man/systemd-stub.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   5
   6 <refentry id="systemd-stub" conditional='ENABLE_BOOTLOADER'
   7     xmlns:xi="http://www.w3.org/2001/XInclude">
   8   <refentryinfo>
   9     <title>systemd-stub</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>systemd-stub</refentrytitle>
  15     <manvolnum>7</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>systemd-stub</refname>
  20     <refname>sd-stub</refname>
  21     <refname>linuxx64.efi.stub</refname>
  22     <refname>linuxia32.efi.stub</refname>
  23     <refname>linuxaa64.efi.stub</refname>
  24     <refpurpose>A simple UEFI kernel boot stub</refpurpose>
  25   </refnamediv>
  26
  27   <refsynopsisdiv>
  28     <para><filename>/usr/lib/systemd/boot/efi/linuxx64.efi.stub</filename></para>
  29     <para><filename>/usr/lib/systemd/boot/efi/linuxia32.efi.stub</filename></para>
  30     <para><filename>/usr/lib/systemd/boot/efi/linuxaa64.efi.stub</filename></para>
  31     <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename></para>
  32     <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.cred</filename></para>
  33     <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.raw</filename></para>
  34     <para><filename><replaceable>ESP</replaceable>/loader/addons/*.addon.efi</filename></para>
  35     <para><filename><replaceable>ESP</replaceable>/loader/credentials/*.cred</filename></para>
  36   </refsynopsisdiv>
  37
  38   <refsect1>
  39     <title>Description</title>
  40
  41     <para><command>systemd-stub</command> (stored in per-architecture files
  42     <filename>linuxx64.efi.stub</filename>, <filename>linuxia32.efi.stub</filename>,
  43     <filename>linuxaa64.efi.stub</filename> on disk) is a simple UEFI boot stub. An UEFI boot stub is
  44     attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware
  45     environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux
  46     kernel is executable as regular UEFI binary, and is able to do various preparations before switching the
  47     system into the Linux world.</para>
  48
  49     <para>The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary
  50     itself. This allows combining various resources inside a single PE binary image (usually called "Unified
  51     Kernel Image", or "UKI" for short), which may then be signed via UEFI SecureBoot as a whole, covering all
  52     individual resources at once. Specifically it may include:</para>
  53
  54     <itemizedlist>
  55       <listitem><para>The ELF Linux kernel images will be looked for in the <literal>.linux</literal> PE
  56       section of the executed image.</para></listitem>
  57
  58       <listitem><para>OS release information, i.e. the
  59       <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file of
  60       the OS the kernel belongs to, in the <literal>.osrel</literal> PE section.</para></listitem>
  61
  62       <listitem><para>Kernel version information, i.e. the output of <command>uname -r</command> for the
  63       kernel included in the UKI, in the <literal>.uname</literal> PE section.</para></listitem>
  64
  65       <listitem><para>The initrd will be loaded from the <literal>.initrd</literal> PE section.
  66       </para></listitem>
  67
  68       <listitem><para>A compiled binary DeviceTree will be looked for in the <literal>.dtb</literal> PE
  69       section.</para></listitem>
  70
  71       <listitem><para>Kernel version information, i.e. the output of <command>uname -r</command> for the
  72       kernel included in the UKI, in the <literal>.uname</literal> PE section.</para></listitem>
  73
  74       <listitem><para>The kernel command line to pass to the invoked kernel will be looked for in the
  75       <literal>.cmdline</literal> PE section.</para></listitem>
  76
  77       <listitem><para>A boot splash (in Windows <filename>.BMP</filename> format) to show on screen before
  78       invoking the kernel will be looked for in the <literal>.splash</literal> PE section.</para></listitem>
  79
  80       <listitem><para>A set of cryptographic signatures for expected TPM2 PCR values when this kernel is
  81       booted, in JSON format, in the <literal>.pcrsig</literal> section. This is useful for implementing TPM2
  82       policies that bind disk encryption and similar to kernels that are signed by a specific
  83       key.</para></listitem>
  84
  85       <listitem><para>A public key in PEM format matching this TPM2 PCR signature data in the
  86       <literal>.pcrpkey</literal> section.</para></listitem>
  87     </itemizedlist>
  88
  89     <para>If UEFI SecureBoot is enabled and the <literal>.cmdline</literal> section is present in the executed
  90     image, any attempts to override the kernel command line by passing one as invocation parameters to the
  91     EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
  92     SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
  93     is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is
  94     present).</para>
  95
  96     <para>If a DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing
  97     DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
  98     <literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para>
  99
 100     <para>The contents of seven of these eight PE sections are measured into TPM PCR 11, that is otherwise
 101     not used. Thus, it can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section
 102     is not included in this PCR measurement, since it's supposed to contain signatures for the expected
 103     results for these measurements, i.e. of the outputs of the measurement operation, and thus cannot also be
 104     input to it.</para>
 105
 106     <para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> are present in a unified kernel
 107     image their contents are passed to the booted kernel in an synthetic initrd cpio archive that places them in the
 108     <filename>/.extra/tpm2-pcr-signature.json</filename> and
 109     <filename>/.extra/tpm2-pcr-public-key.pem</filename> files. Typically, a
 110     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
 111     ensures they are copied into <filename>/run/systemd/tpm2-pcr-signature.json</filename> and
 112     <filename>/run/systemd/tpm2-pcr-public-key.pem</filename> where they remain accessible even after the
 113     system transitions out of the initrd environment into the host file system. Tools such
 114     <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 115     <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 116     and <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 117     will automatically use files present under these paths to unlock protected resources (encrypted storage
 118     or credentials) or bind encryption to booted kernels.</para>
 119   </refsect1>
 120
 121   <refsect1>
 122     <title>Companion Files</title>
 123
 124     <para>The <command>systemd-stub</command> UEFI boot stub automatically collects two types of auxiliary
 125     companion files optionally placed in drop-in directories on the same partition as the EFI binary,
 126     dynamically generates <command>cpio</command> initrd archives from them, and passes them to the kernel.
 127     Specifically:</para>
 128
 129     <itemizedlist>
 130       <listitem><para>For a kernel binary called <filename><replaceable>foo</replaceable>.efi</filename>, it
 131       will look for files with the <filename>.cred</filename> suffix in a directory named
 132       <filename><replaceable>foo</replaceable>.efi.extra.d/</filename> next to it. A <command>cpio</command>
 133       archive is generated from all files found that way, placing them in the
 134       <filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may
 135       then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
 136       authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System
 137       Partition. See
 138       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 139       and
 140       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 141       for
 142       details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM
 143       PCR 12 (if a TPM is present).</para></listitem>
 144
 145       <listitem><para>Similarly, files <filename><replaceable>foo</replaceable>.efi.extra.d/*.raw</filename>
 146       are packed up in a <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename>
 147       directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension
 148       images to the initrd. See
 149       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
 150       details on system extension images. The generated <command>cpio</command> archive containing these
 151       system extension images is measured into TPM PCR 13 (if a TPM is present).</para></listitem>
 152
 153       <listitem><para>Similarly, files
 154       <filename><replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename>
 155       are loaded and verified as PE binaries, and a <literal>.cmdline</literal> section is parsed from them.
 156       In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
 157       Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a
 158       a <literal>.uname</literal> section, the addon will be rejected if they do not exactly match. It is
 159       recommended to always add a <literal>.sbat</literal> section to all signed addons, so that they may be
 160       revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
 161       <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will
 162       add a SBAT policy by default if none is passed when building addons. For more information on SBAT see
 163       <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">Shim's documentation</ulink>.
 164       Addons are supposed to be used to pass additional kernel command line parameters, regardless of the
 165       kernel image being booted, for example to allow platform vendors to ship platform-specific
 166       configuration. The loaded command line addon files are sorted, loaded, measured into TPM PCR 12 (if a
 167       TPM is present) and appended to the kernel command line. UKI command line options are listed first,
 168       then options from addons in <filename>/loader/addons/*.addon.efi</filename> are appended next, and
 169       finally UKI-specific addons are appended last. Addons are always loaded in the same order based on the
 170       filename, so that, given the same set of addons, the same set of measurements can be expected in
 171       PCR12, however note that the filename is not protected by the PE signature, and as such an attacker
 172       with write access to the ESP could potentially rename these files to change the order in which they
 173       are loaded, in a way that could alter the functionality of the kernel, as some options might be order
 174       dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of an
 175       attestation service so that improper use of your signed addons can be detected and dealt with using
 176       one of the aforementioned revocation mechanisms.</para></listitem>
 177
 178       <listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
 179       <command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
 180       directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
 181       the initrd, regardless of the kernel being booted. The generated <command>cpio</command> archive is
 182       measured into TPM PCR 12 (if a TPM is present).</para></listitem>
 183
 184       <listitem><para>Additionally, files <filename>/loader/addons/*.addon.efi</filename> are loaded and
 185       verified as PE binaries, and a <literal>.cmdline</literal> section is parsed from them. This is
 186       supposed to be used to pass additional command line parameters to the kernel, regardless of the kernel
 187       being booted.</para></listitem>
 188     </itemizedlist>
 189
 190     <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
 191     images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
 192     further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
 193     by <command>systemd-creds encrypt -T</command> (see
 194     <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 195     details); in case of the system extension images by using signed Verity images.</para>
 196   </refsect1>
 197
 198   <refsect1>
 199     <title>TPM PCR Notes</title>
 200
 201     <para>Note that when a unified kernel using <command>systemd-stub</command> is invoked the firmware will
 202     measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the
 203     core kernel, the embedded initrd and kernel command line (see above for a full list).</para>
 204
 205     <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
 206     every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
 207     measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials will be measured to both PCR
 208     9 and PCR 12; the initrd synthesized from system extensions will be measured to both PCR 4 and PCR
 209     9. Let's summarize the OS resources and the PCRs they are measured to:</para>
 210
 211     <table>
 212       <title>OS Resource PCR Summary</title>
 213
 214       <tgroup cols='2' align='left' colsep='1' rowsep='1'>
 215         <colspec colname="pcr" />
 216         <colspec colname="definition" />
 217
 218         <thead>
 219           <row>
 220             <entry>OS Resource</entry>
 221             <entry>Measurement PCR</entry>
 222           </row>
 223         </thead>
 224
 225         <tbody>
 226           <row>
 227             <entry><command>systemd-stub</command> code (the entry point of the unified PE binary)</entry>
 228             <entry>4</entry>
 229           </row>
 230
 231           <row>
 232             <entry>Core kernel code (embedded in unified PE binary)</entry>
 233             <entry>4 + 11</entry>
 234           </row>
 235
 236           <row>
 237             <entry>OS release information (embedded in the unified PE binary)</entry>
 238             <entry>4 + 11</entry>
 239           </row>
 240
 241           <row>
 242             <entry>Main initrd (embedded in unified PE binary)</entry>
 243             <entry>4 + 9 + 11</entry>
 244           </row>
 245
 246           <row>
 247             <entry>Default kernel command line (embedded in unified PE binary)</entry>
 248             <entry>4 + 11</entry>
 249           </row>
 250
 251           <row>
 252             <entry>Overridden kernel command line</entry>
 253             <entry>12</entry>
 254           </row>
 255
 256           <row>
 257             <entry>Boot splash (embedded in the unified PE binary)</entry>
 258             <entry>4 + 11</entry>
 259           </row>
 260
 261           <row>
 262             <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry>
 263             <entry>4 + 9</entry>
 264           </row>
 265
 266           <row>
 267             <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry>
 268             <entry>4 + 9 + 11</entry>
 269           </row>
 270
 271           <row>
 272             <entry>Credentials (synthesized initrd from companion files)</entry>
 273             <entry>9 + 12</entry>
 274           </row>
 275
 276           <row>
 277             <entry>System Extensions (synthesized initrd from companion files)</entry>
 278             <entry>9 + 13</entry>
 279           </row>
 280         </tbody>
 281       </tgroup>
 282     </table>
 283   </refsect1>
 284
 285   <refsect1>
 286     <title>EFI Variables</title>
 287
 288     <para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
 289     vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
 290     stub and the OS:</para>
 291
 292     <variablelist class='efi-variables'>
 293       <varlistentry>
 294         <term><varname>LoaderDevicePartUUID</varname></term>
 295
 296         <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
 297         from. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 298         uses this information to automatically find the disk booted from, in order to discover various other
 299         partitions on the same disk automatically.</para>
 300
 301         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 302       </varlistentry>
 303
 304       <varlistentry>
 305         <term><varname>LoaderFirmwareInfo</varname></term>
 306         <term><varname>LoaderFirmwareType</varname></term>
 307
 308         <listitem><para>Brief firmware information. Use
 309         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
 310         data.</para>
 311
 312         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 313       </varlistentry>
 314
 315       <varlistentry>
 316         <term><varname>LoaderImageIdentifier</varname></term>
 317
 318         <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
 319         directory. Use
 320         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 321         this data.</para>
 322
 323         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 324       </varlistentry>
 325
 326       <varlistentry>
 327         <term><varname>StubInfo</varname></term>
 328
 329         <listitem><para>Brief stub information. Use
 330         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 331         this data.</para>
 332
 333         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 334       </varlistentry>
 335
 336       <varlistentry>
 337         <term><varname>StubPcrKernelImage</varname></term>
 338
 339         <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
 340         database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
 341         <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
 342         unset otherwise.</para>
 343
 344         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 345       </varlistentry>
 346
 347       <varlistentry>
 348         <term><varname>StubPcrKernelParameters</varname></term>
 349
 350         <listitem><para>The PCR register index the kernel command line and credentials are measured into,
 351         formatted as decimal ASCII string (e.g. <literal>12</literal>). This variable is set if a measurement
 352         was successfully completed, and remains unset otherwise.</para>
 353
 354         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 355       </varlistentry>
 356
 357       <varlistentry>
 358         <term><varname>StubPcrInitRDSysExts</varname></term>
 359
 360         <listitem><para>The PCR register index the systemd extensions for the initrd, which are picked up
 361         from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
 362         <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
 363         unset otherwise.</para>
 364
 365         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 366       </varlistentry>
 367     </variablelist>
 368
 369     <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
 370     them if they aren't set already. Some of these variables are defined by the <ulink
 371     url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
 372   </refsect1>
 373
 374   <refsect1>
 375     <title>initrd Resources</title>
 376
 377     <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
 378     the initial file system hierarchy in the initrd execution environment:</para>
 379
 380     <variablelist>
 381       <varlistentry>
 382         <term><filename>/</filename></term>
 383
 384         <listitem><para>The main initrd from the <literal>.initrd</literal> PE section of the unified kernel image.</para>
 385
 386         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 387       </varlistentry>
 388
 389       <varlistentry>
 390         <term><filename>/.extra/credentials/*.cred</filename></term>
 391         <listitem><para>Credential files (suffix <literal>.cred</literal>) that are placed next to the
 392         unified kernel image (as described above) are copied into the
 393         <filename>/.extra/credentials/</filename> directory in the initrd execution
 394         environment.</para>
 395
 396         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 397       </varlistentry>
 398
 399       <varlistentry>
 400         <term><filename>/.extra/global_credentials/*.cred</filename></term>
 401         <listitem><para>Similar, credential files in the <filename>/loader/credentials/</filename> directory
 402         in the file system the unified kernel image is placed in are copied into the
 403         <filename>/.extra/global_credentials/</filename> directory in the initrd execution
 404         environment.</para>
 405
 406         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 407       </varlistentry>
 408
 409       <varlistentry>
 410         <term><filename>/.extra/sysext/*.raw</filename></term>
 411         <listitem><para>System extension image files (suffix <literal>.raw</literal>) that are placed next to
 412         the unified kernel image (as described above) are copied into the
 413         <filename>/.extra/sysext/</filename> directory in the initrd execution environment.</para>
 414
 415         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 416       </varlistentry>
 417
 418       <varlistentry>
 419         <term><filename>/.extra/tpm2-pcr-signature.json</filename></term>
 420         <listitem><para>The TPM2 PCR signature JSON object included in the <literal>.pcrsig</literal> PE
 421         section of the unified kernel image is copied into the
 422         <filename>/.extra/tpm2-pcr-signature.json</filename> file in the initrd execution
 423         environment.</para>
 424
 425         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 426       </varlistentry>
 427
 428       <varlistentry>
 429         <term><filename>/.extra/tpm2-pcr-pkey.pem</filename></term>
 430         <listitem><para>The PEM public key included in the <literal>.pcrpkey</literal> PE section of the
 431         unified kernel image is copied into the <filename>/.extra/tpm2-pcr-public-key.pem</filename> file in
 432         the initrd execution environment.</para>
 433
 434         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 435       </varlistentry>
 436     </variablelist>
 437
 438     <para>Note that all these files are located in the <literal>tmpfs</literal> file system the kernel sets
 439     up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
 440     environment into the host file system. If these resources shall be kept around over this transition they
 441     need to be copied to a place that survives the transition first, for example via a suitable
 442     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
 443     default, this is done for the TPM2 PCR signature and public key files.</para>
 444   </refsect1>
 445
 446   <refsect1>
 447     <title>SMBIOS Type 11 Strings</title>
 448
 449     <para><command>systemd-stub</command> can be configured using SMBIOS Type 11 strings. Applicable strings
 450     consist of a name, followed by <literal>=</literal>, followed by the value.
 451     <command>systemd-stub</command> will search the table for a string with a specific name, and if found,
 452     use its value. The following strings are read:</para>
 453
 454     <variablelist>
 455       <varlistentry>
 456         <term><varname>io.systemd.stub.kernel-cmdline-extra</varname></term>
 457         <listitem><para>If set, the value of this string is added to the list of kernel command line
 458         arguments that are measured in PCR12 and passed to the kernel.</para>
 459
 460         <xi:include href="version-info.xml" xpointer="v254"/></listitem>
 461       </varlistentry>
 462     </variablelist>
 463   </refsect1>
 464
 465   <refsect1>
 466     <title>Assembling Kernel Images</title>
 467
 468     <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
 469     <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
 470   </refsect1>
 471
 472   <refsect1>
 473     <title>See Also</title>
 474     <para>
 475       <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 476       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 477       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 478       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 479       <ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink>,
 480       <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
 481       <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 482       <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 483     </para>
 484   </refsect1>
 485 </refentry>