchdogDevice=. For details, see systemd-system.conf(5). Added in version 236.

systemd.watchdog_sec=

Overrides the watchdog timeout settings otherwise configured with RuntimeWatchdog=, RebootWatchdog= and KExecWatchdogSec=. Takes a time value (if no unit is specified, seconds is the implicitly assumed time unit) or the special strings "off" or "default". For details, see systemd-system.conf(5). Added in version 250.

systemd.watchdog_pre_sec=

Overrides the watchdog pre-timeout settings otherwise configured with RuntimeWatchdogPreSec=. Takes a time value (if no unit is specified, seconds is the implicitly assumed time unit) or the special strings "off" or "default". For details, see systemd-system.conf(5). Added in version 251.

systemd.watchdog_pretimeout_governor=

Overrides the watchdog pre-timeout settings otherwise configured with RuntimeWatchdogPreGovernor=. Takes a string value. For details, see systemd-system.conf(5). Added in version 251.

systemd.cpu_affinity=

Overrides the CPU affinity mask for the service manager and the default for all child processes it forks. This takes precedence over CPUAffinity=, see systemd-system.conf(5) for details. Added in version 245.

modules_load=, rd.modules_load=

Load a specific kernel module early at boot. For details, see systemd-modules-load.service(8). Added in version 187.

nameserver=, domain=

Configures DNS server information and search domains, see systemd-resolved.service(8) for details. Added in version 253.

resume=, resumeflags=

Enable resume from hibernation using the specified device and timeout options. All fstab(5)-style device identifiers are supported. For details, see systemd-hibernate-resume-generator(8). Added in version 217.

resume_offset=

Configure the page offset of the swap space from the resume device. For details, see systemd-hibernate-resume-generator(8). Added in version 254.

systemd.firstboot=

Takes a boolean argument, defaults to on. If off, systemd-firstboot.service(8) and systemd-homed-firstboot.service(1) will not query the user for basic system settings, even if the system boots up for the first time and the relevant settings are not initialized yet. Not to be confused with systemd.condition_first_boot= (see below), which overrides the result of the ConditionFirstBoot= unit file condition, and thus controls more than just systemd-firstboot.service behaviour. Added in version 233.

systemd.condition_needs_update=

Takes a boolean argument. If specified, overrides the result of ConditionNeedsUpdate= unit condition checks. See systemd.unit(5) for details. Added in version 246.

systemd.condition_first_boot=

Takes a boolean argument. If specified, overrides the result of ConditionFirstBoot= unit condition checks. See systemd.unit(5) for details. Not to be confused with systemd.firstboot= which only controls behaviour of the systemd-firstboot.service system service but has no effect on the condition check (see above). Added in version 246.

systemd.clock_usec=

Takes a decimal, numeric timestamp in μs since January 1st 1970, 00:00am, to set the system clock to. The system time is set to the specified timestamp early during boot. It is not propagated to the hardware clock (RTC). Added in version 246.

systemd.random_seed=

Takes a base64 encoded random seed value to credit with full entropy to the kernel's random pool during early service manager initialization. This option is useful in testing environments where delays due to random pool initialization in entropy starved virtual machines shall be avoided. Note that if this option is used the seed is accessible to unprivileged programs from /proc/cmdline. This option is hence a security risk when used outside of test systems, since the (possibly) only seed used for initialization of the kernel's entropy pool might be easily acquired by unprivileged programs.

It is recommended to pass 512 bytes of randomized data (as that matches the Linux kernel pool size), which may be generated with a command like the following:

```
dd if=/dev/urandom bs=512 count=1 status=none | base64 -w 0
```

Again: do not use this option outside of testing environments, it is a security risk elsewhere, as secret key material derived from the entropy pool can possibly be reconstructed by unprivileged programs.

Added in version 246.

systemd.allow_userspace_verity=

Takes a boolean argument. Controls whether disk images that are Verity protected may be authenticated in userspace signature checks via /etc/verity.d/ (and related directories) public key drop-ins, or whether in-kernel signature checking only. Defaults to on. Added in version 256.

systemd.hostname=

Accepts a hostname to set during early boot. If specified takes precedence over what is set in /etc/hostname. Note that this does not bar later runtime changes to the hostname, it simply controls the initial hostname set during early boot. Added in version 246.

systemd.tty.term.tty=, systemd.tty.rows.tty=, systemd.tty.columns.tty=

These arguments allow configuring default values for $TERM, TTYRows=, and TTYColumns= for tty tty. Additionally, systemd.tty.term.console will configure the $TERM value used by systemd if not set explicitly using TERM on the kernel command line. The tty name should be specified without the /dev/ prefix (e.g. "systemd.tty.rows.ttyS0=80"). Added in version 254.

systemd.battery_check=

Accepts a boolean argument. If false the boot-time battery charge check implemented by systemd-battery-check.service(8) is disabled. Added in version 254.

ifname=, net.ifname_policy=

Controls interface naming policies, implemented by systemd-network-generator.service(8). Added in version 245.

systemd.tpm2_wait=

Controls whether to wait for a TPM2 device at boot, implemented by systemd-tpm2-generator(8). Added in version 256.

## HISTORY
systemd 252

Kernel command-line arguments systemd.unified_cgroup_hierarchy and systemd.legacy_systemd_cgroup_controller were deprecated. Please switch to the unified cgroup hierarchy. Added in version 252.

## SEE ALSO
systemd(1), systemd-system.conf(5), bootparam(7), systemd.system-credentials(7), smbios-type-11(7), systemd-debug-generator(8), systemd-fsck@.service(8), systemd-quotacheck.service(8), systemd-journald.service(8), systemd-vconsole-setup.service(8), systemd-udevd.service(8), plymouth(8), systemd-cryptsetup-generator(8), systemd-veritysetup-generator(8), systemd-fstab-generator(8), systemd-getty-generator(8), systemd-gpt-auto-generator(8), systemd-volatile-root.service(8), systemd-modules-load.service(8), systemd-backlight@.service(8), systemd-rfkill.service(8), systemd-hibernate-resume-generator(8), systemd-firstboot.service(8), bootctl(1)

## NOTES
1.

kernel-parameters.html

https://docs.kernel.org/admin-guide/kernel-parameters.html


package: systemd

title: smbios-type-11(7)

summary: SMBIOS Type 11 strings

text:
# smbios-type-11(7)

## NAME
smbios-type-11 - SMBIOS Type 11 strings

## SYNOPSIS
/sys/firmware/dmi/entries/11-*/raw

## DESCRIPTION
Various OS components process SMBIOS Type 11 vendor strings that a virtual machine manager (VMM) may set and a virtual machine (VM) receives. SMBIOS Type 11 vendor strings may play a similar role as kernel-command-line(7) parameters but generally are under control of the VMM rather than the boot loader or UKI.

For details on SMBIOS Type 11 see the System Management BIOS[1] specifications.

## STRINGS
The following strings are supported:

io.systemd.credential:CREDENTIAL=VALUE, io.systemd.credential.binary:CREDENTIAL=VALUE

This allows passing additional system credentials into the system, in textual or binary (Base64) form. See systemd.exec(5) and System and Service Credentials[2] for details. Added in version 252.

io.systemd.stub.kernel-cmdline-extra=CMDLINE

This allows configuration of additional kernel command line options, and is read by the kernel UEFI stub. For details see systemd-stub(7). Added in version 254.

io.systemd.boot.kernel-cmdline-extra=CMDLINE

This allows configuration of additional kernel command line options for Boot Loader Specification Type 1 entries, and is read by systemd-boot. For details see systemd-boot(7). Added in version 256.

## SEE ALSO
systemd(1), kernel-command-line(7), systemd.system-credentials(7)

## NOTES
1.

System Management BIOS

https://www.dmtf.org/standards/smbios/

2.

System and Service Credentials

https://systemd.io/CREDENTIALS


systemd-stub

title: systemd-stub(7)

summary: A simple UEFI kernel boot stub

text:
# systemd-stub(7)

## 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 (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:

•A ".linux" section with the ELF Linux kernel image. This section is required.

•An optional ".osrel" section with OS release information, i.e. the contents of the os-release(5) file of the OS the kernel belongs to.

•An optional ".cmdline" section with the kernel command line to pass to the invoked kernel.

•An optional ".initrd" section with the initrd.

•An optional ".ucode" section with an initrd containing microcode, to be handed to the kernel before any other initrd. This initrd must not be compressed.

•An optional ".splash" section with an image (in the Windows .BMP format) to show on screen before invoking the kernel.

•An optional ".dtb" section with a compiled binary DeviceTree.

•Zero or more ".dtbauto" sections. systemd-stub will always use the first matching one. The match is performed by taking the first DeviceTree's compatible string supplied by the firmware in configuration tables and comparing it with the first compatible string from each of the ".dtbauto" sections. If the firmware does not provide a DeviceTree, the match is done using the .hwids section instead. After selecting a ".hwids" section (see the description below), the compatible string from that section will be used to perform the same matching procedure. If a match is found, that ".dtbauto" section will be loaded and will override .dtb if present.

•Zero or more ".hwids" sections with hardware IDs of the machines to match DeviceTrees. systemd-stub will use the SMBIOS data to calculate hardware IDs of the machine (as per specification[1]), and then it will try to find any of them in each of the ".hwids" sections. The first matching section will be used.

•An optional ".uname" section with the kernel version information, i.e. the output of uname -r for the kernel included in the ".linux" section.

•An optional ".sbat" section with SBAT[2] revocation metadata.

•An optional ".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.

•An optional ".pcrpkey" section with a public key in the PEM format matching the signature data in the ".pcrsig" section.

In a basic UKI, the sections listed above appear at most once, with the exception of ".dtbauto" and ".hwids" 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.

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).

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 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.

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

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[3].

## 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[4], 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 configuration 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 specific sections are loaded from them. Addons are used to pass additional kernel command line parameters (".cmdline" section), or DeviceTree blobs (".dtb" section), additional initrds (".initrd" section), and microcode updates (".ucode" section). Addons allow those resources to be passed regardless of the kernel version being booted, for example allowing 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 only loaded if the check passes. 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[2].

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. Microcode addons are passed to the kernel in inverse order (UKI specific addons, global addons, UKI embedded section). This is because the microcode update driver stops on the first matching filename. 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 version 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", ".dtb", ".initrd", and ".ucode" sections are parsed from them. This is supposed to be used to pass additional command line parameters, DeviceTree blobs, initrds, and microcode updates to the kernel, regardless of the kernel version 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.

## MULTI-PROFILE UKIS
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".

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.

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

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

Table 1. Multi-Profile UKI Example

Section

Profile

".linux"

Base profile

".osrel"

".cmdline"

".initrd"

".profile"

Profile @0

".profile"

Profile @1

".cmdline"

".profile"

Profile @2

".cmdline"

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.)

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.

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'").

## 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), including all UKI profiles.

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.

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:

Table 2. OS Resource PCR Summary

OS Resource

Measurement PCR

systemd-stub code (the entry point of the unified PE binary)

4

Core kernel code (embedded in unified PE binary)

4 + 11

OS release information (embedded in the unified PE binary)

4 + 11

Main initrd (embedded in unified PE binary)

4 + 9 + 11

Microcode initrd (embedded in unified PE binary)

4 + 9 + 11

Default kernel command line (embedded in unified PE binary)

4 + 11

Overridden kernel command line

12

Boot splash (embedded in the unified PE binary)

4 + 11

TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)

4 + 9

TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)

4 + 9 + 11

Credentials (synthesized initrd from companion files)

9 + 12

System Extensions (synthesized initrd from companion files)

9 + 13

Configuration Extensions (synthesized initrd from companion files)

9 + 12

Selected profile unless zero

12

## 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 partition the boot loader has been started from on the current boot (usually a EFI System Partition). If already set by the boot loader, this will remain untouched by systemd-stub. If not set yet, this will be set to the partition UUID of the partition the unified kernel is started from, in order to support systems that directly boot into a unified kernel image, bypassing any boot loader. 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 224.

LoaderFirmwareInfo, LoaderFirmwareType

Brief firmware information. Use bootctl(1) to view this data. Added in version 250.

LoaderImageIdentifier

The file system path to the EFI executable of the boot loader for the current boot, relative to the partition's root directory (i.e. relative to the partition indicated by LoaderDevicePartUUID, see above). If not set yet, this will be set to the file system path of the EFI executable of the booted unified kernel, in order to support systems that directly boot into a unified kernel image, bypassing any boot loader. Use bootctl(1) to view this data. Added in version 237.

StubDevicePartUUID, StubImageIdentifier

Similar to LoaderDevicePartUUID and LoaderImageIdentifier, but indicates the location of the unified kernel image EFI binary rather than the location of the boot loader binary, regardless if booted via a boot loader or not. Added in version 257.

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.

StubProfile

The numeric index of the selected profile, without the "@", formatted as decimal string. Set both on single-profile and multi-profile UKIs. (In the former case this variable will be set to "0" unconditionally.) Added in version 257.

Note that some of the variables above may also be set by the boot loader. The stub will only set them if they are not set already. Some of these variables are defined by the Boot Loader Interface[5].

## 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-public-key.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.

/.extra/profile, /.extra/os-release

The contents of the ".profile" and ".osrel" sections of the selected profile, if any. Added in version 257.

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[6], Boot Loader Interface[5], ukify(1), systemd-measure(1), TPM2 PCR Measurements Made by systemd[7]

## NOTES
1.

specification

https://learn.microsoft.com/en-us/windows-hardware/drivers/install/specifying-hardware-ids-for-a-computer

2.

SBAT

https://github.com/rhboot/shim/blob/main/SBAT.md

3.

UKI specification

https://uapi-group.org/specifications/specs/unified_kernel_image/

4.

Automatic Boot Assessment

https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT

5.

Boot Loader Interface

https://systemd.io/BOOT_LOADER_INTERFACE

6.

Boot Loader Specification

https://uapi-group.org/specifications/specs/boot_loader_specification

7.

TPM2 PCR Measurements Made by systemd

https://systemd.io/TPM2_PCR_MEASUREMENTS