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 the following PE sections:</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
  63       image. (Required)</para></listitem>
  64
  65       <listitem><para>An <literal>.osrel</literal> section with OS release information, i.e. the contents of
  66       the <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
  67       of the OS the kernel belongs to.</para></listitem>
  68
  69       <listitem><para>A <literal>.cmdline</literal> section with the kernel command line to pass to the
  70       invoked kernel.</para></listitem>
  71
  72       <listitem><para>An <literal>.initrd</literal> section with the initrd.</para></listitem>
  73
  74       <listitem><para>A <literal>.ucode</literal> section with an initrd containing microcode, to be handed
  75       to the kernel before any other initrd. This initrd must not be compressed.</para></listitem>
  76
  77       <listitem><para>A <literal>.splash</literal> section with an image (in the Windows
  78       <filename>.BMP</filename> format) to show on screen before invoking the kernel.</para></listitem>
  79
  80       <listitem><para>A <literal>.dtb</literal> section with a compiled binary DeviceTree.</para></listitem>
  81
  82       <listitem><para>A <literal>.uname</literal> section with the kernel version information, i.e. the
  83       output of <command>uname -r</command> for the kernel included in the <literal>.linux</literal>
  84       section.</para></listitem>
  85
  86       <listitem><para>An <literal>.sbat</literal> section with
  87       <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT</ulink> revocation
  88       metadata.</para></listitem>
  89
  90       <listitem><para>A <literal>.pcrsig</literal> section with a set of cryptographic signatures for the
  91       expected TPM2 PCR values after the kernel has been booted, in JSON format. This is useful for
  92       implementing TPM2 policies that bind disk encryption and similar to kernels that are signed by a
  93       specific key.</para></listitem>
  94
  95       <listitem><para>A <literal>.pcrpkey</literal> section with a public key in the PEM format matching the
  96       signature data in the <literal>.pcrsig</literal> section.</para></listitem>
  97     </itemizedlist>
  98
  99     <para>Generally, the sections above should appear at most once in a UKI. That said, a concept of
 100     "profiles" is defined, that allows multiple sets of these sections to exist in a single UKI file, of
 101     which one can be selected at boot. For this an additional PE section <literal>.profile</literal> is
 102     defined which can be used as separator between multiple sets of these settings. The
 103     <literal>.profile</literal> section itself may contain meta-information about the section, and follows a
 104     similar structure as the contents of the <literal>.osrel</literal> section. For further details about
 105     multi-profile UKIs, see below.</para> <para>If UEFI SecureBoot is enabled and the
 106     <literal>.cmdline</literal> section is present in the executed image, any attempts to override the kernel
 107     command line by passing one as invocation parameters to the EFI binary are ignored. Thus, in order to
 108     allow overriding the kernel command line, either disable UEFI SecureBoot, or don't include a kernel
 109     command line PE section in the kernel image file. If a command line is accepted via EFI invocation
 110     parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is present).</para> <para>If a
 111     DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing DeviceTree in the
 112     corresponding EFI configuration table. systemd-stub will ask the firmware via the
 113     <literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para> <para>The
 114     contents of 11 of these 12 sections are measured into TPM PCR 11. It is otherwise not used and thus the
 115     result can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section is not
 116     included in this PCR measurement, since it is supposed to contain signatures for the output of the
 117     measurement operation, and thus cannot also be input to it. If an UKI contains multiple profiles, only
 118     the PE sections of the selected profile (and those of the base profile, except if overridden) are
 119     measured.</para>
 120
 121     <para>If non-zero, the selected numeric profile is measured into PCR 12.</para>
 122
 123     <para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> sections are present in a
 124     unified kernel image their contents are passed to the booted kernel in an synthetic initrd cpio archive
 125     that places them in the <filename>/.extra/tpm2-pcr-signature.json</filename> and
 126     <filename>/.extra/tpm2-pcr-public-key.pem</filename> files. Typically, a
 127     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
 128     ensures they are copied into <filename>/run/systemd/tpm2-pcr-signature.json</filename> and
 129     <filename>/run/systemd/tpm2-pcr-public-key.pem</filename> where they remain accessible even after the
 130     system transitions out of the initrd environment into the host file system. Tools such
 131     <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 132     <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 133     and <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 134     will automatically use files present under these paths to unlock protected resources (encrypted storage
 135     or credentials) or bind encryption to booted kernels.</para>
 136
 137     <para>For further details about the UKI concept, see the <ulink
 138     url="https://uapi-group.org/specifications/specs/unified_kernel_image/">UKI specification</ulink>.</para>
 139   </refsect1>
 140
 141   <refsect1>
 142     <title>Companion Files</title>
 143
 144     <para>The <command>systemd-stub</command> UEFI boot stub automatically collects three types of auxiliary
 145     companion files optionally placed in drop-in directories on the same partition as the EFI binary,
 146     dynamically generates <command>cpio</command> initrd archives from them, and passes them to the kernel.
 147     Specifically:</para>
 148
 149     <itemizedlist>
 150       <listitem><para>For a kernel binary called <filename><replaceable>foo</replaceable>.efi</filename>, it
 151       will look for files with the <filename>.cred</filename> suffix in a directory named
 152       <filename><replaceable>foo</replaceable>.efi.extra.d/</filename> next to it. If the kernel binary
 153       uses a counter for the purpose of
 154       <ulink url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot Assessment</ulink>, this
 155       counter will be ignored. For example, <filename><replaceable>foo</replaceable>+3-0.efi</filename>
 156       will look in directory <filename><replaceable>foo</replaceable>.efi.extra.d/</filename>.
 157       A <command>cpio</command>
 158       archive is generated from all files found that way, placing them in the
 159       <filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may
 160       then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
 161       authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System
 162       Partition. See
 163       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 164       and
 165       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 166       for
 167       details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM
 168       PCR 12 (if a TPM is present).</para></listitem>
 169
 170       <listitem><para>Similarly, files
 171       <filename><replaceable>foo</replaceable>.efi.extra.d/*.sysext.raw</filename> are packed up in a
 172       <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename> directory in the
 173       initrd file hierarchy. This is supposed to be used to pass additional system extension images to the
 174       initrd. See
 175       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
 176       details on system extension images. The generated <command>cpio</command> archive containing these
 177       system extension images is measured into TPM PCR 13 (if a TPM is present).</para></listitem>
 178
 179       <!-- Note: the actual suffix we look for for sysexts is just *.raw (not *.sysext.raw), for
 180            compatibility reasons with old versions. But we want people to name their system extensions
 181            properly, hence we document the *.sysext.raw suffix only. -->
 182
 183       <listitem><para>Similarly, files
 184       <filename><replaceable>foo</replaceable>.efi.extra.d/*.confext.raw</filename> are packed up in a
 185       <command>cpio</command> archive and placed in the <filename>/.extra/confext/</filename> directory in
 186       the initrd file hierarchy. This is supposed to be used to pass additional configuration extension
 187       images to the initrd. See
 188       <citerefentry><refentrytitle>systemd-confext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
 189       details on configuration extension images. The generated <command>cpio</command> archive containing
 190       these system extension images is measured into TPM PCR 12 (if a TPM is present).</para></listitem>
 191
 192       <listitem><para>Similarly, files
 193       <filename><replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename> are loaded and verified as
 194       PE binaries, and a <literal>.cmdline</literal> or <literal>.ucode</literal> section is parsed from them.
 195       Addons are supposed to be used to pass additional kernel command line parameters, or Devicetree blobs,
 196       and microcode updates, regardless of the kernel image being booted, for example to allow platform vendors
 197       to ship platform-specific configuration.</para>
 198
 199       <para>In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
 200       Shim's MOK, and will be rejected otherwise. Additionally, if both the addon and the UKI contain a
 201       <literal>.uname</literal> section, the addon will be rejected if they do not match exactly. It is
 202       recommended to always add a <literal>.sbat</literal> section to all signed addons, so that they may be
 203       revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
 204       <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will add
 205       a SBAT policy by default if none is passed when building addons. For more information on SBAT see
 206       <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">Shim documentation</ulink>.</para>
 207
 208       <para>Addon files are sorted, loaded, and measured into TPM PCR 12 (if a TPM is present) and appended
 209       to the kernel command line. UKI command line options are listed first, then options from addons in
 210       <filename>/loader/addons/*.addon.efi</filename>, and finally UKI-specific addons. Device tree blobs are
 211       loaded and measured following the same algorithm. Microcode addons are passed to the kernel in inverse
 212       order (UKI specific addons, global addons, UKI embedded section). This is because the microcode update
 213       driver stops on the first matching filename. Addons are always loaded in the same order based on
 214       the filename, so that, given the same set of addons, the same set of measurements can be expected in
 215       PCR12. However, note that the filename is not protected by the PE signature, and as such an attacker
 216       with write access to the ESP could potentially rename these files to change the order in which they are
 217       loaded, in a way that could alter the functionality of the kernel, as some options might be
 218       order-dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of
 219       an attestation service so that improper use of your signed addons can be detected and dealt with using
 220       one of the aforementioned revocation mechanisms.</para></listitem>
 221
 222       <listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
 223       <command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
 224       directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
 225       the initrd, regardless of the kernel being booted. The generated <command>cpio</command> archive is
 226       measured into TPM PCR 12 (if a TPM is present).</para></listitem>
 227
 228       <listitem><para>Additionally, files <filename>/loader/addons/*.addon.efi</filename> are loaded and
 229       verified as PE binaries, and <literal>.cmdline</literal>, <literal>.dtb</literal> and/or
 230       <literal>.ucode</literal>sections are parsed from them. This is supposed to be used to pass additional
 231       command line parameters, Devicetree blobs and microcode updates to the kernel, regardless of the
 232       kernel being booted.</para></listitem>
 233     </itemizedlist>
 234
 235     <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
 236     images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
 237     further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
 238     by <command>systemd-creds encrypt -T</command> (see
 239     <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 240     details); in case of the system extension images by using signed Verity images.</para>
 241   </refsect1>
 242
 243   <refsect1>
 244     <title>Multi-Profile UKIs</title>
 245
 246     <para>In many contexts it is useful to allow invocation of a single UKI in multiple different modes (or
 247     "profiles") without compromising the cryptographic integrity, measurements and so on of the boot
 248     process. For example, a single UKI might provide three distinct profiles: a regular boot one, one that
 249     invokes a "factory reset" operation, and one that boots into a storage target mode (as in
 250     <citerefentry><refentrytitle>systemd-storagetm.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>). Each
 251     profile would then use the same <literal>.linux</literal> and <literal>.initrd</literal> sections, but would
 252     have a separate <literal>.cmdline</literal> section. For example the latter two profiles would extend the
 253     regular kernel command line with <literal>systemd.unit=factory-reset.target</literal> or
 254     <literal>rd.systemd.unit=storagetm.target</literal>.</para>
 255
 256     <para>A single UKI may support multiple profiles by means of the special <literal>.profile</literal> PE
 257     section. This section acts as separator between the PE sections of the individual
 258     profiles. <literal>.profile</literal> PE sections hence may appear multiple times in a single UKI, and
 259     the other PE sections listed above may appear multiple times too, if <literal>.profile</literal> are
 260     used, but only once before the first <literal>.profile</literal> section, once between each subsequent
 261     pair, and once after the last appearance of <literal>.profile</literal>. The sections listed before the
 262     first <literal>.profile</literal> are considered the "base" profile of the UKI. Each
 263     <literal>.profile</literal> section then introduces a new profile, which are numbered starting from
 264     zero. The PE sections following each <literal>.profile</literal> are specific to that profile. When
 265     booting into a specific profile the base section's profiles are used in combination with the specific
 266     profile's sections: if the same section is defined in both, the per-profile section overrides the base
 267     profile's version, otherwise the per-profile sections is used together with the base profile
 268     sections.</para> <para>A UKI that contains no <literal>.profile</literal> is consider equivalent to one
 269     that just contains a single <literal>.profile</literal>, as having only a single profile @0.</para>
 270
 271     <para>Here's a simple example for a multi-profile UKI's sections, inspired by the setup suggested above:</para>
 272
 273     <table>
 274       <title>Multi-Profile UKI Example</title>
 275
 276       <tgroup cols='2' align='left' colsep='1' rowsep='1'>
 277         <colspec colname="section" />
 278         <colspec colname="profile" />
 279
 280         <thead>
 281           <row>
 282             <entry>Section</entry>
 283             <entry>Profile</entry>
 284           </row>
 285         </thead>
 286
 287         <tbody>
 288           <row>
 289             <entry><literal>.linux</literal></entry>
 290             <entry morerows="3" valign="middle">Base profile</entry>
 291           </row>
 292           <row>
 293             <entry><literal>.osrel</literal></entry>
 294           </row>
 295           <row>
 296             <entry><literal>.cmdline</literal></entry>
 297           </row>
 298           <row>
 299             <entry><literal>.initrd</literal></entry>
 300           </row>
 301           <row>
 302             <entry><literal>.profile</literal></entry>
 303             <entry>Profile @0</entry>
 304           </row>
 305           <row>
 306             <entry><literal>.profile</literal></entry>
 307             <entry morerows="1" valign="middle">Profile @1</entry>
 308           </row>
 309           <row>
 310             <entry><literal>.cmdline</literal></entry>
 311           </row>
 312           <row>
 313             <entry><literal>.profile</literal></entry>
 314             <entry morerows="1" valign="middle">Profile @2</entry>
 315           </row>
 316           <row>
 317             <entry><literal>.cmdline</literal></entry>
 318           </row>
 319         </tbody>
 320       </tgroup>
 321     </table>
 322
 323     <para>The section list above would define three profiles. The first four sections make up the base
 324     profile. A <literal>.profile</literal> section then introduces profile @0. It doesn't override any
 325     sections (or add any) from the base section, hence it is immediately followed by another
 326     <literal>.profile</literal> section that then introduces section @1. This profile overrides the kernel
 327     command line. Finally, the last two sections define section @2, again overriding the command line. (Note
 328     that in this example the first <literal>.cmdline</literal> could also moved behind the first
 329     <literal>.profile</literal> with equivalent effect. To keep things nicely extensible, it's probably a
 330     good idea to keep the generic command line in the base section instead of profile 0, in case later added
 331     profiles might want to reuse it.)</para>
 332
 333     <para>The profile to boot may be controlled via the UKI's own command line: if the first argument starts
 334     with <literal>@</literal>, followed by a positive integer number in decimal, it selects the profile to
 335     boot into. If the first argument is not specified like that, the UKI will automatically boot into profile
 336     0.</para>
 337
 338     <para>A <literal>.profile</literal> section may contain meta-information about the profile. It follows a
 339     similar format as <literal>.osrel</literal> (i.e. an environment-variable-assignment-block-like list of
 340     newline separated strings). Currently two fields are defined: <literal>ID=</literal> is supposed to carry
 341     a short identifying string that identifies the profile
 342     (e.g. <literal>ID=factory-reset</literal>). <literal>TITLE=</literal> should contain a human readable
 343     string that may appear in the boot menu entry for this profile (e.g. <literal>TITLE='Factory Reset this
 344     Device'</literal>).</para>
 345   </refsect1>
 346
 347   <refsect1>
 348     <title>TPM PCR Notes</title>
 349
 350     <para>Note that when a unified kernel using <command>systemd-stub</command> is invoked the firmware will
 351     measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the
 352     core kernel, the embedded initrd and kernel command line (see above for a full list), including all UKI
 353     profiles.</para>
 354
 355     <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
 356     every type of initrd (of the selected UKI profile) will possibly be measured two or three times: the
 357     initrds embedded in the kernel image will be measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized
 358     from credentials (and the one synthesized from configuration extensions) will be measured to both PCR 9
 359     and PCR 12; the initrd synthesized from system extensions will be measured to both PCR 4 and PCR 9. Let's
 360     summarize the OS resources and the PCRs they are measured to:</para>
 361
 362     <table>
 363       <title>OS Resource PCR Summary</title>
 364
 365       <tgroup cols='2' align='left' colsep='1' rowsep='1'>
 366         <colspec colname="pcr" />
 367         <colspec colname="definition" />
 368
 369         <thead>
 370           <row>
 371             <entry>OS Resource</entry>
 372             <entry>Measurement PCR</entry>
 373           </row>
 374         </thead>
 375
 376         <tbody>
 377           <row>
 378             <entry><command>systemd-stub</command> code (the entry point of the unified PE binary)</entry>
 379             <entry>4</entry>
 380           </row>
 381
 382           <row>
 383             <entry>Core kernel code (embedded in unified PE binary)</entry>
 384             <entry>4 + 11</entry>
 385           </row>
 386
 387           <row>
 388             <entry>OS release information (embedded in the unified PE binary)</entry>
 389             <entry>4 + 11</entry>
 390           </row>
 391
 392           <row>
 393             <entry>Main initrd (embedded in unified PE binary)</entry>
 394             <entry>4 + 9 + 11</entry>
 395           </row>
 396
 397           <row>
 398             <entry>Microcode initrd (embedded in unified PE binary)</entry>
 399             <entry>4 + 9 + 11</entry>
 400           </row>
 401
 402           <row>
 403             <entry>Default kernel command line (embedded in unified PE binary)</entry>
 404             <entry>4 + 11</entry>
 405           </row>
 406
 407           <row>
 408             <entry>Overridden kernel command line</entry>
 409             <entry>12</entry>
 410           </row>
 411
 412           <row>
 413             <entry>Boot splash (embedded in the unified PE binary)</entry>
 414             <entry>4 + 11</entry>
 415           </row>
 416
 417           <row>
 418             <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry>
 419             <entry>4 + 9</entry>
 420           </row>
 421
 422           <row>
 423             <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry>
 424             <entry>4 + 9 + 11</entry>
 425           </row>
 426
 427           <row>
 428             <entry>Credentials (synthesized initrd from companion files)</entry>
 429             <entry>9 + 12</entry>
 430           </row>
 431
 432           <row>
 433             <entry>System Extensions (synthesized initrd from companion files)</entry>
 434             <entry>9 + 13</entry>
 435           </row>
 436
 437           <row>
 438             <entry>Configuration Extensions (synthesized initrd from companion files)</entry>
 439             <entry>9 + 12</entry>
 440           </row>
 441
 442           <row>
 443             <entry>Selected profile unless zero</entry>
 444             <entry>12</entry>
 445           </row>
 446         </tbody>
 447       </tgroup>
 448     </table>
 449   </refsect1>
 450
 451   <refsect1>
 452     <title>EFI Variables</title>
 453
 454     <para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
 455     vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
 456     stub and the OS:</para>
 457
 458     <variablelist class='efi-variables'>
 459       <varlistentry>
 460         <term><varname>LoaderDevicePartUUID</varname></term>
 461
 462         <listitem><para>Contains the partition UUID of the partition the boot loader has been started from on
 463         the current boot (usually a EFI System Partition). If already set by the boot loader, this will
 464         remain untouched by <command>systemd-stub</command>. If not set yet, this will be set to the
 465         partition UUID of the partition the unified kernel is started from, in order to support systems that
 466         directly boot into a unified kernel image, bypassing any boot loader.
 467         <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 468         uses this information to automatically find the disk booted from, in order to discover various other
 469         partitions on the same disk automatically.</para>
 470
 471         <xi:include href="version-info.xml" xpointer="v224"/></listitem>
 472       </varlistentry>
 473
 474       <varlistentry>
 475         <term><varname>LoaderFirmwareInfo</varname></term>
 476         <term><varname>LoaderFirmwareType</varname></term>
 477
 478         <listitem><para>Brief firmware information. Use
 479         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
 480         data.</para>
 481
 482         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 483       </varlistentry>
 484
 485       <varlistentry>
 486         <term><varname>LoaderImageIdentifier</varname></term>
 487
 488         <listitem><para>The file system path to the EFI executable of the boot loader for the current boot,
 489         relative to the partition's root directory (i.e. relative to the partition indicated by
 490         <varname>LoaderDevicePartUUID</varname>, see above). If not set yet, this will be set to the file
 491         system path of the EFI executable of the booted unified kernel, in order to support systems that
 492         directly boot into a unified kernel image, bypassing any boot loader.  Use
 493         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 494         this data.</para>
 495
 496         <xi:include href="version-info.xml" xpointer="v237"/></listitem>
 497       </varlistentry>
 498
 499       <varlistentry>
 500         <term><varname>StubDevicePartUUID</varname></term>
 501         <term><varname>StubImageIdentifier</varname></term>
 502
 503         <listitem><para>Similar to <varname>LoaderDevicePartUUID</varname> and
 504         <varname>StubImageIdentifier</varname>, but indicates the location of the unified kernel image EFI
 505         binary rather than the location of the boot loader binary, regardless if booted via a boot loader
 506         or not.</para>
 507
 508         <xi:include href="version-info.xml" xpointer="v257"/></listitem>
 509       </varlistentry>
 510
 511       <varlistentry>
 512         <term><varname>StubInfo</varname></term>
 513
 514         <listitem><para>Brief stub information. Use
 515         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 516         this data.</para>
 517
 518         <xi:include href="version-info.xml" xpointer="v250"/></listitem>
 519       </varlistentry>
 520
 521       <varlistentry>
 522         <term><varname>StubPcrKernelImage</varname></term>
 523
 524         <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
 525         database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
 526         <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
 527         unset otherwise.</para>
 528
 529         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 530       </varlistentry>
 531
 532       <varlistentry>
 533         <term><varname>StubPcrKernelParameters</varname></term>
 534
 535         <listitem><para>The PCR register index the kernel command line and credentials are measured into,
 536         formatted as decimal ASCII string (e.g. <literal>12</literal>). This variable is set if a measurement
 537         was successfully completed, and remains unset otherwise.</para>
 538
 539         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 540       </varlistentry>
 541
 542       <varlistentry>
 543         <term><varname>StubPcrInitRDSysExts</varname></term>
 544
 545         <listitem><para>The PCR register index the system extensions for the initrd, which are picked up from
 546         the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
 547         <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
 548         unset otherwise.</para>
 549
 550         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 551       </varlistentry>
 552
 553       <varlistentry>
 554         <term><varname>StubPcrInitRDConfExts</varname></term>
 555
 556         <listitem><para>The PCR register index the configuration extensions for the initrd, which are picked
 557         up from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
 558         <literal>12</literal>). This variable is set if a measurement was successfully completed, and remains
 559         unset otherwise.</para>
 560
 561         <xi:include href="version-info.xml" xpointer="v255"/></listitem>
 562       </varlistentry>
 563
 564       <varlistentry>
 565         <term><varname>StubProfile</varname></term>
 566
 567         <listitem><para>The numeric index of the selected profile, without the <literal>@</literal>,
 568         formatted as decimal string. Set both on single-profile and multi-profile UKIs. (In the former case
 569         this variable will be set to <literal>0</literal> unconditionally.)</para>
 570
 571         <xi:include href="version-info.xml" xpointer="v257"/></listitem>
 572       </varlistentry>
 573     </variablelist>
 574
 575     <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
 576     them if they aren't set already. Some of these variables are defined by the <ulink
 577     url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
 578   </refsect1>
 579
 580   <refsect1>
 581     <title>initrd Resources</title>
 582
 583     <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
 584     the initial file system hierarchy in the initrd execution environment:</para>
 585
 586     <variablelist>
 587       <varlistentry>
 588         <term><filename>/</filename></term>
 589
 590         <listitem><para>The main initrd from the <literal>.initrd</literal> PE section of the unified kernel
 591         image.</para>
 592
 593         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 594       </varlistentry>
 595
 596       <varlistentry>
 597         <term><filename>/.extra/credentials/*.cred</filename></term>
 598         <listitem><para>Credential files (suffix <literal>.cred</literal>) that are placed next to the
 599         unified kernel image (as described above) are copied into the
 600         <filename>/.extra/credentials/</filename> directory in the initrd execution
 601         environment.</para>
 602
 603         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 604       </varlistentry>
 605
 606       <varlistentry>
 607         <term><filename>/.extra/global_credentials/*.cred</filename></term>
 608         <listitem><para>Similarly, credential files in the <filename>/loader/credentials/</filename>
 609         directory in the file system the unified kernel image is placed in are copied into the
 610         <filename>/.extra/global_credentials/</filename> directory in the initrd execution
 611         environment.</para>
 612
 613         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 614       </varlistentry>
 615
 616       <varlistentry>
 617         <term><filename>/.extra/sysext/*.sysext.raw</filename></term>
 618         <listitem><para>System extension image files (suffix <literal>.sysext.raw</literal>) that are placed
 619         next to the unified kernel image (as described above) are copied into the
 620         <filename>/.extra/sysext/</filename> directory in the initrd execution environment.</para>
 621
 622         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 623       </varlistentry>
 624
 625       <varlistentry>
 626         <term><filename>/.extra/confext/*.confext.raw</filename></term>
 627         <listitem><para>Configuration extension image files (suffix <literal>.confext.raw</literal>) that are
 628         placed next to the unified kernel image (as described above) are copied into the
 629         <filename>/.extra/confext/</filename> directory in the initrd execution environment.</para>
 630
 631         <xi:include href="version-info.xml" xpointer="v255"/></listitem>
 632       </varlistentry>
 633
 634       <varlistentry>
 635         <term><filename>/.extra/tpm2-pcr-signature.json</filename></term>
 636         <listitem><para>The TPM2 PCR signature JSON object included in the <literal>.pcrsig</literal> PE
 637         section of the unified kernel image is copied into the
 638         <filename>/.extra/tpm2-pcr-signature.json</filename> file in the initrd execution environment.</para>
 639
 640         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 641       </varlistentry>
 642
 643       <varlistentry>
 644         <term><filename>/.extra/tpm2-pcr-public-key.pem</filename></term>
 645         <listitem><para>The PEM public key included in the <literal>.pcrpkey</literal> PE section of the
 646         unified kernel image is copied into the <filename>/.extra/tpm2-pcr-public-key.pem</filename> file in
 647         the initrd execution environment.</para>
 648
 649         <xi:include href="version-info.xml" xpointer="v252"/></listitem>
 650       </varlistentry>
 651
 652       <varlistentry>
 653         <term><filename>/.extra/profile</filename></term>
 654         <term><filename>/.extra/os-release</filename></term>
 655         <listitem><para>The contents of the <literal>.profile</literal> and <literal>.osrel</literal>
 656         sections of the selected profile, if any.</para>
 657
 658         <xi:include href="version-info.xml" xpointer="v257"/></listitem>
 659       </varlistentry>
 660     </variablelist>
 661
 662     <para>Note that all these files are located in the <literal>tmpfs</literal> file system the kernel sets
 663     up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
 664     environment into the host file system. If these resources shall be kept around over this transition they
 665     need to be copied to a place that survives the transition first, for example via a suitable
 666     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
 667     default, this is done for the TPM2 PCR signature and public key files.</para>
 668   </refsect1>
 669
 670   <refsect1>
 671     <title>SMBIOS Type 11 Strings</title>
 672
 673     <para><command>systemd-stub</command> can be configured using SMBIOS Type 11 strings. Applicable strings
 674     consist of a name, followed by <literal>=</literal>, followed by the value. Unless
 675     <command>systemd-stub</command> detects it is running inside a confidential computing environment,
 676     <command>systemd-stub</command> will search the table for a string with a specific name, and if found,
 677     use its value. The following strings are read:</para>
 678
 679     <variablelist>
 680       <varlistentry>
 681         <term><varname>io.systemd.stub.kernel-cmdline-extra</varname></term>
 682         <listitem><para>If set, the value of this string is added to the list of kernel command line
 683         arguments that are measured in PCR12 and passed to the kernel.</para>
 684
 685         <xi:include href="version-info.xml" xpointer="v254"/></listitem>
 686       </varlistentry>
 687     </variablelist>
 688   </refsect1>
 689
 690   <refsect1>
 691     <title>Assembling Kernel Images</title>
 692
 693     <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
 694     <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
 695   </refsect1>
 696
 697   <refsect1>
 698     <title>See Also</title>
 699     <para><simplelist type="inline">
 700       <member><citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
 701       <member><citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 702       <member><citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 703       <member><citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 704       <member><ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink></member>
 705       <member><ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink></member>
 706       <member><citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 707       <member><citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 708       <member><ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink></member>
 709     </simplelist></para>
 710   </refsect1>
 711 </refentry>