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 or Devicetree blobs,
 171       regardless of the kernel image being booted, for example to allow platform vendors to ship
 172       platform-specific configuration. The loaded command line addon files are sorted, loaded, and measured
 173       into TPM PCR 12 (if a TPM is present) and appended to the kernel command line. UKI command line options
 174       are listed first, then options from addons in <filename>/loader/addons/*.addon.efi</filename>, and
 175       finally UKI-specific addons. Device tree blobs are loaded and measured following the same algorithm.
 176       Addons are always loaded in the same order based on the filename, so that, given the same set of
 177       addons, the same set of measurements can be expected in PCR12. However, note that the filename is not
 178       protected by the PE signature, and as such an attacker with write access to the ESP could potentially
 179       rename these files to change the order in which they are loaded, in a way that could alter the
 180       functionality of the kernel, as some options might be order dependent. If you sign such addons, you
 181       should pay attention to the PCR12 values and make use of an attestation service so that improper use
 182       of your signed addons can be detected and dealt with using one of the aforementioned revocation
 183       mechanisms.</para></listitem>
 184
 185       <listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
 186       <command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
 187       directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
 188       the initrd, regardless of the kernel being booted. The generated <command>cpio</command> archive is
 189       measured into TPM PCR 12 (if a TPM is present).</para></listitem>
 190
 191       <listitem><para>Additionally, files <filename>/loader/addons/*.addon.efi</filename> are loaded and
 192       verified as PE binaries, and <literal>.cmdline</literal> and/or <literal>.dtb</literal> sections are
 193       parsed from them. This is supposed to be used to pass additional command line parameters or Devicetree
 194       blobs to the kernel, regardless of the kernel being booted.</para></listitem>
 195     </itemizedlist>
 196
 197     <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
 198     images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
 199     further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
 200     by <command>systemd-creds encrypt -T</command> (see
 201     <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 202     details); in case of the system extension images by using signed Verity images.</para>
 203   </refsect1>
 204
 205   <refsect1>
 206     <title>TPM PCR Notes</title>
 207
 208     <para>Note that when a unified kernel using <command>systemd-stub</command> is invoked the firmware will
 209     measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the
 210     core kernel, the embedded initrd and kernel command line (see above for a full list).</para>
 211
 212     <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
 213     every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
 214     measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials will be measured to both PCR
 215     9 and PCR 12; the initrd synthesized from system extensions will be measured to both PCR 4 and PCR
 216     9. Let's summarize the OS resources and the PCRs they are measured to:</para>
 217
 218     <table>
 219       <title>OS Resource PCR Summary</title>
 220
 221       <tgroup cols='2' align='left' colsep='1' rowsep='1'>
 222         <colspec colname="pcr" />
 223         <colspec colname="definition" />
 224
 225         <thead>
 226           <row>
 227             <entry>OS Resource</entry>
 228             <entry>Measurement PCR</entry>
 229           </row>
 230         </thead>
 231
 232         <tbody>
 233           <row>
 234             <entry><command>systemd-stub</command> code (the entry point of the unified PE binary)</entry>
 235             <entry>4</entry>
 236           </row>
 237
 238           <row>
 239             <entry>Core kernel code (embedded in unified PE binary)</entry>
 240             <entry>4 + 11</entry>
 241           </row>
 242
 243           <row>
 244             <entry>OS release information (embedded in the unified PE binary)</entry>
 245             <entry>4 + 11</entry>
 246           </row>
 247
 248           <row>
 249             <entry>Main initrd (embedded in unified PE binary)</entry>
 250             <entry>4 + 9 + 11</entry>
 251           </row>
 252
 253           <row>
 254             <entry>Default kernel command line (embedded in unified PE binary)</entry>
 255             <entry>4 + 11</entry>
 256           </row>
 257
 258           <row>
 259             <entry>Overridden kernel command line</entry>
 260             <entry>12</entry>
 261           </row>
 262
 263           <row>
 264             <entry>Boot splash (embedded in the unified PE binary)</entry>
 265             <entry>4 + 11</entry>
 266           </row>
 267
 268           <row>
 269             <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry>
 270             <entry>4 + 9</entry>
 271           </row>
 272
 273           <row>
 274             <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry>
 275             <entry>4 + 9 + 11</entry>
 276           </row>
 277
 278           <row>
 279             <entry>Credentials (synthesized initrd from companion files)</entry>
 280             <entry>9 + 12</entry>
 281           </row>
 282
 283           <row>
 284             <entry>System Extensions (synthesized initrd from companion files)</entry>
 285             <entry>9 + 13</entry>
 286           </row>
 287         </tbody>
 288       </tgroup>
 289     </table>
 290   </refsect1>
 291
 292   <refsect1>
 293     <title>EFI Variables</title>
 294
 295     <para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
 296     vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
 297     stub and the OS:</para>
 298
 299     <variablelist class='efi-variables'>
 300       <varlistentry>
 301         <term><varname>LoaderDevicePartUUID</varname></term>
 302
 303         <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
 304         from. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 305         uses this information to automatically find the disk booted from, in order to discover various other
 306         partitions on the same disk automatically.</para>
 307
 308         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 309       </varlistentry>
 310
 311       <varlistentry>
 312         <term><varname>LoaderFirmwareInfo</varname></term>
 313         <term><varname>LoaderFirmwareType</varname></term>
 314
 315         <listitem><para>Brief firmware information. Use
 316         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
 317         data.</para>
 318
 319         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 320       </varlistentry>
 321
 322       <varlistentry>
 323         <term><varname>LoaderImageIdentifier</varname></term>
 324
 325         <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
 326         directory. Use
 327         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 328         this data.</para>
 329
 330         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 331       </varlistentry>
 332
 333       <varlistentry>
 334         <term><varname>StubInfo</varname></term>
 335
 336         <listitem><para>Brief stub information. Use
 337         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 338         this data.</para>
 339
 340         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 341       </varlistentry>
 342
 343       <varlistentry>
 344         <term><varname>StubPcrKernelImage</varname></term>
 345
 346         <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
 347         database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
 348         <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
 349         unset otherwise.</para>
 350
 351         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 352       </varlistentry>
 353
 354       <varlistentry>
 355         <term><varname>StubPcrKernelParameters</varname></term>
 356
 357         <listitem><para>The PCR register index the kernel command line and credentials are measured into,
 358         formatted as decimal ASCII string (e.g. <literal>12</literal>). This variable is set if a measurement
 359         was successfully completed, and remains unset otherwise.</para>
 360
 361         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 362       </varlistentry>
 363
 364       <varlistentry>
 365         <term><varname>StubPcrInitRDSysExts</varname></term>
 366
 367         <listitem><para>The PCR register index the systemd extensions for the initrd, which are picked up
 368         from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
 369         <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
 370         unset otherwise.</para>
 371
 372         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 373       </varlistentry>
 374     </variablelist>
 375
 376     <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
 377     them if they aren't set already. Some of these variables are defined by the <ulink
 378     url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
 379   </refsect1>
 380
 381   <refsect1>
 382     <title>initrd Resources</title>
 383
 384     <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
 385     the initial file system hierarchy in the initrd execution environment:</para>
 386
 387     <variablelist>
 388       <varlistentry>
 389         <term><filename>/</filename></term>
 390
 391         <listitem><para>The main initrd from the <literal>.initrd</literal> PE section of the unified kernel image.</para>
 392
 393         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 394       </varlistentry>
 395
 396       <varlistentry>
 397         <term><filename>/.extra/credentials/*.cred</filename></term>
 398         <listitem><para>Credential files (suffix <literal>.cred</literal>) that are placed next to the
 399         unified kernel image (as described above) are copied into the
 400         <filename>/.extra/credentials/</filename> directory in the initrd execution
 401         environment.</para>
 402
 403         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 404       </varlistentry>
 405
 406       <varlistentry>
 407         <term><filename>/.extra/global_credentials/*.cred</filename></term>
 408         <listitem><para>Similar, credential files in the <filename>/loader/credentials/</filename> directory
 409         in the file system the unified kernel image is placed in are copied into the
 410         <filename>/.extra/global_credentials/</filename> directory in the initrd execution
 411         environment.</para>
 412
 413         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 414       </varlistentry>
 415
 416       <varlistentry>
 417         <term><filename>/.extra/sysext/*.raw</filename></term>
 418         <listitem><para>System extension image files (suffix <literal>.raw</literal>) that are placed next to
 419         the unified kernel image (as described above) are copied into the
 420         <filename>/.extra/sysext/</filename> directory in the initrd execution environment.</para>
 421
 422         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 423       </varlistentry>
 424
 425       <varlistentry>
 426         <term><filename>/.extra/tpm2-pcr-signature.json</filename></term>
 427         <listitem><para>The TPM2 PCR signature JSON object included in the <literal>.pcrsig</literal> PE
 428         section of the unified kernel image is copied into the
 429         <filename>/.extra/tpm2-pcr-signature.json</filename> file in the initrd execution
 430         environment.</para>
 431
 432         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 433       </varlistentry>
 434
 435       <varlistentry>
 436         <term><filename>/.extra/tpm2-pcr-pkey.pem</filename></term>
 437         <listitem><para>The PEM public key included in the <literal>.pcrpkey</literal> PE section of the
 438         unified kernel image is copied into the <filename>/.extra/tpm2-pcr-public-key.pem</filename> file in
 439         the initrd execution environment.</para>
 440
 441         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 442       </varlistentry>
 443     </variablelist>
 444
 445     <para>Note that all these files are located in the <literal>tmpfs</literal> file system the kernel sets
 446     up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
 447     environment into the host file system. If these resources shall be kept around over this transition they
 448     need to be copied to a place that survives the transition first, for example via a suitable
 449     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
 450     default, this is done for the TPM2 PCR signature and public key files.</para>
 451   </refsect1>
 452
 453   <refsect1>
 454     <title>SMBIOS Type 11 Strings</title>
 455
 456     <para><command>systemd-stub</command> can be configured using SMBIOS Type 11 strings. Applicable strings
 457     consist of a name, followed by <literal>=</literal>, followed by the value.
 458     <command>systemd-stub</command> will search the table for a string with a specific name, and if found,
 459     use its value. The following strings are read:</para>
 460
 461     <variablelist>
 462       <varlistentry>
 463         <term><varname>io.systemd.stub.kernel-cmdline-extra</varname></term>
 464         <listitem><para>If set, the value of this string is added to the list of kernel command line
 465         arguments that are measured in PCR12 and passed to the kernel.</para>
 466
 467         <xi:include href="version-info.xml" xpointer="v254"/></listitem>
 468       </varlistentry>
 469     </variablelist>
 470   </refsect1>
 471
 472   <refsect1>
 473     <title>Assembling Kernel Images</title>
 474
 475     <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
 476     <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
 477   </refsect1>
 478
 479   <refsect1>
 480     <title>See Also</title>
 481     <para>
 482       <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 483       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 484       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 485       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 486       <ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink>,
 487       <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
 488       <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 489       <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 490       <ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink>
 491     </para>
 492   </refsect1>
 493 </refentry>