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