ukify

Name

ukify — Combine components into a signed Unified Kernel Image for UEFI systems

Synopsis

/usr/lib/systemd/ukify [OPTIONS...] build

ukify [OPTIONS...] genkey

Description¶

Note: this command is experimental for now. While it is intended to become a regular component of systemd, it might still change in behaviour and interface.

ukify is a tool whose primary purpose is to combine components (usually a kernel, an initrd, and a UEFI boot stub) to create a Unified Kernel Image (UKI) — a PE binary that can be executed by the firmware to start the embedded linux kernel. See systemd-stub(7) for details about the stub.

Commands¶

The following commands are understood:

build¶

This command creates a Unified Kernel Image. The two primary options that should be specified for the build verb are Linux=/--linux=, and Initrd=/--initrd=. Initrd= accepts multiple whitespace-separated paths and --initrd= can be specified multiple times.

Additional sections will be inserted into the UKI, either automatically or only if a specific option is provided. See the discussions of Cmdline=/--cmdline=, OSRelease=/--os-release=, DeviceTree=/--devicetree=, Splash=/--splash=, PCRPKey=/--pcrpkey=, Uname=/--uname=, SBAT=/--sbat=, and --section= below.

ukify can also be used to assemble a PE binary that is not executable but contains auxiliary data, for example additional kernel command line entries.

If PCR signing keys are provided via the PCRPrivateKey=/--pcr-private-key= and PCRPublicKey=/--pcr-public-key= options, PCR values that will be seen after booting with the given kernel, initrd, and other sections, will be calculated, signed, and embedded in the UKI. systemd-measure(1) is used to perform this calculation and signing.

The calculation of PCR values is done for specific boot phase paths. Those can be specified with the Phases=/--phases= option. If not specified, the default provided by systemd-measure is used. It is also possible to specify the PCRPrivateKey=/--pcr-private-key=, PCRPublicKey=/--pcr-public-key=, and Phases=/--phases= arguments more than once. Signatures will then be performed with each of the specified keys. On the command line, when both --phases= and --pcr-private-key= are used, they must be specified the same number of times, and then the n-th boot phase path set will be signed by the n-th key. This can be used to build different trust policies for different phases of the boot. In the config file, PCRPrivateKey=, PCRPublicKey=, and Phases= are grouped into separate sections, describing separate boot phases.

If a SecureBoot signing key is provided via the SecureBootPrivateKey=/--secureboot-private-key= option, the resulting PE binary will be signed as a whole, allowing the resulting UKI to be trusted by SecureBoot. Also see the discussion of automatic enrollment in systemd-boot(7).

If the stub and/or the kernel contain ".sbat" sections they will be merged in the UKI so that revocation updates affecting either are considered when the UKI is loaded by Shim. For more information on SBAT see Shim's documentation.

genkey¶

This command creates the keys for PCR signing and the key and certificate used for SecureBoot signing. The same configuration options that determine what keys and in which paths will be needed for signing when build is used, here determine which keys will be created. See the discussion of PCRPrivateKey=/--pcr-private-key=, PCRPublicKey=/--pcr-public-key=, and SecureBootPrivateKey=/--secureboot-private-key= below.

The output files must not exist.

Configuration settings¶

Settings can appear in configuration files (the syntax with SomeSetting=value) and on the command line (the syntax with --some-setting=value). For some command line parameters, a single-letter shortcut is also allowed. In the configuration files, the setting must be in the appropriate section, so the descriptions are grouped by section below. When the same setting appears in the configuration file and on the command line, generally the command line setting has higher priority and overwrites the config file setting completely. If some setting behaves differently, this is described below.

The LINUX and INITRD positional arguments, or the equivalent Linux= and Initrd= settings, are optional. If more than one initrd is specified, they will all be combined into a single PE section. This is useful to, for example, prepend microcode before the actual initrd.

The following options and settings are understood:

Commandline-only options¶

--config=PATH¶: Load configuration from the given config file. In general, settings specified in the config file have lower precedence than the settings specified via options. In cases where the commandline option does not fully override the config file setting are explicitly mentioned in the descriptions of individual options.
--measure, --no-measure¶: Enable or disable a call to systemd-measure(1) to print pre-calculated PCR values. Defaults to false.
--section=NAME:TEXT|@PATH¶: Specify an arbitrary additional section "NAME". Note that the name is used as-is, and if the section name should start with a dot, it must be included in NAME. The argument may be a literal string, or "@" followed by a path name. This option may be specified more than once. Any sections specified in this fashion will be inserted (in order) before the ".linux" section which is always last.
--tools=DIRS¶: Specify one or more directories with helper tools. ukify will look for helper tools in those directories first, and if not found, try to load them from $PATH in the usual fashion.
--output=FILENAME¶: The output filename. If not specified, the name of the LINUX argument, with the suffix ".unsigned.efi" or ".signed.efi" will be used, depending on whether signing for SecureBoot was performed.
--summary¶: Print a summary of loaded config and exit. This is useful to check how the options form the configuration file and the commandline are combined.
-h, --help¶: Print a short help text and exit.
--version¶: Print a short version string and exit.

[UKI] section¶

Linux=LINUX, --linux=LINUX¶: A path to the kernel binary.
Initrd=INITRD..., --initrd=LINUX¶: Zero or more initrd paths. In the configuration file, items are separated by whitespace. The initrds are combined in the order of specification, with the initrds specified in the config file first.
Cmdline=TEXT|@PATH, --cmdline=TEXT|@PATH¶: The kernel command line (the ".cmdline" section). The argument may be a literal string, or "@" followed by a path name. If not specified, no command line will be embedded.
OSRelease=TEXT|@PATH, --os-release=TEXT|@PATH¶: The os-release description (the ".osrel" section). The argument may be a literal string, or "@" followed by a path name. If not specified, the os-release(5) file will be picked up from the host system.
DeviceTree=PATH, --devicetree=PATH¶: The devicetree description (the ".dtb" section). The argument is a path to a compiled binary DeviceTree file. If not specified, the section will not be present.
Splash=PATH, --splash=PATH¶: A picture to display during boot (the ".splash" section). The argument is a path to a BMP file. If not specified, the section will not be present.
PCRPKey=PATH, --pcrpkey=PATH¶: A path to a public key to embed in the ".pcrpkey" section. If not specified, and there's exactly one PCRPublicKey=/--pcr-public-key= argument, that key will be used. Otherwise, the section will not be present.
Uname=VERSION, --uname=VERSION¶: Specify the kernel version (as in uname -r, the ".uname" section). If not specified, an attempt will be made to extract the version string from the kernel image. It is recommended to pass this explicitly if known, because the extraction is based on heuristics and not very reliable. If not specified and extraction fails, the section will not be present.
PCRBanks=PATH, --pcr-banks=PATH¶: A comma or space-separated list of PCR banks to sign a policy for. If not present, all known banks will be used ("sha1", "sha256", "sha384", "sha512"), which will fail if not supported by the system.
SecureBootSigningTool=SIGNER, --signtool=SIGNER¶: Whether to use "sbsign" or "pesign". Depending on this choice, different parameters are required in order to sign an image. Defaults to "sbsign".
SecureBootPrivateKey=SB_KEY, --secureboot-private-key=SB_KEY¶: A path to a private key to use for signing of the resulting binary. If the SigningEngine=/--signing-engine= option is used, this may also be an engine-specific designation. This option is required by SecureBootSigningTool=sbsign/--signtool=sbsign.
SecureBootCertificate=SB_CERT, --secureboot-certificate=SB_CERT¶: A path to a certificate to use for signing of the resulting binary. If the SigningEngine=/--signing-engine= option is used, this may also be an engine-specific designation. This option is required by SecureBootSigningTool=sbsign/--signtool=sbsign.
SecureBootCertificateDir=SB_PATH, --secureboot-certificate-dir=SB_PATH¶: A path to a nss certificate database directory to use for signing of the resulting binary. Takes effect when SecureBootSigningTool=pesign/--signtool=pesign is used. Defaults to /etc/pki/pesign.
SecureBootCertificateName=SB_CERTNAME, --secureboot-certificate-name=SB_CERTNAME¶: The name of the nss certificate database entry to use for signing of the resulting binary. This option is required by SecureBootSigningTool=pesign/--signtool=pesign.
SecureBootCertificateValidity=DAYS, --secureboot-certificate-validity=DAYS¶: Period of validity (in days) for a certificate created by genkey. Defaults to 3650, i.e. 10 years.
SigningEngine=ENGINE, --signing-engine=ENGINE¶: An "engine" for signing of the resulting binary. This option is currently passed verbatim to the --engine= option of sbsign(1).
SignKernel=BOOL, --sign-kernel, --no-sign-kernel¶: Override the detection of whether to sign the Linux binary itself before it is embedded in the combined image. If not specified, it will be signed if a SecureBoot signing key is provided via the SecureBootPrivateKey=/--secureboot-private-key= option and the binary has not already been signed. If SignKernel=/--sign-kernel is true, and the binary has already been signed, the signature will be appended anyway.
SBAT=TEXT|@PATH, --sbat=TEXT|@PATH¶: SBAT metadata associated with the UKI or addon. SBAT policies are useful to revoke whole groups of UKIs or addons with a single, static policy update that does not take space in DBX/MOKX. If not specified manually, a default metadata entry consisting of "uki,1,UKI,uki,1,https://www.freedesktop.org/software/systemd/man/systemd-stub.html" will be used, to ensure it is always possible to revoke UKIs and addons. For more information on SBAT see Shim's documentation.

[PCRSignature:`NAME`] section¶

In the config file, those options are grouped by section. On the commandline, they must be specified in the same order. The sections specified in both sources are combined.

PCRPrivateKey=PATH, --pcr-private-key=PATH¶

A private key to use for signing PCR policies. On the commandline, this option may be specified more than once, in which case multiple signatures will be made.

PCRPublicKey=PATH, --pcr-public-key=PATH¶

A public key to use for signing PCR policies.

On the commandline, this option may be specified more than once, similarly to the --pcr-private-key= option. If not present, the public keys will be extracted from the private keys. On the commandline, if present, the this option must be specified the same number of times as the --pcr-private-key= option.

Phases=LIST, --phases=LIST¶

A comma or space-separated list of colon-separated phase paths to sign a policy for. Each set of boot phase paths will be signed with the corresponding private key. If not present, the default of systemd-measure(1) will be used.

On the commandline, when this argument is present, it must appear the same number of times as the --pcr-private-key= option.

Examples¶

Example 1. Minimal invocation

$ ukify build \
      --linux=/lib/modules/6.0.9-300.fc37.x86_64/vmlinuz \
      --initrd=/some/path/initramfs-6.0.9-300.fc37.x86_64.img \
      --cmdline='quiet rw'

This creates an unsigned UKI ./vmlinuz.unsigned.efi.

Example 2. All the bells and whistles

$ /usr/lib/systemd/ukify build \
      --linux=/lib/modules/6.0.9-300.fc37.x86_64/vmlinuz \
      --initrd=early_cpio \
      --initrd=/some/path/initramfs-6.0.9-300.fc37.x86_64.img \
      --sbat='sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
      uki.author.myimage,1,UKI for System,uki.author.myimage,1,https://www.freedesktop.org/software/systemd/man/systemd-stub.html' \
      --pcr-private-key=pcr-private-initrd-key.pem \
      --pcr-public-key=pcr-public-initrd-key.pem \
      --phases='enter-initrd' \
      --pcr-private-key=pcr-private-system-key.pem \
      --pcr-public-key=pcr-public-system-key.pem \
      --phases='enter-initrd:leave-initrd enter-initrd:leave-initrd:sysinit \
                enter-initrd:leave-initrd:sysinit:ready' \
      --pcr-banks=sha384,sha512 \
      --secureboot-private-key=sb.key \
      --secureboot-certificate=sb.cert \
      --sign-kernel \
      --cmdline='quiet rw rhgb'

This creates a signed UKI ./vmlinuz.signed.efi. The initrd section contains two concatenated parts, early_cpio and initramfs-6.0.9-300.fc37.x86_64.img. The policy embedded in the ".pcrsig" section will be signed for the initrd (the enter-initrd phase) with the key pcr-private-initrd-key.pem, and for the main system (phases leave-initrd, sysinit, ready) with the key pcr-private-system-key.pem. The Linux binary and the resulting combined image will be signed with the SecureBoot key sb.key.

Example 3. All the bells and whistles, via a config file

This is the same as the previous example, but this time the configuration is stored in a file:

$ cat ukify.conf
[UKI]
Initrd=early_cpio
Cmdline=quiet rw rhgb

SecureBootPrivateKey=sb.key
SecureBootCertificate=sb.cert
SignKernel=yes
PCRBanks=sha384,sha512

[PCRSignature:initrd]
PCRPrivateKey=pcr-private-initrd-key.pem
PCRPublicKey=pcr-public-initrd-key.pem
Phases=enter-initrd

[PCRSignature:system]
PCRPrivateKey=pcr-private-system-key.pem
PCRPublicKey=pcr-public-system-key.pem
Phases=enter-initrd:leave-initrd
       enter-initrd:leave-initrd:sysinit
       enter-initrd:leave-initrd:sysinit:ready

$ /usr/lib/systemd/ukify -c ukify.conf build \
        --linux=/lib/modules/6.0.9-300.fc37.x86_64/vmlinuz \
        --initrd=/some/path/initramfs-6.0.9-300.fc37.x86_64.img

One "initrd" (early_cpio) is specified in the config file, and the other initrd (initramfs-6.0.9-300.fc37.x86_64.img) is specified on the commandline. This may be useful for example when the first initrd contains microcode for the CPU and does not need to be updated when the kernel version changes, unlike the actual initrd.

Example 4. Kernel command line auxiliary PE

ukify build \
      --secureboot-private-key=sb.key \
      --secureboot-certificate=sb.cert \
      --cmdline='debug' \
      --sbat='sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
      uki.addon.author,1,UKI Addon for System,uki.addon.author,1,https://www.freedesktop.org/software/systemd/man/systemd-stub.html'
      --output=debug.cmdline

This creates a signed PE binary that contains the additional kernel command line parameter "debug" with SBAT metadata referring to the owner of the addon.

Example 5. Decide signing policy and create certificate and keys

First, let's create an config file that specifies what signatures shall be made:

# cat >/etc/kernel/uki.conf <<EOF
[UKI]
SecureBootPrivateKey=/etc/kernel/secure-boot.key.pem
SecureBootCertificate=/etc/kernel/secure-boot.cert.pem

[PCRSignature:initrd]
Phases=enter-initrd
PCRPrivateKey=/etc/kernel/pcr-initrd.key.pem
PCRPublicKey=/etc/kernel/pcr-initrd.pub.pem

[PCRSignature:system]
Phases=enter-initrd:leave-initrd enter-initrd:leave-initrd:sysinit
       enter-initrd:leave-initrd:sysinit:ready
PCRPrivateKey=/etc/kernel/pcr-system.key.pem
PCRPublicKey=/etc/kernel/pcr-system.pub.pem
EOF

Next, we can generate the certificate and keys:

# /usr/lib/systemd/ukify genkey --config=/etc/kernel/uki.conf
Writing SecureBoot private key to /etc/kernel/secure-boot.key.pem
Writing SecureBoot certificate to /etc/kernel/secure-boot.cert.pem
Writing private key for PCR signing to /etc/kernel/pcr-initrd.key.pem
Writing public key for PCR signing to /etc/kernel/pcr-initrd.pub.pem
Writing private key for PCR signing to /etc/kernel/pcr-system.key.pem
Writing public key for PCR signing to /etc/kernel/pcr-system.pub.pem

(Both operations need to be done as root to allow write access to /etc/kernel/.)

Subsequent invocations of using the config file (/usr/lib/systemd/ukify build --config=/etc/kernel/uki.conf) will use this certificate and key files. Note that the kernel-install(8) plugin 60-ukify.install uses /etc/kernel/uki.conf by default, so after this file has been created, installations of kernels that create a UKI on the local machine using kernel-install would perform signing using this config.