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><simplelist>
  29       <member><filename>/usr/lib/systemd/boot/efi/linuxx64.efi.stub</filename></member>
  30       <member><filename>/usr/lib/systemd/boot/efi/linuxia32.efi.stub</filename></member>
  31       <member><filename>/usr/lib/systemd/boot/efi/linuxaa64.efi.stub</filename></member>
  32       <member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename></member>
  33       <member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.cred</filename></member>
  34       <member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.raw</filename></member>
  35       <member><filename><replaceable>ESP</replaceable>/loader/addons/*.addon.efi</filename></member>
  36       <member><filename><replaceable>ESP</replaceable>/loader/credentials/*.cred</filename></member>
  37     </simplelist></para>
  38   </refsynopsisdiv>
  39
  40   <refsect1>
  41     <title>Description</title>
  42
  43     <para><command>systemd-stub</command> (stored in per-architecture files
  44     <filename>linuxx64.efi.stub</filename>, <filename>linuxia32.efi.stub</filename>,
  45     <filename>linuxaa64.efi.stub</filename> on disk) is a simple UEFI boot stub. An UEFI boot stub is
  46     attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware
  47     environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux
  48     kernel is executable as regular UEFI binary, and is able to do various preparations before switching the
  49     system into the Linux world.</para>
  50
  51     <para>The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary
  52     itself. This allows combining various resources inside a single PE binary image (usually called "Unified
  53     Kernel Image", or "UKI" for short), which may then be signed via UEFI SecureBoot as a whole, covering all
  54     individual resources at once. Specifically it may include:</para>
  55
  56     <itemizedlist>
  57       <!-- Let's keep this in the canonical order we also measure the sections by, i.e. as in
  58            src/fundamental/uki.h's UnifiedSection enum -->
  59
  60       <listitem><para>A <literal>.linux</literal> section with the ELF Linux kernel image.</para></listitem>
  61
  62       <listitem><para>An <literal>.osrel</literal> section with OS release information, i.e. the contents of
  63       the <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
  64       of the OS the kernel belongs to.</para></listitem>
  65
  66       <listitem><para>A <literal>.cmdline</literal> section with the kernel command line to pass to the
  67       invoked kernel.</para></listitem>
  68
  69       <listitem><para>An <literal>.initrd</literal> section with the initrd.</para></listitem>
  70
  71       <listitem><para>A <literal>.splash</literal> section with an image (in the Windows
  72       <filename>.BMP</filename> format) to show on screen before invoking the kernel.</para></listitem>
  73
  74       <listitem><para>A <literal>.dtb</literal> section with a compiled binary DeviceTree.</para></listitem>
  75
  76       <listitem><para>A <literal>.uname</literal> section with the kernel version information, i.e. the
  77       output of <command>uname -r</command> for the kernel included in the <literal>.linux</literal>
  78       section.</para></listitem>
  79
  80       <listitem><para>An <literal>.sbat</literal> section with
  81       <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT</ulink> revocation
  82       metadata.</para></listitem>
  83
  84       <listitem><para>A <literal>.pcrsig</literal> section with a set of cryptographic signatures for the
  85       expected TPM2 PCR values after the kernel has been booted, in JSON format. This is useful for
  86       implementing TPM2 policies that bind disk encryption and similar to kernels that are signed by a
  87       specific key.</para></listitem>
  88
  89       <listitem><para>A <literal>.pcrpkey</literal> section with a public key in the PEM format matching the
  90       signature data in the the <literal>.pcrsig</literal> section.</para></listitem>
  91     </itemizedlist>
  92
  93     <para>If UEFI SecureBoot is enabled and the <literal>.cmdline</literal> section is present in the executed
  94     image, any attempts to override the kernel command line by passing one as invocation parameters to the
  95     EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
  96     SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
  97     is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is
  98     present).</para>
  99
 100     <para>If a DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing
 101     DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
 102     <literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para>
 103
 104     <para>The contents of eight of these nine sections are measured into TPM PCR 11. It is otherwise not used
 105     and thus the result can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section
 106     is not included in this PCR measurement, since it is supposed to contain signatures for the output of the
 107     measurement operation, and thus cannot also be input to it.</para>
 108
 109     <para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> sections are present in a
 110     unified kernel image their contents are passed to the booted kernel in an synthetic initrd cpio archive
 111     that places them in the <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. If the kernel binary
 139       uses a counter for the purpose of
 140       <ulink url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot Assessment</ulink>, this
 141       counter will be ignored. For example, <filename><replaceable>foo</replaceable>+3-0.efi</filename>
 142       will look in directory <filename><replaceable>foo</replaceable>.efi.extra.d/</filename>.
 143       A <command>cpio</command>
 144       archive is generated from all files found that way, placing them in the
 145       <filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may
 146       then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
 147       authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System
 148       Partition. See
 149       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 150       and
 151       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 152       for
 153       details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM
 154       PCR 12 (if a TPM is present).</para></listitem>
 155
 156       <listitem><para>Similarly, files <filename><replaceable>foo</replaceable>.efi.extra.d/*.raw</filename>
 157       are packed up in a <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename>
 158       directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension
 159       images to the initrd. See
 160       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
 161       details on system extension images. The generated <command>cpio</command> archive containing these
 162       system extension images is measured into TPM PCR 13 (if a TPM is present).</para></listitem>
 163
 164       <listitem><para>Similarly, files
 165       <filename><replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename> are loaded and verified as
 166       PE binaries, and a <literal>.cmdline</literal> section is parsed from them. Addons are supposed to be
 167       used to pass additional kernel command line parameters or Devicetree blobs, regardless of the kernel
 168       image being booted, for example to allow platform vendors to ship platform-specific
 169       configuration.</para>
 170
 171       <para>In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
 172       Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a a
 173       <literal>.uname</literal> section, the addon will be rejected if they do not match exactly. It is
 174       recommended to always add a <literal>.sbat</literal> section to all signed addons, so that they may be
 175       revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
 176       <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will add
 177       a SBAT policy by default if none is passed when building addons. For more information on SBAT see
 178       <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">Shim documentation</ulink>.</para>
 179
 180       <para>Addon files are sorted, loaded, and measured into TPM PCR 12 (if a TPM is present) and appended
 181       to the kernel command line. UKI command line options are listed first, then options from addons in
 182       <filename>/loader/addons/*.addon.efi</filename>, and finally UKI-specific addons. Device tree blobs are
 183       loaded and measured following the same algorithm. Addons are always loaded in the same order based on
 184       the filename, so that, given the same set of addons, the same set of measurements can be expected in
 185       PCR12. However, note that the filename is not protected by the PE signature, and as such an attacker
 186       with write access to the ESP could potentially rename these files to change the order in which they are
 187       loaded, in a way that could alter the functionality of the kernel, as some options might be
 188       order-dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of
 189       an attestation service so that improper use of your signed addons can be detected and dealt with using
 190       one of the aforementioned revocation mechanisms.</para></listitem>
 191
 192       <listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
 193       <command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
 194       directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
 195       the initrd, regardless of the kernel being booted. The generated <command>cpio</command> archive is
 196       measured into TPM PCR 12 (if a TPM is present).</para></listitem>
 197
 198       <listitem><para>Additionally, files <filename>/loader/addons/*.addon.efi</filename> are loaded and
 199       verified as PE binaries, and <literal>.cmdline</literal> and/or <literal>.dtb</literal> sections are
 200       parsed from them. This is supposed to be used to pass additional command line parameters or Devicetree
 201       blobs to the kernel, regardless of the kernel being booted.</para></listitem>
 202     </itemizedlist>
 203
 204     <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
 205     images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
 206     further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
 207     by <command>systemd-creds encrypt -T</command> (see
 208     <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 209     details); in case of the system extension images by using signed Verity images.</para>
 210   </refsect1>
 211
 212   <refsect1>
 213     <title>TPM PCR Notes</title>
 214
 215     <para>Note that when a unified kernel using <command>systemd-stub</command> is invoked the firmware will
 216     measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the
 217     core kernel, the embedded initrd and kernel command line (see above for a full list).</para>
 218
 219     <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
 220     every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
 221     measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials will be measured to both PCR
 222     9 and PCR 12; the initrd synthesized from system extensions will be measured to both PCR 4 and PCR
 223     9. Let's summarize the OS resources and the PCRs they are measured to:</para>
 224
 225     <table>
 226       <title>OS Resource PCR Summary</title>
 227
 228       <tgroup cols='2' align='left' colsep='1' rowsep='1'>
 229         <colspec colname="pcr" />
 230         <colspec colname="definition" />
 231
 232         <thead>
 233           <row>
 234             <entry>OS Resource</entry>
 235             <entry>Measurement PCR</entry>
 236           </row>
 237         </thead>
 238
 239         <tbody>
 240           <row>
 241             <entry><command>systemd-stub</command> code (the entry point of the unified PE binary)</entry>
 242             <entry>4</entry>
 243           </row>
 244
 245           <row>
 246             <entry>Core kernel code (embedded in unified PE binary)</entry>
 247             <entry>4 + 11</entry>
 248           </row>
 249
 250           <row>
 251             <entry>OS release information (embedded in the unified PE binary)</entry>
 252             <entry>4 + 11</entry>
 253           </row>
 254
 255           <row>
 256             <entry>Main initrd (embedded in unified PE binary)</entry>
 257             <entry>4 + 9 + 11</entry>
 258           </row>
 259
 260           <row>
 261             <entry>Default kernel command line (embedded in unified PE binary)</entry>
 262             <entry>4 + 11</entry>
 263           </row>
 264
 265           <row>
 266             <entry>Overridden kernel command line</entry>
 267             <entry>12</entry>
 268           </row>
 269
 270           <row>
 271             <entry>Boot splash (embedded in the unified PE binary)</entry>
 272             <entry>4 + 11</entry>
 273           </row>
 274
 275           <row>
 276             <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry>
 277             <entry>4 + 9</entry>
 278           </row>
 279
 280           <row>
 281             <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry>
 282             <entry>4 + 9 + 11</entry>
 283           </row>
 284
 285           <row>
 286             <entry>Credentials (synthesized initrd from companion files)</entry>
 287             <entry>9 + 12</entry>
 288           </row>
 289
 290           <row>
 291             <entry>System Extensions (synthesized initrd from companion files)</entry>
 292             <entry>9 + 13</entry>
 293           </row>
 294         </tbody>
 295       </tgroup>
 296     </table>
 297   </refsect1>
 298
 299   <refsect1>
 300     <title>EFI Variables</title>
 301
 302     <para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
 303     vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
 304     stub and the OS:</para>
 305
 306     <variablelist class='efi-variables'>
 307       <varlistentry>
 308         <term><varname>LoaderDevicePartUUID</varname></term>
 309
 310         <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
 311         from. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 312         uses this information to automatically find the disk booted from, in order to discover various other
 313         partitions on the same disk automatically.</para>
 314
 315         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 316       </varlistentry>
 317
 318       <varlistentry>
 319         <term><varname>LoaderFirmwareInfo</varname></term>
 320         <term><varname>LoaderFirmwareType</varname></term>
 321
 322         <listitem><para>Brief firmware information. Use
 323         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
 324         data.</para>
 325
 326         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 327       </varlistentry>
 328
 329       <varlistentry>
 330         <term><varname>LoaderImageIdentifier</varname></term>
 331
 332         <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
 333         directory. Use
 334         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 335         this data.</para>
 336
 337         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 338       </varlistentry>
 339
 340       <varlistentry>
 341         <term><varname>StubInfo</varname></term>
 342
 343         <listitem><para>Brief stub information. Use
 344         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 345         this data.</para>
 346
 347         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 348       </varlistentry>
 349
 350       <varlistentry>
 351         <term><varname>StubPcrKernelImage</varname></term>
 352
 353         <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
 354         database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
 355         <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
 356         unset otherwise.</para>
 357
 358         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 359       </varlistentry>
 360
 361       <varlistentry>
 362         <term><varname>StubPcrKernelParameters</varname></term>
 363
 364         <listitem><para>The PCR register index the kernel command line and credentials are measured into,
 365         formatted as decimal ASCII string (e.g. <literal>12</literal>). This variable is set if a measurement
 366         was successfully completed, and remains unset otherwise.</para>
 367
 368         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 369       </varlistentry>
 370
 371       <varlistentry>
 372         <term><varname>StubPcrInitRDSysExts</varname></term>
 373
 374         <listitem><para>The PCR register index the systemd extensions for the initrd, which are picked up
 375         from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
 376         <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
 377         unset otherwise.</para>
 378
 379         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 380       </varlistentry>
 381     </variablelist>
 382
 383     <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
 384     them if they aren't set already. Some of these variables are defined by the <ulink
 385     url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
 386   </refsect1>
 387
 388   <refsect1>
 389     <title>initrd Resources</title>
 390
 391     <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
 392     the initial file system hierarchy in the initrd execution environment:</para>
 393
 394     <variablelist>
 395       <varlistentry>
 396         <term><filename>/</filename></term>
 397
 398         <listitem><para>The main initrd from the <literal>.initrd</literal> PE section of the unified kernel
 399         image.</para>
 400
 401         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 402       </varlistentry>
 403
 404       <varlistentry>
 405         <term><filename>/.extra/credentials/*.cred</filename></term>
 406         <listitem><para>Credential files (suffix <literal>.cred</literal>) that are placed next to the
 407         unified kernel image (as described above) are copied into the
 408         <filename>/.extra/credentials/</filename> directory in the initrd execution
 409         environment.</para>
 410
 411         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 412       </varlistentry>
 413
 414       <varlistentry>
 415         <term><filename>/.extra/global_credentials/*.cred</filename></term>
 416         <listitem><para>Similarly, credential files in the <filename>/loader/credentials/</filename>
 417         directory in the file system the unified kernel image is placed in are copied into the
 418         <filename>/.extra/global_credentials/</filename> directory in the initrd execution
 419         environment.</para>
 420
 421         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 422       </varlistentry>
 423
 424       <varlistentry>
 425         <term><filename>/.extra/sysext/*.raw</filename></term>
 426         <listitem><para>System extension image files (suffix <literal>.raw</literal>) that are placed next to
 427         the unified kernel image (as described above) are copied into the
 428         <filename>/.extra/sysext/</filename> directory in the initrd execution environment.</para>
 429
 430         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 431       </varlistentry>
 432
 433       <varlistentry>
 434         <term><filename>/.extra/tpm2-pcr-signature.json</filename></term>
 435         <listitem><para>The TPM2 PCR signature JSON object included in the <literal>.pcrsig</literal> PE
 436         section of the unified kernel image is copied into the
 437         <filename>/.extra/tpm2-pcr-signature.json</filename> file in the initrd execution environment.</para>
 438
 439         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 440       </varlistentry>
 441
 442       <varlistentry>
 443         <term><filename>/.extra/tpm2-pcr-pkey.pem</filename></term>
 444         <listitem><para>The PEM public key included in the <literal>.pcrpkey</literal> PE section of the
 445         unified kernel image is copied into the <filename>/.extra/tpm2-pcr-public-key.pem</filename> file in
 446         the initrd execution environment.</para>
 447
 448         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 449       </varlistentry>
 450     </variablelist>
 451
 452     <para>Note that all these files are located in the <literal>tmpfs</literal> file system the kernel sets
 453     up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
 454     environment into the host file system. If these resources shall be kept around over this transition they
 455     need to be copied to a place that survives the transition first, for example via a suitable
 456     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
 457     default, this is done for the TPM2 PCR signature and public key files.</para>
 458   </refsect1>
 459
 460   <refsect1>
 461     <title>SMBIOS Type 11 Strings</title>
 462
 463     <para><command>systemd-stub</command> can be configured using SMBIOS Type 11 strings. Applicable strings
 464     consist of a name, followed by <literal>=</literal>, followed by the value.
 465     <command>systemd-stub</command> will search the table for a string with a specific name, and if found,
 466     use its value. The following strings are read:</para>
 467
 468     <variablelist>
 469       <varlistentry>
 470         <term><varname>io.systemd.stub.kernel-cmdline-extra</varname></term>
 471         <listitem><para>If set, the value of this string is added to the list of kernel command line
 472         arguments that are measured in PCR12 and passed to the kernel.</para>
 473
 474         <xi:include href="version-info.xml" xpointer="v254"/></listitem>
 475       </varlistentry>
 476     </variablelist>
 477   </refsect1>
 478
 479   <refsect1>
 480     <title>Assembling Kernel Images</title>
 481
 482     <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
 483     <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
 484   </refsect1>
 485
 486   <refsect1>
 487     <title>See Also</title>
 488     <para><simplelist type="inline">
 489       <member><citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
 490       <member><citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 491       <member><citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 492       <member><citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 493       <member><ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink></member>
 494       <member><ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink></member>
 495       <member><citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 496       <member><citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 497       <member><ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink></member>
 498     </simplelist></para>
 499   </refsect1>
 500 </refentry>