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       <!-- Let's keep this in the canonical order we also measure the sections by, i.e. as in
  56            src/fundamental/uki.h's UnifiedSection enum -->
  57
  58       <listitem><para>The ELF Linux kernel images will be looked for in the <literal>.linux</literal> PE
  59       section of the executed image.</para></listitem>
  60
  61       <listitem><para>OS release information, i.e. the
  62       <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file of
  63       the OS the kernel belongs to, in the <literal>.osrel</literal> PE section.</para></listitem>
  64
  65       <listitem><para>The kernel command line to pass to the invoked kernel will be looked for in the
  66       <literal>.cmdline</literal> PE section.</para></listitem>
  67
  68       <listitem><para>The initrd will be loaded from the <literal>.initrd</literal> PE
  69       section.</para></listitem>
  70
  71       <listitem><para>A boot splash (in Windows <filename>.BMP</filename> format) to show on screen before
  72       invoking the kernel will be looked for in the <literal>.splash</literal> PE section.</para></listitem>
  73
  74       <listitem><para>A compiled binary DeviceTree will be looked for in the <literal>.dtb</literal> PE
  75       section.</para></listitem>
  76
  77       <listitem><para>Kernel version information, i.e. the output of <command>uname -r</command> for the
  78       kernel included in the UKI, in the <literal>.uname</literal> PE section.</para></listitem>
  79
  80       <listitem><para><ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT</ulink> revocation
  81       metadata, in the <literal>.sbat</literal> PE section.</para></listitem>
  82
  83       <listitem><para>A set of cryptographic signatures for expected TPM2 PCR values when this kernel is
  84       booted, in JSON format, in the <literal>.pcrsig</literal> section. This is useful for implementing TPM2
  85       policies that bind disk encryption and similar to kernels that are signed by a specific
  86       key.</para></listitem>
  87
  88       <listitem><para>A public key in PEM format matching this TPM2 PCR signature data in the
  89       <literal>.pcrpkey</literal> section.</para></listitem>
  90     </itemizedlist>
  91
  92     <para>If UEFI SecureBoot is enabled and the <literal>.cmdline</literal> section is present in the executed
  93     image, any attempts to override the kernel command line by passing one as invocation parameters to the
  94     EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
  95     SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
  96     is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is
  97     present).</para>
  98
  99     <para>If a DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing
 100     DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
 101     <literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para>
 102
 103     <para>The contents of seven of these eight PE sections are measured into TPM PCR 11, that is otherwise
 104     not used. Thus, it can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section
 105     is not included in this PCR measurement, since it's supposed to contain signatures for the expected
 106     results for these measurements, i.e. of the outputs of the measurement operation, and thus cannot also be
 107     input to it.</para>
 108
 109     <para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> are present in a unified kernel
 110     image their contents are passed to the booted kernel in an synthetic initrd cpio archive that places them in the
 111     <filename>/.extra/tpm2-pcr-signature.json</filename> and
 112     <filename>/.extra/tpm2-pcr-public-key.pem</filename> files. Typically, a
 113     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
 114     ensures they are copied into <filename>/run/systemd/tpm2-pcr-signature.json</filename> and
 115     <filename>/run/systemd/tpm2-pcr-public-key.pem</filename> where they remain accessible even after the
 116     system transitions out of the initrd environment into the host file system. Tools such
 117     <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 118     <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 119     and <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 120     will automatically use files present under these paths to unlock protected resources (encrypted storage
 121     or credentials) or bind encryption to booted kernels.</para>
 122
 123     <para>For further details about the UKI concept, see the <ulink
 124     url="https://uapi-group.org/specifications/specs/unified_kernel_image/">UKI specification</ulink>.</para>
 125   </refsect1>
 126
 127   <refsect1>
 128     <title>Companion Files</title>
 129
 130     <para>The <command>systemd-stub</command> UEFI boot stub automatically collects three types of auxiliary
 131     companion files optionally placed in drop-in directories on the same partition as the EFI binary,
 132     dynamically generates <command>cpio</command> initrd archives from them, and passes them to the kernel.
 133     Specifically:</para>
 134
 135     <itemizedlist>
 136       <listitem><para>For a kernel binary called <filename><replaceable>foo</replaceable>.efi</filename>, it
 137       will look for files with the <filename>.cred</filename> suffix in a directory named
 138       <filename><replaceable>foo</replaceable>.efi.extra.d/</filename> next to it. A <command>cpio</command>
 139       archive is generated from all files found that way, placing them in the
 140       <filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may
 141       then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
 142       authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System
 143       Partition. See
 144       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 145       and
 146       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 147       for
 148       details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM
 149       PCR 12 (if a TPM is present).</para></listitem>
 150
 151       <listitem><para>Similarly, files <filename><replaceable>foo</replaceable>.efi.extra.d/*.raw</filename>
 152       are packed up in a <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename>
 153       directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension
 154       images to the initrd. See
 155       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
 156       details on system extension images. The generated <command>cpio</command> archive containing these
 157       system extension images is measured into TPM PCR 13 (if a TPM is present).</para></listitem>
 158
 159       <listitem><para>Similarly, files
 160       <filename><replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename>
 161       are loaded and verified as PE binaries, and a <literal>.cmdline</literal> section is parsed from them.
 162       In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
 163       Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a
 164       a <literal>.uname</literal> section, the addon will be rejected if they do not exactly match. It is
 165       recommended to always add a <literal>.sbat</literal> section to all signed addons, so that they may be
 166       revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
 167       <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will
 168       add a SBAT policy by default if none is passed when building addons. For more information on SBAT see
 169       <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">Shim's documentation</ulink>.
 170       Addons are supposed to be used to pass additional kernel command line parameters, regardless of the
 171       kernel image being booted, for example to allow platform vendors to ship platform-specific
 172       configuration. The loaded command line addon files are sorted, loaded, measured into TPM PCR 12 (if a
 173       TPM is present) and appended to the kernel command line. UKI command line options are listed first,
 174       then options from addons in <filename>/loader/addons/*.addon.efi</filename> are appended next, and
 175       finally UKI-specific addons are appended last. Addons are always loaded in the same order based on the
 176       filename, so that, given the same set of addons, the same set of measurements can be expected in
 177       PCR12, however note that the filename is not protected by the PE signature, and as such an attacker
 178       with write access to the ESP could potentially rename these files to change the order in which they
 179       are loaded, in a way that could alter the functionality of the kernel, as some options might be order
 180       dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of an
 181       attestation service so that improper use of your signed addons can be detected and dealt with using
 182       one of the aforementioned revocation mechanisms.</para></listitem>
 183
 184       <listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
 185       <command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
 186       directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
 187       the initrd, regardless of the kernel being booted. The generated <command>cpio</command> archive is
 188       measured into TPM PCR 12 (if a TPM is present).</para></listitem>
 189
 190       <listitem><para>Additionally, files <filename>/loader/addons/*.addon.efi</filename> are loaded and
 191       verified as PE binaries, and a <literal>.cmdline</literal> section is parsed from them. This is
 192       supposed to be used to pass additional command line parameters to the kernel, regardless of the kernel
 193       being booted.</para></listitem>
 194     </itemizedlist>
 195
 196     <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
 197     images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
 198     further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
 199     by <command>systemd-creds encrypt -T</command> (see
 200     <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 201     details); in case of the system extension images by using signed Verity images.</para>
 202   </refsect1>
 203
 204   <refsect1>
 205     <title>TPM PCR Notes</title>
 206
 207     <para>Note that when a unified kernel using <command>systemd-stub</command> is invoked the firmware will
 208     measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the
 209     core kernel, the embedded initrd and kernel command line (see above for a full list).</para>
 210
 211     <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
 212     every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
 213     measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials will be measured to both PCR
 214     9 and PCR 12; the initrd synthesized from system extensions will be measured to both PCR 4 and PCR
 215     9. Let's summarize the OS resources and the PCRs they are measured to:</para>
 216
 217     <table>
 218       <title>OS Resource PCR Summary</title>
 219
 220       <tgroup cols='2' align='left' colsep='1' rowsep='1'>
 221         <colspec colname="pcr" />
 222         <colspec colname="definition" />
 223
 224         <thead>
 225           <row>
 226             <entry>OS Resource</entry>
 227             <entry>Measurement PCR</entry>
 228           </row>
 229         </thead>
 230
 231         <tbody>
 232           <row>
 233             <entry><command>systemd-stub</command> code (the entry point of the unified PE binary)</entry>
 234             <entry>4</entry>
 235           </row>
 236
 237           <row>
 238             <entry>Core kernel code (embedded in unified PE binary)</entry>
 239             <entry>4 + 11</entry>
 240           </row>
 241
 242           <row>
 243             <entry>OS release information (embedded in the unified PE binary)</entry>
 244             <entry>4 + 11</entry>
 245           </row>
 246
 247           <row>
 248             <entry>Main initrd (embedded in unified PE binary)</entry>
 249             <entry>4 + 9 + 11</entry>
 250           </row>
 251
 252           <row>
 253             <entry>Default kernel command line (embedded in unified PE binary)</entry>
 254             <entry>4 + 11</entry>
 255           </row>
 256
 257           <row>
 258             <entry>Overridden kernel command line</entry>
 259             <entry>12</entry>
 260           </row>
 261
 262           <row>
 263             <entry>Boot splash (embedded in the unified PE binary)</entry>
 264             <entry>4 + 11</entry>
 265           </row>
 266
 267           <row>
 268             <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry>
 269             <entry>4 + 9</entry>
 270           </row>
 271
 272           <row>
 273             <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry>
 274             <entry>4 + 9 + 11</entry>
 275           </row>
 276
 277           <row>
 278             <entry>Credentials (synthesized initrd from companion files)</entry>
 279             <entry>9 + 12</entry>
 280           </row>
 281
 282           <row>
 283             <entry>System Extensions (synthesized initrd from companion files)</entry>
 284             <entry>9 + 13</entry>
 285           </row>
 286         </tbody>
 287       </tgroup>
 288     </table>
 289   </refsect1>
 290
 291   <refsect1>
 292     <title>EFI Variables</title>
 293
 294     <para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
 295     vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
 296     stub and the OS:</para>
 297
 298     <variablelist class='efi-variables'>
 299       <varlistentry>
 300         <term><varname>LoaderDevicePartUUID</varname></term>
 301
 302         <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
 303         from. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 304         uses this information to automatically find the disk booted from, in order to discover various other
 305         partitions on the same disk automatically.</para>
 306
 307         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 308       </varlistentry>
 309
 310       <varlistentry>
 311         <term><varname>LoaderFirmwareInfo</varname></term>
 312         <term><varname>LoaderFirmwareType</varname></term>
 313
 314         <listitem><para>Brief firmware information. Use
 315         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
 316         data.</para>
 317
 318         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 319       </varlistentry>
 320
 321       <varlistentry>
 322         <term><varname>LoaderImageIdentifier</varname></term>
 323
 324         <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
 325         directory. Use
 326         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 327         this data.</para>
 328
 329         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 330       </varlistentry>
 331
 332       <varlistentry>
 333         <term><varname>StubInfo</varname></term>
 334
 335         <listitem><para>Brief stub information. Use
 336         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 337         this data.</para>
 338
 339         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 340       </varlistentry>
 341
 342       <varlistentry>
 343         <term><varname>StubPcrKernelImage</varname></term>
 344
 345         <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
 346         database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
 347         <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
 348         unset otherwise.</para>
 349
 350         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 351       </varlistentry>
 352
 353       <varlistentry>
 354         <term><varname>StubPcrKernelParameters</varname></term>
 355
 356         <listitem><para>The PCR register index the kernel command line and credentials are measured into,
 357         formatted as decimal ASCII string (e.g. <literal>12</literal>). This variable is set if a measurement
 358         was successfully completed, and remains unset otherwise.</para>
 359
 360         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 361       </varlistentry>
 362
 363       <varlistentry>
 364         <term><varname>StubPcrInitRDSysExts</varname></term>
 365
 366         <listitem><para>The PCR register index the systemd extensions for the initrd, which are picked up
 367         from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
 368         <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
 369         unset otherwise.</para>
 370
 371         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 372       </varlistentry>
 373     </variablelist>
 374
 375     <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
 376     them if they aren't set already. Some of these variables are defined by the <ulink
 377     url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
 378   </refsect1>
 379
 380   <refsect1>
 381     <title>initrd Resources</title>
 382
 383     <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
 384     the initial file system hierarchy in the initrd execution environment:</para>
 385
 386     <variablelist>
 387       <varlistentry>
 388         <term><filename>/</filename></term>
 389
 390         <listitem><para>The main initrd from the <literal>.initrd</literal> PE section of the unified kernel image.</para>
 391
 392         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 393       </varlistentry>
 394
 395       <varlistentry>
 396         <term><filename>/.extra/credentials/*.cred</filename></term>
 397         <listitem><para>Credential files (suffix <literal>.cred</literal>) that are placed next to the
 398         unified kernel image (as described above) are copied into the
 399         <filename>/.extra/credentials/</filename> directory in the initrd execution
 400         environment.</para>
 401
 402         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 403       </varlistentry>
 404
 405       <varlistentry>
 406         <term><filename>/.extra/global_credentials/*.cred</filename></term>
 407         <listitem><para>Similar, credential files in the <filename>/loader/credentials/</filename> directory
 408         in the file system the unified kernel image is placed in are copied into the
 409         <filename>/.extra/global_credentials/</filename> directory in the initrd execution
 410         environment.</para>
 411
 412         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 413       </varlistentry>
 414
 415       <varlistentry>
 416         <term><filename>/.extra/sysext/*.raw</filename></term>
 417         <listitem><para>System extension image files (suffix <literal>.raw</literal>) that are placed next to
 418         the unified kernel image (as described above) are copied into the
 419         <filename>/.extra/sysext/</filename> directory in the initrd execution environment.</para>
 420
 421         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 422       </varlistentry>
 423
 424       <varlistentry>
 425         <term><filename>/.extra/tpm2-pcr-signature.json</filename></term>
 426         <listitem><para>The TPM2 PCR signature JSON object included in the <literal>.pcrsig</literal> PE
 427         section of the unified kernel image is copied into the
 428         <filename>/.extra/tpm2-pcr-signature.json</filename> file in the initrd execution
 429         environment.</para>
 430
 431         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 432       </varlistentry>
 433
 434       <varlistentry>
 435         <term><filename>/.extra/tpm2-pcr-pkey.pem</filename></term>
 436         <listitem><para>The PEM public key included in the <literal>.pcrpkey</literal> PE section of the
 437         unified kernel image is copied into the <filename>/.extra/tpm2-pcr-public-key.pem</filename> file in
 438         the initrd execution environment.</para>
 439
 440         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 441       </varlistentry>
 442     </variablelist>
 443
 444     <para>Note that all these files are located in the <literal>tmpfs</literal> file system the kernel sets
 445     up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
 446     environment into the host file system. If these resources shall be kept around over this transition they
 447     need to be copied to a place that survives the transition first, for example via a suitable
 448     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
 449     default, this is done for the TPM2 PCR signature and public key files.</para>
 450   </refsect1>
 451
 452   <refsect1>
 453     <title>SMBIOS Type 11 Strings</title>
 454
 455     <para><command>systemd-stub</command> can be configured using SMBIOS Type 11 strings. Applicable strings
 456     consist of a name, followed by <literal>=</literal>, followed by the value.
 457     <command>systemd-stub</command> will search the table for a string with a specific name, and if found,
 458     use its value. The following strings are read:</para>
 459
 460     <variablelist>
 461       <varlistentry>
 462         <term><varname>io.systemd.stub.kernel-cmdline-extra</varname></term>
 463         <listitem><para>If set, the value of this string is added to the list of kernel command line
 464         arguments that are measured in PCR12 and passed to the kernel.</para>
 465
 466         <xi:include href="version-info.xml" xpointer="v254"/></listitem>
 467       </varlistentry>
 468     </variablelist>
 469   </refsect1>
 470
 471   <refsect1>
 472     <title>Assembling Kernel Images</title>
 473
 474     <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
 475     <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
 476   </refsect1>
 477
 478   <refsect1>
 479     <title>See Also</title>
 480     <para>
 481       <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 482       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 483       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 484       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 485       <ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink>,
 486       <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
 487       <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 488       <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 489       <ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink>
 490     </para>
 491   </refsect1>
 492 </refentry>