UKIFY(1)

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

SYNOPSIS

ukify [OPTIONS...] build

ukify [OPTIONS...] genkey

ukify [OPTIONS...] inspect FILE...

DESCRIPTION

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)[1] — 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 Microcode=/--microcode=, 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 SigningEngine=/--signing-engine= is specified, then the private keys arguments will be passed verbatim to OpenSSL as URIs, and the public key arguments will be loaded as X.509 certificates, so that signing can be performed with an OpenSSL engine.

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 documentation[2].

  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.

  inspect

Display information about the sections in a given binary or binaries. If --all is given, all sections are shown. Otherwise, if --section= option is specified at least once, only those sections are shown. Otherwise, well-known sections that are typically included in an UKI are shown. For each section, its name, size, and sha256-digest is printed. For text sections, the contents are printed.

Also see the description of -j/--json= and --section=.

Other tools that may be useful for inspect UKIs: llvm-objdump(1) -p and pe-inspect.

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.

If no config file is provided via the option --config=PATH, ukify will try to look for a default configuration file in the following paths in this order: /etc/systemd/ukify.conf, /run/systemd/ukify.conf, /usr/local/lib/systemd/ukify.conf, and /usr/lib/systemd/ukify.conf, and then load the first one found. ukify will proceed normally if no configuration file is specified and no default one is found.

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:

  Command line-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 command line option does not fully override the config file setting are explicitly mentioned in the descriptions of individual options.

    Added in version 254.

--measure, --no-measure
    Enable or disable a call to systemd-measure(1) to print pre-calculated PCR values. Defaults to false.

    Added in version 253.

--section=NAME:TEXT|@PATH, --section=NAME:text|binary[@PATH]
    For all verbs except inspect, the first syntax is used. Specify an arbitrary additional section "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.

    For the inspect verb, the second syntax is used. The section NAME will be inspected (if found). If the second argument is "text", the contents will be printed. If the third argument is given, the contents will be saved to file PATH.

    Note that the name is used as-is, and if the section name should start with a dot, it must be included in NAME.

    Added in version 253.

--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.

    Added in version 253.

--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.

    Added in version 253.

--summary
    Print a summary of loaded config and exit. This is useful to check how the options from the configuration file and the command line are combined.

    Added in version 254.

--all
    Print all sections (with inspect verb).

    Added in version 255.

--json
    Generate JSON output (with inspect verb).

    Added in version 255.

-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.

    Added in version 254.

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.

    Added in version 254.

Microcode=UCODE, --microcode=UCODE
    Path to initrd containing microcode updates. If not specified, the section will not be present.

    Added in version 256.

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.

    Added in version 253.

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.

    Added in version 253.

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.

    Added in version 253.

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.

    Added in version 253.

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.

    Added in version 253.

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.

    Added in version 253.

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.

    Added in version 253.

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".

    Added in version 254.

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.

    Added in version 253.

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.

    Added in version 253.

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.

    Added in version 254.

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.

    Added in version 254.

SecureBootCertificateValidity=DAYS, --secureboot-certificate-validity=DAYS
    Period of validity (in days) for a certificate created by genkey. Defaults to 3650, i.e. 10 years.

    Added in version 254.

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).

    Added in version 253.

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.

    Added in version 253.

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://uapi-group.org/specifications/specs/unified_kernel_image/" for UKIs and "uki-addon,1,UKI Addon,addon,1,https://www.freedesktop.org/software/systemd/man/latest/systemd-stub.html" for addons will be used, to ensure it is always possible to revoke them. For more information on SBAT see Shim documentation[2].

    Added in version 254.

  [PCRSignature:NAME] section

In the config file, those options are grouped by section. On the command line, 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 command line, this option may be specified more than once, in which case multiple signatures will be made.

    Added in version 253.

PCRPublicKey=PATH, --pcr-public-key=PATH
    A public key to use for signing PCR policies.

    On the command line, 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 command line, if present, this option must be specified the same number of times as the --pcr-private-key= option.

    Added in version 253.

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

    Added in version 253.

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

    $ 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://uapi-group.org/specifications/specs/unified_kernel_image/' \
          --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
    $ 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 command line. 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 PE addon

    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.addon.efi

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 a configuration 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:

    # 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 using the config file (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 will perform signing using this config.

SEE ALSO

systemd(1), systemd-stub(7), systemd-boot(7), systemd-measure(1), systemd-pcrphase.service(8)

NOTES

1.  Unified Kernel Image (UKI)
    https://uapi-group.org/specifications/specs/unified_kernel_image/

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