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>A <literal>.linux</literal> section with the ELF Linux kernel image.</para></listitem>
  59
  60       <listitem><para>An <literal>.osrel</literal> section with OS release information, i.e. the contents of
  61       the <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
  62       of the OS the kernel belongs to.</para></listitem>
  63
  64       <listitem><para>A <literal>.cmdline</literal> section with the kernel command line to pass to the
  65       invoked kernel.</para></listitem>
  66
  67       <listitem><para>An <literal>.initrd</literal> section with the initrd.</para></listitem>
  68
  69       <listitem><para>A <literal>.splash</literal> section with an image (in the Windows
  70       <filename>.BMP</filename> format) to show on screen before invoking the kernel.</para></listitem>
  71
  72       <listitem><para>A <literal>.dtb</literal> section with a compiled binary DeviceTree.</para></listitem>
  73
  74       <listitem><para>A <literal>.uname</literal> section with the kernel version information, i.e. the
  75       output of <command>uname -r</command> for the kernel included in the <literal>.linux</literal>
  76       section.</para></listitem>
  77
  78       <listitem><para>An <literal>.sbat</literal> section with
  79       <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT</ulink> revocation
  80       metadata.</para></listitem>
  81
  82       <listitem><para>A <literal>.pcrsig</literal> section with a set of cryptographic signatures for the
  83       expected TPM2 PCR values after the kernel has been booted, in JSON format. This is useful for
  84       implementing TPM2 policies that bind disk encryption and similar to kernels that are signed by a
  85       specific key.</para></listitem>
  86
  87       <listitem><para>A <literal>.pcrpkey</literal> section with a public key in the PEM format matching the
  88       signature data in the the <literal>.pcrsig</literal> section.</para></listitem>
  89     </itemizedlist>
  90
  91     <para>If UEFI SecureBoot is enabled and the <literal>.cmdline</literal> section is present in the executed
  92     image, any attempts to override the kernel command line by passing one as invocation parameters to the
  93     EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
  94     SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
  95     is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is
  96     present).</para>
  97
  98     <para>If a DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing
  99     DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
 100     <literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para>
 101
 102     <para>The contents of eight of these nine sections are measured into TPM PCR 11. It is otherwise not used
 103     and thus the result can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section
 104     is not included in this PCR measurement, since it is supposed to contain signatures for the output of the
 105     measurement operation, and thus cannot also be input to it.</para>
 106
 107     <para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> sections are present in a
 108     unified kernel image their contents are passed to the booted kernel in an synthetic initrd cpio archive
 109     that places them in the <filename>/.extra/tpm2-pcr-signature.json</filename> and
 110     <filename>/.extra/tpm2-pcr-public-key.pem</filename> files. Typically, a
 111     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
 112     ensures they are copied into <filename>/run/systemd/tpm2-pcr-signature.json</filename> and
 113     <filename>/run/systemd/tpm2-pcr-public-key.pem</filename> where they remain accessible even after the
 114     system transitions out of the initrd environment into the host file system. Tools such
 115     <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 116     <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 117     and <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 118     will automatically use files present under these paths to unlock protected resources (encrypted storage
 119     or credentials) or bind encryption to booted kernels.</para>
 120
 121     <para>For further details about the UKI concept, see the <ulink
 122     url="https://uapi-group.org/specifications/specs/unified_kernel_image/">UKI specification</ulink>.</para>
 123   </refsect1>
 124
 125   <refsect1>
 126     <title>Companion Files</title>
 127
 128     <para>The <command>systemd-stub</command> UEFI boot stub automatically collects three types of auxiliary
 129     companion files optionally placed in drop-in directories on the same partition as the EFI binary,
 130     dynamically generates <command>cpio</command> initrd archives from them, and passes them to the kernel.
 131     Specifically:</para>
 132
 133     <itemizedlist>
 134       <listitem><para>For a kernel binary called <filename><replaceable>foo</replaceable>.efi</filename>, it
 135       will look for files with the <filename>.cred</filename> suffix in a directory named
 136       <filename><replaceable>foo</replaceable>.efi.extra.d/</filename> next to it. If the kernel binary
 137       uses a counter for the purpose of
 138       <ulink url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot Assessment</ulink>, this
 139       counter will be ignored. For example, <filename><replaceable>foo</replaceable>+3-0.efi</filename>
 140       will look in directory <filename><replaceable>foo</replaceable>.efi.extra.d/</filename>.
 141       A <command>cpio</command>
 142       archive is generated from all files found that way, placing them in the
 143       <filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may
 144       then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
 145       authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System
 146       Partition. See
 147       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 148       and
 149       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 150       for
 151       details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM
 152       PCR 12 (if a TPM is present).</para></listitem>
 153
 154       <listitem><para>Similarly, files <filename><replaceable>foo</replaceable>.efi.extra.d/*.raw</filename>
 155       are packed up in a <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename>
 156       directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension
 157       images to the initrd. See
 158       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
 159       details on system extension images. The generated <command>cpio</command> archive containing these
 160       system extension images is measured into TPM PCR 13 (if a TPM is present).</para></listitem>
 161
 162       <listitem><para>Similarly, files
 163       <filename><replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename> are loaded and verified as
 164       PE binaries, and a <literal>.cmdline</literal> section is parsed from them. Addons are supposed to be
 165       used to pass additional kernel command line parameters or Devicetree blobs, regardless of the kernel
 166       image being booted, for example to allow platform vendors to ship platform-specific
 167       configuration.</para>
 168
 169       <para>In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
 170       Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a a
 171       <literal>.uname</literal> section, the addon will be rejected if they do not match exactly. It is
 172       recommended to always add a <literal>.sbat</literal> section to all signed addons, so that they may be
 173       revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
 174       <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will add
 175       a SBAT policy by default if none is passed when building addons. For more information on SBAT see
 176       <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">Shim documentation</ulink>.</para>
 177
 178       <para>Addon files are sorted, loaded, and measured into TPM PCR 12 (if a TPM is present) and appended
 179       to the kernel command line. UKI command line options are listed first, then options from addons in
 180       <filename>/loader/addons/*.addon.efi</filename>, and finally UKI-specific addons. Device tree blobs are
 181       loaded and measured following the same algorithm. Addons are always loaded in the same order based on
 182       the filename, so that, given the same set of addons, the same set of measurements can be expected in
 183       PCR12. However, note that the filename is not protected by the PE signature, and as such an attacker
 184       with write access to the ESP could potentially rename these files to change the order in which they are
 185       loaded, in a way that could alter the functionality of the kernel, as some options might be
 186       order-dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of
 187       an attestation service so that improper use of your signed addons can be detected and dealt with using
 188       one of the aforementioned revocation mechanisms.</para></listitem>
 189
 190       <listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
 191       <command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
 192       directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
 193       the initrd, regardless of the kernel being booted. The generated <command>cpio</command> archive is
 194       measured into TPM PCR 12 (if a TPM is present).</para></listitem>
 195
 196       <listitem><para>Additionally, files <filename>/loader/addons/*.addon.efi</filename> are loaded and
 197       verified as PE binaries, and <literal>.cmdline</literal> and/or <literal>.dtb</literal> sections are
 198       parsed from them. This is supposed to be used to pass additional command line parameters or Devicetree
 199       blobs to the kernel, regardless of the kernel being booted.</para></listitem>
 200     </itemizedlist>
 201
 202     <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
 203     images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
 204     further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
 205     by <command>systemd-creds encrypt -T</command> (see
 206     <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 207     details); in case of the system extension images by using signed Verity images.</para>
 208   </refsect1>
 209
 210   <refsect1>
 211     <title>TPM PCR Notes</title>
 212
 213     <para>Note that when a unified kernel using <command>systemd-stub</command> is invoked the firmware will
 214     measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the
 215     core kernel, the embedded initrd and kernel command line (see above for a full list).</para>
 216
 217     <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
 218     every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
 219     measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials will be measured to both PCR
 220     9 and PCR 12; the initrd synthesized from system extensions will be measured to both PCR 4 and PCR
 221     9. Let's summarize the OS resources and the PCRs they are measured to:</para>
 222
 223     <table>
 224       <title>OS Resource PCR Summary</title>
 225
 226       <tgroup cols='2' align='left' colsep='1' rowsep='1'>
 227         <colspec colname="pcr" />
 228         <colspec colname="definition" />
 229
 230         <thead>
 231           <row>
 232             <entry>OS Resource</entry>
 233             <entry>Measurement PCR</entry>
 234           </row>
 235         </thead>
 236
 237         <tbody>
 238           <row>
 239             <entry><command>systemd-stub</command> code (the entry point of the unified PE binary)</entry>
 240             <entry>4</entry>
 241           </row>
 242
 243           <row>
 244             <entry>Core kernel code (embedded in unified PE binary)</entry>
 245             <entry>4 + 11</entry>
 246           </row>
 247
 248           <row>
 249             <entry>OS release information (embedded in the unified PE binary)</entry>
 250             <entry>4 + 11</entry>
 251           </row>
 252
 253           <row>
 254             <entry>Main initrd (embedded in unified PE binary)</entry>
 255             <entry>4 + 9 + 11</entry>
 256           </row>
 257
 258           <row>
 259             <entry>Default kernel command line (embedded in unified PE binary)</entry>
 260             <entry>4 + 11</entry>
 261           </row>
 262
 263           <row>
 264             <entry>Overridden kernel command line</entry>
 265             <entry>12</entry>
 266           </row>
 267
 268           <row>
 269             <entry>Boot splash (embedded in the unified PE binary)</entry>
 270             <entry>4 + 11</entry>
 271           </row>
 272
 273           <row>
 274             <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry>
 275             <entry>4 + 9</entry>
 276           </row>
 277
 278           <row>
 279             <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry>
 280             <entry>4 + 9 + 11</entry>
 281           </row>
 282
 283           <row>
 284             <entry>Credentials (synthesized initrd from companion files)</entry>
 285             <entry>9 + 12</entry>
 286           </row>
 287
 288           <row>
 289             <entry>System Extensions (synthesized initrd from companion files)</entry>
 290             <entry>9 + 13</entry>
 291           </row>
 292         </tbody>
 293       </tgroup>
 294     </table>
 295   </refsect1>
 296
 297   <refsect1>
 298     <title>EFI Variables</title>
 299
 300     <para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
 301     vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
 302     stub and the OS:</para>
 303
 304     <variablelist class='efi-variables'>
 305       <varlistentry>
 306         <term><varname>LoaderDevicePartUUID</varname></term>
 307
 308         <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
 309         from. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 310         uses this information to automatically find the disk booted from, in order to discover various other
 311         partitions on the same disk automatically.</para>
 312
 313         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 314       </varlistentry>
 315
 316       <varlistentry>
 317         <term><varname>LoaderFirmwareInfo</varname></term>
 318         <term><varname>LoaderFirmwareType</varname></term>
 319
 320         <listitem><para>Brief firmware information. Use
 321         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
 322         data.</para>
 323
 324         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 325       </varlistentry>
 326
 327       <varlistentry>
 328         <term><varname>LoaderImageIdentifier</varname></term>
 329
 330         <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
 331         directory. Use
 332         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 333         this data.</para>
 334
 335         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 336       </varlistentry>
 337
 338       <varlistentry>
 339         <term><varname>StubInfo</varname></term>
 340
 341         <listitem><para>Brief stub information. Use
 342         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 343         this data.</para>
 344
 345         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 346       </varlistentry>
 347
 348       <varlistentry>
 349         <term><varname>StubPcrKernelImage</varname></term>
 350
 351         <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
 352         database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
 353         <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
 354         unset otherwise.</para>
 355
 356         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 357       </varlistentry>
 358
 359       <varlistentry>
 360         <term><varname>StubPcrKernelParameters</varname></term>
 361
 362         <listitem><para>The PCR register index the kernel command line and credentials are measured into,
 363         formatted as decimal ASCII string (e.g. <literal>12</literal>). This variable is set if a measurement
 364         was successfully completed, and remains unset otherwise.</para>
 365
 366         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 367       </varlistentry>
 368
 369       <varlistentry>
 370         <term><varname>StubPcrInitRDSysExts</varname></term>
 371
 372         <listitem><para>The PCR register index the systemd extensions for the initrd, which are picked up
 373         from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
 374         <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
 375         unset otherwise.</para>
 376
 377         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 378       </varlistentry>
 379     </variablelist>
 380
 381     <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
 382     them if they aren't set already. Some of these variables are defined by the <ulink
 383     url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
 384   </refsect1>
 385
 386   <refsect1>
 387     <title>initrd Resources</title>
 388
 389     <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
 390     the initial file system hierarchy in the initrd execution environment:</para>
 391
 392     <variablelist>
 393       <varlistentry>
 394         <term><filename>/</filename></term>
 395
 396         <listitem><para>The main initrd from the <literal>.initrd</literal> PE section of the unified kernel
 397         image.</para>
 398
 399         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 400       </varlistentry>
 401
 402       <varlistentry>
 403         <term><filename>/.extra/credentials/*.cred</filename></term>
 404         <listitem><para>Credential files (suffix <literal>.cred</literal>) that are placed next to the
 405         unified kernel image (as described above) are copied into the
 406         <filename>/.extra/credentials/</filename> directory in the initrd execution
 407         environment.</para>
 408
 409         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 410       </varlistentry>
 411
 412       <varlistentry>
 413         <term><filename>/.extra/global_credentials/*.cred</filename></term>
 414         <listitem><para>Similarly, credential files in the <filename>/loader/credentials/</filename>
 415         directory in the file system the unified kernel image is placed in are copied into the
 416         <filename>/.extra/global_credentials/</filename> directory in the initrd execution
 417         environment.</para>
 418
 419         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 420       </varlistentry>
 421
 422       <varlistentry>
 423         <term><filename>/.extra/sysext/*.raw</filename></term>
 424         <listitem><para>System extension image files (suffix <literal>.raw</literal>) that are placed next to
 425         the unified kernel image (as described above) are copied into the
 426         <filename>/.extra/sysext/</filename> directory in the initrd execution environment.</para>
 427
 428         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 429       </varlistentry>
 430
 431       <varlistentry>
 432         <term><filename>/.extra/tpm2-pcr-signature.json</filename></term>
 433         <listitem><para>The TPM2 PCR signature JSON object included in the <literal>.pcrsig</literal> PE
 434         section of the unified kernel image is copied into the
 435         <filename>/.extra/tpm2-pcr-signature.json</filename> file in the initrd execution environment.</para>
 436
 437         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 438       </varlistentry>
 439
 440       <varlistentry>
 441         <term><filename>/.extra/tpm2-pcr-pkey.pem</filename></term>
 442         <listitem><para>The PEM public key included in the <literal>.pcrpkey</literal> PE section of the
 443         unified kernel image is copied into the <filename>/.extra/tpm2-pcr-public-key.pem</filename> file in
 444         the initrd execution environment.</para>
 445
 446         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 447       </varlistentry>
 448     </variablelist>
 449
 450     <para>Note that all these files are located in the <literal>tmpfs</literal> file system the kernel sets
 451     up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
 452     environment into the host file system. If these resources shall be kept around over this transition they
 453     need to be copied to a place that survives the transition first, for example via a suitable
 454     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
 455     default, this is done for the TPM2 PCR signature and public key files.</para>
 456   </refsect1>
 457
 458   <refsect1>
 459     <title>SMBIOS Type 11 Strings</title>
 460
 461     <para><command>systemd-stub</command> can be configured using SMBIOS Type 11 strings. Applicable strings
 462     consist of a name, followed by <literal>=</literal>, followed by the value.
 463     <command>systemd-stub</command> will search the table for a string with a specific name, and if found,
 464     use its value. The following strings are read:</para>
 465
 466     <variablelist>
 467       <varlistentry>
 468         <term><varname>io.systemd.stub.kernel-cmdline-extra</varname></term>
 469         <listitem><para>If set, the value of this string is added to the list of kernel command line
 470         arguments that are measured in PCR12 and passed to the kernel.</para>
 471
 472         <xi:include href="version-info.xml" xpointer="v254"/></listitem>
 473       </varlistentry>
 474     </variablelist>
 475   </refsect1>
 476
 477   <refsect1>
 478     <title>Assembling Kernel Images</title>
 479
 480     <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
 481     <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
 482   </refsect1>
 483
 484   <refsect1>
 485     <title>See Also</title>
 486     <para>
 487       <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 488       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 489       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 490       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 491       <ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink>,
 492       <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
 493       <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 494       <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 495       <ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink>
 496     </para>
 497   </refsect1>
 498 </refentry>