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/4.5/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>/.../<replaceable>foo</replaceable>.efi.extra.d/*.sysext.raw</filename></member>
  36       <member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.confext.raw</filename></member>
  37       <member><filename><replaceable>ESP</replaceable>/loader/addons/*.addon.efi</filename></member>
  38       <member><filename><replaceable>ESP</replaceable>/loader/credentials/*.cred</filename></member>
  39     </simplelist></para>
  40   </refsynopsisdiv>
  41
  42   <refsect1>
  43     <title>Description</title>
  44
  45     <para><command>systemd-stub</command> (stored in per-architecture files
  46     <filename>linuxx64.efi.stub</filename>, <filename>linuxia32.efi.stub</filename>,
  47     <filename>linuxaa64.efi.stub</filename> on disk) is a simple UEFI boot stub. An UEFI boot stub is
  48     attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware
  49     environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux
  50     kernel is executable as regular UEFI binary, and is able to do various preparations before switching the
  51     system into the Linux world.</para>
  52
  53     <para>The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary
  54     itself. This allows combining various resources inside a single PE binary image (usually called "Unified
  55     Kernel Image", or "UKI" for short), which may then be signed via UEFI SecureBoot as a whole, covering all
  56     individual resources at once. Specifically it may include:</para>
  57
  58     <itemizedlist>
  59       <!-- Let's keep this in the canonical order we also measure the sections by, i.e. as in
  60            src/fundamental/uki.h's UnifiedSection enum -->
  61
  62       <listitem><para>A <literal>.linux</literal> section with the ELF Linux kernel image.</para></listitem>
  63
  64       <listitem><para>An <literal>.osrel</literal> section with OS release information, i.e. the contents of
  65       the <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
  66       of the OS the kernel belongs to.</para></listitem>
  67
  68       <listitem><para>A <literal>.cmdline</literal> section with the kernel command line to pass to the
  69       invoked kernel.</para></listitem>
  70
  71       <listitem><para>An <literal>.initrd</literal> section with the initrd.</para></listitem>
  72
  73       <listitem><para>A <literal>.splash</literal> section with an image (in the Windows
  74       <filename>.BMP</filename> format) to show on screen before invoking the kernel.</para></listitem>
  75
  76       <listitem><para>A <literal>.dtb</literal> section with a compiled binary DeviceTree.</para></listitem>
  77
  78       <listitem><para>A <literal>.uname</literal> section with the kernel version information, i.e. the
  79       output of <command>uname -r</command> for the kernel included in the <literal>.linux</literal>
  80       section.</para></listitem>
  81
  82       <listitem><para>An <literal>.sbat</literal> section with
  83       <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT</ulink> revocation
  84       metadata.</para></listitem>
  85
  86       <listitem><para>A <literal>.pcrsig</literal> section with a set of cryptographic signatures for the
  87       expected TPM2 PCR values after the kernel has been booted, in JSON format. This is useful for
  88       implementing TPM2 policies that bind disk encryption and similar to kernels that are signed by a
  89       specific key.</para></listitem>
  90
  91       <listitem><para>A <literal>.pcrpkey</literal> section with a public key in the PEM format matching the
  92       signature data in the the <literal>.pcrsig</literal> section.</para></listitem>
  93     </itemizedlist>
  94
  95     <para>If UEFI SecureBoot is enabled and the <literal>.cmdline</literal> section is present in the executed
  96     image, any attempts to override the kernel command line by passing one as invocation parameters to the
  97     EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
  98     SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
  99     is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is
 100     present).</para>
 101
 102     <para>If a DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing
 103     DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
 104     <literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para>
 105
 106     <para>The contents of eight of these nine sections are measured into TPM PCR 11. It is otherwise not used
 107     and thus the result can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section
 108     is not included in this PCR measurement, since it is supposed to contain signatures for the output of the
 109     measurement operation, and thus cannot also be input to it.</para>
 110
 111     <para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> sections are present in a
 112     unified kernel image their contents are passed to the booted kernel in an synthetic initrd cpio archive
 113     that places them in the <filename>/.extra/tpm2-pcr-signature.json</filename> and
 114     <filename>/.extra/tpm2-pcr-public-key.pem</filename> files. Typically, a
 115     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
 116     ensures they are copied into <filename>/run/systemd/tpm2-pcr-signature.json</filename> and
 117     <filename>/run/systemd/tpm2-pcr-public-key.pem</filename> where they remain accessible even after the
 118     system transitions out of the initrd environment into the host file system. Tools such
 119     <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 120     <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 121     and <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 122     will automatically use files present under these paths to unlock protected resources (encrypted storage
 123     or credentials) or bind encryption to booted kernels.</para>
 124
 125     <para>For further details about the UKI concept, see the <ulink
 126     url="https://uapi-group.org/specifications/specs/unified_kernel_image/">UKI specification</ulink>.</para>
 127   </refsect1>
 128
 129   <refsect1>
 130     <title>Companion Files</title>
 131
 132     <para>The <command>systemd-stub</command> UEFI boot stub automatically collects three types of auxiliary
 133     companion files optionally placed in drop-in directories on the same partition as the EFI binary,
 134     dynamically generates <command>cpio</command> initrd archives from them, and passes them to the kernel.
 135     Specifically:</para>
 136
 137     <itemizedlist>
 138       <listitem><para>For a kernel binary called <filename><replaceable>foo</replaceable>.efi</filename>, it
 139       will look for files with the <filename>.cred</filename> suffix in a directory named
 140       <filename><replaceable>foo</replaceable>.efi.extra.d/</filename> next to it. If the kernel binary
 141       uses a counter for the purpose of
 142       <ulink url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot Assessment</ulink>, this
 143       counter will be ignored. For example, <filename><replaceable>foo</replaceable>+3-0.efi</filename>
 144       will look in directory <filename><replaceable>foo</replaceable>.efi.extra.d/</filename>.
 145       A <command>cpio</command>
 146       archive is generated from all files found that way, placing them in the
 147       <filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may
 148       then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
 149       authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System
 150       Partition. See
 151       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 152       and
 153       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 154       for
 155       details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM
 156       PCR 12 (if a TPM is present).</para></listitem>
 157
 158       <listitem><para>Similarly, files
 159       <filename><replaceable>foo</replaceable>.efi.extra.d/*.sysext.raw</filename> are packed up in a
 160       <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename> directory in the
 161       initrd file hierarchy. This is supposed to be used to pass additional system extension images to the
 162       initrd. See
 163       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
 164       details on system extension images. The generated <command>cpio</command> archive containing these
 165       system extension images is measured into TPM PCR 13 (if a TPM is present).</para></listitem>
 166
 167       <!-- Note: the actual suffix we look for for sysexts is just *.raw (not *.sysext.raw), for
 168            compatibility reasons with old versions. But we want people to name their system extensions
 169            properly, hence we document the *.sysext.raw suffix only. -->
 170
 171       <listitem><para>Similarly, files
 172       <filename><replaceable>foo</replaceable>.efi.extra.d/*.confext.raw</filename> are packed up in a
 173       <command>cpio</command> archive and placed in the <filename>/.extra/confext/</filename> directory in
 174       the initrd file hierarchy. This is supposed to be used to pass additional configuration extension
 175       images to the initrd. See
 176       <citerefentry><refentrytitle>systemd-confext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
 177       details on configuration extension images. The generated <command>cpio</command> archive containing
 178       these system extension images is measured into TPM PCR 12 (if a TPM is present).</para></listitem>
 179
 180       <listitem><para>Similarly, files
 181       <filename><replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename> are loaded and verified as
 182       PE binaries, and a <literal>.cmdline</literal> section is parsed from them. Addons are supposed to be
 183       used to pass additional kernel command line parameters or Devicetree blobs, regardless of the kernel
 184       image being booted, for example to allow platform vendors to ship platform-specific
 185       configuration.</para>
 186
 187       <para>In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
 188       Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a a
 189       <literal>.uname</literal> section, the addon will be rejected if they do not match exactly. It is
 190       recommended to always add a <literal>.sbat</literal> section to all signed addons, so that they may be
 191       revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
 192       <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will add
 193       a SBAT policy by default if none is passed when building addons. For more information on SBAT see
 194       <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">Shim documentation</ulink>.</para>
 195
 196       <para>Addon files are sorted, loaded, and measured into TPM PCR 12 (if a TPM is present) and appended
 197       to the kernel command line. UKI command line options are listed first, then options from addons in
 198       <filename>/loader/addons/*.addon.efi</filename>, and finally UKI-specific addons. Device tree blobs are
 199       loaded and measured following the same algorithm. Addons are always loaded in the same order based on
 200       the filename, so that, given the same set of addons, the same set of measurements can be expected in
 201       PCR12. However, note that the filename is not protected by the PE signature, and as such an attacker
 202       with write access to the ESP could potentially rename these files to change the order in which they are
 203       loaded, in a way that could alter the functionality of the kernel, as some options might be
 204       order-dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of
 205       an attestation service so that improper use of your signed addons can be detected and dealt with using
 206       one of the aforementioned revocation mechanisms.</para></listitem>
 207
 208       <listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
 209       <command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
 210       directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
 211       the initrd, regardless of the kernel being booted. The generated <command>cpio</command> archive is
 212       measured into TPM PCR 12 (if a TPM is present).</para></listitem>
 213
 214       <listitem><para>Additionally, files <filename>/loader/addons/*.addon.efi</filename> are loaded and
 215       verified as PE binaries, and <literal>.cmdline</literal> and/or <literal>.dtb</literal> sections are
 216       parsed from them. This is supposed to be used to pass additional command line parameters or Devicetree
 217       blobs to the kernel, regardless of the kernel being booted.</para></listitem>
 218     </itemizedlist>
 219
 220     <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
 221     images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
 222     further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
 223     by <command>systemd-creds encrypt -T</command> (see
 224     <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 225     details); in case of the system extension images by using signed Verity images.</para>
 226   </refsect1>
 227
 228   <refsect1>
 229     <title>TPM PCR Notes</title>
 230
 231     <para>Note that when a unified kernel using <command>systemd-stub</command> is invoked the firmware will
 232     measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the
 233     core kernel, the embedded initrd and kernel command line (see above for a full list).</para>
 234
 235     <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
 236     every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
 237     measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials (and the one synthesized
 238     from configuration extensions) will be measured to both PCR 9 and PCR 12; the initrd synthesized from
 239     system extensions will be measured to both PCR 4 and PCR 9. Let's summarize the OS resources and the PCRs
 240     they are measured to:</para>
 241
 242     <table>
 243       <title>OS Resource PCR Summary</title>
 244
 245       <tgroup cols='2' align='left' colsep='1' rowsep='1'>
 246         <colspec colname="pcr" />
 247         <colspec colname="definition" />
 248
 249         <thead>
 250           <row>
 251             <entry>OS Resource</entry>
 252             <entry>Measurement PCR</entry>
 253           </row>
 254         </thead>
 255
 256         <tbody>
 257           <row>
 258             <entry><command>systemd-stub</command> code (the entry point of the unified PE binary)</entry>
 259             <entry>4</entry>
 260           </row>
 261
 262           <row>
 263             <entry>Core kernel code (embedded in unified PE binary)</entry>
 264             <entry>4 + 11</entry>
 265           </row>
 266
 267           <row>
 268             <entry>OS release information (embedded in the unified PE binary)</entry>
 269             <entry>4 + 11</entry>
 270           </row>
 271
 272           <row>
 273             <entry>Main initrd (embedded in unified PE binary)</entry>
 274             <entry>4 + 9 + 11</entry>
 275           </row>
 276
 277           <row>
 278             <entry>Default kernel command line (embedded in unified PE binary)</entry>
 279             <entry>4 + 11</entry>
 280           </row>
 281
 282           <row>
 283             <entry>Overridden kernel command line</entry>
 284             <entry>12</entry>
 285           </row>
 286
 287           <row>
 288             <entry>Boot splash (embedded in the unified PE binary)</entry>
 289             <entry>4 + 11</entry>
 290           </row>
 291
 292           <row>
 293             <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry>
 294             <entry>4 + 9</entry>
 295           </row>
 296
 297           <row>
 298             <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry>
 299             <entry>4 + 9 + 11</entry>
 300           </row>
 301
 302           <row>
 303             <entry>Credentials (synthesized initrd from companion files)</entry>
 304             <entry>9 + 12</entry>
 305           </row>
 306
 307           <row>
 308             <entry>System Extensions (synthesized initrd from companion files)</entry>
 309             <entry>9 + 13</entry>
 310           </row>
 311
 312           <row>
 313             <entry>Configuration Extensions (synthesized initrd from companion files)</entry>
 314             <entry>9 + 12</entry>
 315           </row>
 316         </tbody>
 317       </tgroup>
 318     </table>
 319   </refsect1>
 320
 321   <refsect1>
 322     <title>EFI Variables</title>
 323
 324     <para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
 325     vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
 326     stub and the OS:</para>
 327
 328     <variablelist class='efi-variables'>
 329       <varlistentry>
 330         <term><varname>LoaderDevicePartUUID</varname></term>
 331
 332         <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
 333         from. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 334         uses this information to automatically find the disk booted from, in order to discover various other
 335         partitions on the same disk automatically.</para>
 336
 337         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 338       </varlistentry>
 339
 340       <varlistentry>
 341         <term><varname>LoaderFirmwareInfo</varname></term>
 342         <term><varname>LoaderFirmwareType</varname></term>
 343
 344         <listitem><para>Brief firmware information. Use
 345         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
 346         data.</para>
 347
 348         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 349       </varlistentry>
 350
 351       <varlistentry>
 352         <term><varname>LoaderImageIdentifier</varname></term>
 353
 354         <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
 355         directory. Use
 356         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 357         this data.</para>
 358
 359         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 360       </varlistentry>
 361
 362       <varlistentry>
 363         <term><varname>StubInfo</varname></term>
 364
 365         <listitem><para>Brief stub information. Use
 366         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 367         this data.</para>
 368
 369         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 370       </varlistentry>
 371
 372       <varlistentry>
 373         <term><varname>StubPcrKernelImage</varname></term>
 374
 375         <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
 376         database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
 377         <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
 378         unset otherwise.</para>
 379
 380         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 381       </varlistentry>
 382
 383       <varlistentry>
 384         <term><varname>StubPcrKernelParameters</varname></term>
 385
 386         <listitem><para>The PCR register index the kernel command line and credentials are measured into,
 387         formatted as decimal ASCII string (e.g. <literal>12</literal>). This variable is set if a measurement
 388         was successfully completed, and remains unset otherwise.</para>
 389
 390         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 391       </varlistentry>
 392
 393       <varlistentry>
 394         <term><varname>StubPcrInitRDSysExts</varname></term>
 395
 396         <listitem><para>The PCR register index the system extensions for the initrd, which are picked up from
 397         the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
 398         <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
 399         unset otherwise.</para>
 400
 401         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 402       </varlistentry>
 403
 404       <varlistentry>
 405         <term><varname>StubPcrInitRDConfExts</varname></term>
 406
 407         <listitem><para>The PCR register index the configuration extensions for the initrd, which are picked
 408         up from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
 409         <literal>12</literal>). This variable is set if a measurement was successfully completed, and remains
 410         unset otherwise.</para>
 411
 412         <xi:include href="version-info.xml" xpointer="v255"/></listitem>
 413       </varlistentry>
 414     </variablelist>
 415
 416     <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
 417     them if they aren't set already. Some of these variables are defined by the <ulink
 418     url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
 419   </refsect1>
 420
 421   <refsect1>
 422     <title>initrd Resources</title>
 423
 424     <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
 425     the initial file system hierarchy in the initrd execution environment:</para>
 426
 427     <variablelist>
 428       <varlistentry>
 429         <term><filename>/</filename></term>
 430
 431         <listitem><para>The main initrd from the <literal>.initrd</literal> PE section of the unified kernel
 432         image.</para>
 433
 434         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 435       </varlistentry>
 436
 437       <varlistentry>
 438         <term><filename>/.extra/credentials/*.cred</filename></term>
 439         <listitem><para>Credential files (suffix <literal>.cred</literal>) that are placed next to the
 440         unified kernel image (as described above) are copied into the
 441         <filename>/.extra/credentials/</filename> directory in the initrd execution
 442         environment.</para>
 443
 444         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 445       </varlistentry>
 446
 447       <varlistentry>
 448         <term><filename>/.extra/global_credentials/*.cred</filename></term>
 449         <listitem><para>Similarly, credential files in the <filename>/loader/credentials/</filename>
 450         directory in the file system the unified kernel image is placed in are copied into the
 451         <filename>/.extra/global_credentials/</filename> directory in the initrd execution
 452         environment.</para>
 453
 454         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 455       </varlistentry>
 456
 457       <varlistentry>
 458         <term><filename>/.extra/sysext/*.sysext.raw</filename></term>
 459         <listitem><para>System extension image files (suffix <literal>.sysext.raw</literal>) that are placed
 460         next to the unified kernel image (as described above) are copied into the
 461         <filename>/.extra/sysext/</filename> directory in the initrd execution environment.</para>
 462
 463         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 464       </varlistentry>
 465
 466       <varlistentry>
 467         <term><filename>/.extra/confext/*.confext.raw</filename></term>
 468         <listitem><para>Configuration extension image files (suffix <literal>.confext.raw</literal>) that are
 469         placed next to the unified kernel image (as described above) are copied into the
 470         <filename>/.extra/confext/</filename> directory in the initrd execution environment.</para>
 471
 472         <xi:include href="version-info.xml" xpointer="v255"/></listitem>
 473       </varlistentry>
 474
 475       <varlistentry>
 476         <term><filename>/.extra/tpm2-pcr-signature.json</filename></term>
 477         <listitem><para>The TPM2 PCR signature JSON object included in the <literal>.pcrsig</literal> PE
 478         section of the unified kernel image is copied into the
 479         <filename>/.extra/tpm2-pcr-signature.json</filename> file in the initrd execution environment.</para>
 480
 481         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 482       </varlistentry>
 483
 484       <varlistentry>
 485         <term><filename>/.extra/tpm2-pcr-pkey.pem</filename></term>
 486         <listitem><para>The PEM public key included in the <literal>.pcrpkey</literal> PE section of the
 487         unified kernel image is copied into the <filename>/.extra/tpm2-pcr-public-key.pem</filename> file in
 488         the initrd execution environment.</para>
 489
 490         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 491       </varlistentry>
 492     </variablelist>
 493
 494     <para>Note that all these files are located in the <literal>tmpfs</literal> file system the kernel sets
 495     up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
 496     environment into the host file system. If these resources shall be kept around over this transition they
 497     need to be copied to a place that survives the transition first, for example via a suitable
 498     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
 499     default, this is done for the TPM2 PCR signature and public key files.</para>
 500   </refsect1>
 501
 502   <refsect1>
 503     <title>SMBIOS Type 11 Strings</title>
 504
 505     <para><command>systemd-stub</command> can be configured using SMBIOS Type 11 strings. Applicable strings
 506     consist of a name, followed by <literal>=</literal>, followed by the value.
 507     <command>systemd-stub</command> will search the table for a string with a specific name, and if found,
 508     use its value. The following strings are read:</para>
 509
 510     <variablelist>
 511       <varlistentry>
 512         <term><varname>io.systemd.stub.kernel-cmdline-extra</varname></term>
 513         <listitem><para>If set, the value of this string is added to the list of kernel command line
 514         arguments that are measured in PCR12 and passed to the kernel.</para>
 515
 516         <xi:include href="version-info.xml" xpointer="v254"/></listitem>
 517       </varlistentry>
 518     </variablelist>
 519   </refsect1>
 520
 521   <refsect1>
 522     <title>Assembling Kernel Images</title>
 523
 524     <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
 525     <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
 526   </refsect1>
 527
 528   <refsect1>
 529     <title>See Also</title>
 530     <para><simplelist type="inline">
 531       <member><citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
 532       <member><citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 533       <member><citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 534       <member><citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 535       <member><ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink></member>
 536       <member><ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink></member>
 537       <member><citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 538       <member><citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 539       <member><ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink></member>
 540     </simplelist></para>
 541   </refsect1>
 542 </refentry>