path: root/man/systemd-stub.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<refentry id="systemd-stub" conditional='ENABLE_BOOTLOADER'
    xmlns:xi="http://www.w3.org/2001/XInclude">
  <refentryinfo>
    <title>systemd-stub</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd-stub</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd-stub</refname>
    <refname>sd-stub</refname>
    <refname>linuxx64.efi.stub</refname>
    <refname>linuxia32.efi.stub</refname>
    <refname>linuxaa64.efi.stub</refname>
    <refpurpose>A simple UEFI kernel boot stub</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><simplelist>
      <member><filename>/usr/lib/systemd/boot/efi/linuxx64.efi.stub</filename></member>
      <member><filename>/usr/lib/systemd/boot/efi/linuxia32.efi.stub</filename></member>
      <member><filename>/usr/lib/systemd/boot/efi/linuxaa64.efi.stub</filename></member>
      <member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename></member>
      <member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.cred</filename></member>
      <member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.raw</filename></member>
      <member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.sysext.raw</filename></member>
      <member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.confext.raw</filename></member>
      <member><filename><replaceable>ESP</replaceable>/loader/addons/*.addon.efi</filename></member>
      <member><filename><replaceable>ESP</replaceable>/loader/credentials/*.cred</filename></member>
    </simplelist></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><command>systemd-stub</command> (stored in per-architecture files
    <filename>linuxx64.efi.stub</filename>, <filename>linuxia32.efi.stub</filename>,
    <filename>linuxaa64.efi.stub</filename> on disk) is a simple UEFI boot stub. An UEFI boot stub is
    attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware
    environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux
    kernel is executable as regular UEFI binary, and is able to do various preparations before switching the
    system into the Linux world.</para>

    <para>The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary
    itself. This allows combining various resources inside a single PE binary image (a "Unified Kernel Image"
    or "UKI" for short), which may then be signed via UEFI SecureBoot as a whole, covering all individual
    resources at once. Specifically it may include the following PE sections:</para>

    <itemizedlist>
      <!-- Let's keep this in the canonical order we also measure the sections by, i.e. as in
           src/fundamental/uki.h's UnifiedSection enum -->

      <listitem><para>A <literal>.linux</literal> section with the ELF Linux kernel image.
      This section is required.</para></listitem>

      <listitem><para>An optional <literal>.osrel</literal> section with OS release information, i.e. the
      contents of the
      <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file of
      the OS the kernel belongs to.</para></listitem>

      <listitem><para>An optional <literal>.cmdline</literal> section with the kernel command line to pass to
      the invoked kernel.</para></listitem>

      <listitem><para>An optional <literal>.initrd</literal> section with the initrd.</para></listitem>

      <listitem><para>An optional <literal>.ucode</literal> section with an initrd containing microcode, to
      be handed to the kernel before any other initrd. This initrd must not be compressed.</para></listitem>

      <listitem><para>An optional <literal>.splash</literal> section with an image (in the Windows
      <filename>.BMP</filename> format) to show on screen before invoking the kernel.</para></listitem>

      <listitem><para>An optional <literal>.dtb</literal> section with a compiled binary DeviceTree.
      </para></listitem>

      <listitem><para>Zero or more <literal>.dtbauto</literal> sections. <filename>systemd-stub</filename>
      will always use the first matching one. The match is performed by taking the first DeviceTree's
      <varname>compatible</varname> string supplied by the firmware in configuration tables and comparing it
      with the first <varname>compatible</varname> string from each of the <literal>.dtbauto</literal>
      sections. If the firmware does not provide a DeviceTree, the match is done using the
      <varname>.hwids</varname> section instead. After selecting a <literal>.hwids</literal> section (see the
      description below), the <varname>compatible</varname> string from that section will be used to perform
      the same matching procedure. If a match is found, that <literal>.dtbauto</literal> section will be
      loaded and will override <varname>.dtb</varname> if present.</para></listitem>

      <listitem><para>Zero or more <literal>.hwids</literal> sections with hardware IDs of the machines to
      match DeviceTrees. <filename>systemd-stub</filename> will use the SMBIOS data to calculate hardware IDs
      of the machine (as per <ulink
      url="https://learn.microsoft.com/en-us/windows-hardware/drivers/install/specifying-hardware-ids-for-a-computer">specification</ulink>),
      and then it will try to find any of them in each of the <literal>.hwids</literal> sections. The first
      matching section will be used.</para></listitem>

      <listitem><para>An optional <literal>.uname</literal> section with the kernel version information, i.e.
      the output of <command>uname -r</command> for the kernel included in the <literal>.linux</literal>
      section.</para></listitem>

      <listitem><para>An optional <literal>.sbat</literal> section with
      <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT</ulink> revocation metadata.
      </para></listitem>

      <listitem><para>An optional <literal>.pcrsig</literal> section with a set of cryptographic signatures
      for the expected TPM2 PCR values after the kernel has been booted, in JSON format. This is useful for
      implementing TPM2 policies that bind disk encryption and similar to kernels that are signed by a
      specific key.</para></listitem>

      <listitem><para>An optional <literal>.pcrpkey</literal> section with a public key in the PEM format
      matching the signature data in the <literal>.pcrsig</literal> section.</para></listitem>
    </itemizedlist>

    <!-- FIXME: how does .dtauto/.hwids matching interact with profiles? -->

    <para>In a basic UKI, the sections listed above appear at most once, with the exception of
    <literal>.dtbauto</literal> and <literal>.hwids</literal> sections. In a multi-profile UKI,
    multiple sets of these sections are present in a single file and form "profiles",
    one of which can be selected at boot. For this, the PE section <literal>.profile</literal> is
    defined to be used as the separator between sets of sections. The
    <literal>.profile</literal> section itself may contain meta-information about the section, and follows a
    similar structure as the contents of the <literal>.osrel</literal> section. For further details about
    multi-profile UKIs, see below.</para> <para>If UEFI SecureBoot is enabled and the
    <literal>.cmdline</literal> section is present in the executed image, any attempts to override the kernel
    command line by passing one as invocation parameters to the EFI binary are ignored. Thus, in order to
    allow overriding the kernel command line, either disable UEFI SecureBoot, or don't include a kernel
    command line PE section in the kernel image file. If a command line is accepted via EFI invocation
    parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is present).</para> <para>If a
    DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing DeviceTree in the
    corresponding EFI configuration table. systemd-stub will ask the firmware via the
    <literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para> <para>The
    contents of 11 of these 12 sections are measured into TPM PCR 11. It is otherwise not used and thus the
    result can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section is not
    included in this PCR measurement, since it is supposed to contain signatures for the output of the
    measurement operation, and thus cannot also be input to it. If an UKI contains multiple profiles, only
    the PE sections of the selected profile (and those of the base profile, except if overridden) are
    measured.</para>

    <para>If non-zero, the selected numeric profile is measured into PCR 12.</para>

    <para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> sections are present in a
    unified kernel image their contents are passed to the booted kernel in an synthetic initrd cpio archive
    that places them in the <filename>/.extra/tpm2-pcr-signature.json</filename> and
    <filename>/.extra/tpm2-pcr-public-key.pem</filename> files. Typically, a
    <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
    ensures they are copied into <filename>/run/systemd/tpm2-pcr-signature.json</filename> and
    <filename>/run/systemd/tpm2-pcr-public-key.pem</filename> where they remain accessible even after the
    system transitions out of the initrd environment into the host file system. Tools such
    <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    and <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    will automatically use files present under these paths to unlock protected resources (encrypted storage
    or credentials) or bind encryption to booted kernels.</para>

    <para>For further details about the UKI concept, see the <ulink
    url="https://uapi-group.org/specifications/specs/unified_kernel_image/">UKI specification</ulink>.</para>
  </refsect1>

  <refsect1>
    <title>Companion Files</title>

    <para>The <command>systemd-stub</command> UEFI boot stub automatically collects three types of auxiliary
    companion files optionally placed in drop-in directories on the same partition as the EFI binary,
    dynamically generates <command>cpio</command> initrd archives from them, and passes them to the kernel.
    Specifically:</para>

    <itemizedlist>
      <listitem><para>For a kernel binary called <filename><replaceable>foo</replaceable>.efi</filename>, it
      will look for files with the <filename>.cred</filename> suffix in a directory named
      <filename><replaceable>foo</replaceable>.efi.extra.d/</filename> next to it. If the kernel binary
      uses a counter for the purpose of
      <ulink url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot Assessment</ulink>, this
      counter will be ignored. For example, <filename><replaceable>foo</replaceable>+3-0.efi</filename>
      will look in directory <filename><replaceable>foo</replaceable>.efi.extra.d/</filename>.
      A <command>cpio</command>
      archive is generated from all files found that way, placing them in the
      <filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may
      then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
      authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System
      Partition. See
      <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
      and
      <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
      for
      details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM
      PCR 12 (if a TPM is present).</para></listitem>

      <listitem><para>Similarly, files
      <filename><replaceable>foo</replaceable>.efi.extra.d/*.sysext.raw</filename> are packed up in a
      <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename> directory in the
      initrd file hierarchy. This is supposed to be used to pass additional system extension images to the
      initrd. See
      <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
      details on system extension images. The generated <command>cpio</command> archive containing these
      system extension images is measured into TPM PCR 13 (if a TPM is present).</para></listitem>

      <!-- Note: the actual suffix we look for for sysexts is just *.raw (not *.sysext.raw), for
           compatibility reasons with old versions. But we want people to name their system extensions
           properly, hence we document the *.sysext.raw suffix only. -->

      <listitem><para>Similarly, files
      <filename><replaceable>foo</replaceable>.efi.extra.d/*.confext.raw</filename> are packed up in a
      <command>cpio</command> archive and placed in the <filename>/.extra/confext/</filename> directory in
      the initrd file hierarchy. This is supposed to be used to pass additional configuration extension
      images to the initrd. See
      <citerefentry><refentrytitle>systemd-confext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
      details on configuration extension images. The generated <command>cpio</command> archive containing
      these configuration extension images is measured into TPM PCR 12 (if a TPM is present).</para></listitem>

      <listitem><para>Similarly, files
      <filename><replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename> are loaded and verified as
      PE binaries and specific sections are loaded from them. Addons are used to pass additional kernel
      command line parameters (<literal>.cmdline</literal> section), or DeviceTree blobs
      (<literal>.dtb</literal> section), additional initrds (<literal>.initrd</literal> section),
      and microcode updates (<literal>.ucode</literal> section). Addons allow those resources to be passed
      regardless of the kernel version being booted, for example allowing platform vendors to ship
      platform-specific configuration.</para>

      <para>In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
      Shim's MOK, and only loaded if the check passes. Additionally, if both the addon and the UKI contain a
      <literal>.uname</literal> section, the addon will be rejected if they do not match exactly. It is
      recommended to always add a <literal>.sbat</literal> section to all signed addons, so that they may be
      revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
      <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will add
      a SBAT policy by default if none is passed when building addons. For more information on SBAT see
      <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">Shim documentation</ulink>.</para>

      <para>Addon files are sorted, loaded, and measured into TPM PCR 12 (if a TPM is present) and appended
      to the kernel command line. UKI command line options are listed first, then options from addons in
      <filename>/loader/addons/*.addon.efi</filename>, and finally UKI-specific addons. Device tree blobs are
      loaded and measured following the same algorithm. Microcode addons are passed to the kernel in inverse
      order (UKI specific addons, global addons, UKI embedded section). This is because the microcode update
      driver stops on the first matching filename. Addons are always loaded in the same order based on
      the filename, so that, given the same set of addons, the same set of measurements can be expected in
      PCR12. However, note that the filename is not protected by the PE signature, and as such an attacker
      with write access to the ESP could potentially rename these files to change the order in which they are
      loaded, in a way that could alter the functionality of the kernel, as some options might be
      order-dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of
      an attestation service so that improper use of your signed addons can be detected and dealt with using
      one of the aforementioned revocation mechanisms.</para></listitem>

      <listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
      <command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
      directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
      the initrd, regardless of the kernel version being booted. The generated <command>cpio</command>
      archive is measured into TPM PCR 12 (if a TPM is present).</para></listitem>

      <listitem><para>Additionally, files <filename>/loader/addons/*.addon.efi</filename> are loaded and
      verified as PE binaries, and <literal>.cmdline</literal>, <literal>.dtb</literal>,
      <literal>.initrd</literal>, and <literal>.ucode</literal> sections are parsed from them.
      This is supposed to be used to pass additional command line parameters, DeviceTree blobs, initrds,
      and microcode updates to the kernel, regardless of the kernel version being booted.</para></listitem>
    </itemizedlist>

    <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
    images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
    further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
    by <command>systemd-creds encrypt -T</command> (see
    <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
    details); in case of the system extension images by using signed Verity images.</para>
  </refsect1>

  <refsect1>
    <title>Multi-Profile UKIs</title>

    <para>In many contexts it is useful to allow invocation of a single UKI in multiple different modes (or
    "profiles") without compromising the cryptographic integrity, measurements and so on of the boot
    process. For example, a single UKI might provide three distinct profiles: a regular boot one, one that
    invokes a "factory reset" operation, and one that boots into a storage target mode (as in
    <citerefentry><refentrytitle>systemd-storagetm.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>). Each
    profile would then use the same <literal>.linux</literal> and <literal>.initrd</literal> sections, but would
    have a separate <literal>.cmdline</literal> section. For example the latter two profiles would extend the
    regular kernel command line with <literal>systemd.unit=factory-reset.target</literal> or
    <literal>rd.systemd.unit=storagetm.target</literal>.</para>

    <para>A single UKI may support multiple profiles by means of the special <literal>.profile</literal> PE
    section. This section acts as separator between the PE sections of the individual
    profiles. <literal>.profile</literal> PE sections hence may appear multiple times in a single UKI, and
    the other PE sections listed above may appear multiple times too, if <literal>.profile</literal> are
    used, but only once before the first <literal>.profile</literal> section, once between each subsequent
    pair, and once after the last appearance of <literal>.profile</literal>. The sections listed before the
    first <literal>.profile</literal> are considered the "base" profile of the UKI. Each
    <literal>.profile</literal> section then introduces a new profile, which are numbered starting from
    zero. The PE sections following each <literal>.profile</literal> are specific to that profile. When
    booting into a specific profile the base section's profiles are used in combination with the specific
    profile's sections: if the same section is defined in both, the per-profile section overrides the base
    profile's version, otherwise the per-profile sections is used together with the base profile
    sections.</para> <para>A UKI that contains no <literal>.profile</literal> is consider equivalent to one
    that just contains a single <literal>.profile</literal>, as having only a single profile @0.</para>

    <para>Here's a simple example for a multi-profile UKI's sections, inspired by the setup suggested above:</para>

    <table>
      <title>Multi-Profile UKI Example</title>

      <tgroup cols='2' align='left' colsep='1' rowsep='1'>
        <colspec colname="section" />
        <colspec colname="profile" />

        <thead>
          <row>
            <entry>Section</entry>
            <entry>Profile</entry>
          </row>
        </thead>

        <tbody>
          <row>
            <entry><literal>.linux</literal></entry>
            <entry morerows="3" valign="middle">Base profile</entry>
          </row>
          <row>
            <entry><literal>.osrel</literal></entry>
          </row>
          <row>
            <entry><literal>.cmdline</literal></entry>
          </row>
          <row>
            <entry><literal>.initrd</literal></entry>
          </row>
          <row>
            <entry><literal>.profile</literal></entry>
            <entry>Profile @0</entry>
          </row>
          <row>
            <entry><literal>.profile</literal></entry>
            <entry morerows="1" valign="middle">Profile @1</entry>
          </row>
          <row>
            <entry><literal>.cmdline</literal></entry>
          </row>
          <row>
            <entry><literal>.profile</literal></entry>
            <entry morerows="1" valign="middle">Profile @2</entry>
          </row>
          <row>
            <entry><literal>.cmdline</literal></entry>
          </row>
        </tbody>
      </tgroup>
    </table>

    <para>The section list above would define three profiles. The first four sections make up the base
    profile. A <literal>.profile</literal> section then introduces profile @0. It doesn't override any
    sections (or add any) from the base section, hence it is immediately followed by another
    <literal>.profile</literal> section that then introduces section @1. This profile overrides the kernel
    command line. Finally, the last two sections define section @2, again overriding the command line. (Note
    that in this example the first <literal>.cmdline</literal> could also moved behind the first
    <literal>.profile</literal> with equivalent effect. To keep things nicely extensible, it's probably a
    good idea to keep the generic command line in the base section instead of profile 0, in case later added
    profiles might want to reuse it.)</para>

    <para>The profile to boot may be controlled via the UKI's own command line: if the first argument starts
    with <literal>@</literal>, followed by a positive integer number in decimal, it selects the profile to
    boot into. If the first argument is not specified like that, the UKI will automatically boot into profile
    0.</para>

    <para>A <literal>.profile</literal> section may contain meta-information about the profile. It follows a
    similar format as <literal>.osrel</literal> (i.e. an environment-variable-assignment-block-like list of
    newline separated strings). Currently two fields are defined: <literal>ID=</literal> is supposed to carry
    a short identifying string that identifies the profile
    (e.g. <literal>ID=factory-reset</literal>). <literal>TITLE=</literal> should contain a human readable
    string that may appear in the boot menu entry for this profile (e.g. <literal>TITLE='Factory Reset this
    Device'</literal>).</para>
  </refsect1>

  <refsect1>
    <title>TPM PCR Notes</title>

    <para>Note that when a unified kernel using <command>systemd-stub</command> is invoked the firmware will
    measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the
    core kernel, the embedded initrd and kernel command line (see above for a full list), including all UKI
    profiles.</para>

    <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
    every type of initrd (of the selected UKI profile) will possibly be measured two or three times: the
    initrds embedded in the kernel image will be measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized
    from credentials (and the one synthesized from configuration extensions) will be measured to both PCR 9
    and PCR 12; the initrd synthesized from system extensions will be measured to both PCR 4 and PCR 9. Let's
    summarize the OS resources and the PCRs they are measured to:</para>

    <table>
      <title>OS Resource PCR Summary</title>

      <tgroup cols='2' align='left' colsep='1' rowsep='1'>
        <colspec colname="pcr" />
        <colspec colname="definition" />

        <thead>
          <row>
            <entry>OS Resource</entry>
            <entry>Measurement PCR</entry>
          </row>
        </thead>

        <tbody>
          <row>
            <entry><command>systemd-stub</command> code (the entry point of the unified PE binary)</entry>
            <entry>4</entry>
          </row>

          <row>
            <entry>Core kernel code (embedded in unified PE binary)</entry>
            <entry>4 + 11</entry>
          </row>

          <row>
            <entry>OS release information (embedded in the unified PE binary)</entry>
            <entry>4 + 11</entry>
          </row>

          <row>
            <entry>Main initrd (embedded in unified PE binary)</entry>
            <entry>4 + 9 + 11</entry>
          </row>

          <row>
            <entry>Microcode initrd (embedded in unified PE binary)</entry>
            <entry>4 + 9 + 11</entry>
          </row>

          <row>
            <entry>Default kernel command line (embedded in unified PE binary)</entry>
            <entry>4 + 11</entry>
          </row>

          <row>
            <entry>Overridden kernel command line</entry>
            <entry>12</entry>
          </row>

          <row>
            <entry>Boot splash (embedded in the unified PE binary)</entry>
            <entry>4 + 11</entry>
          </row>

          <row>
            <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry>
            <entry>4 + 9</entry>
          </row>

          <row>
            <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry>
            <entry>4 + 9 + 11</entry>
          </row>

          <row>
            <entry>Credentials (synthesized initrd from companion files)</entry>
            <entry>9 + 12</entry>
          </row>

          <row>
            <entry>System Extensions (synthesized initrd from companion files)</entry>
            <entry>9 + 13</entry>
          </row>

          <row>
            <entry>Configuration Extensions (synthesized initrd from companion files)</entry>
            <entry>9 + 12</entry>
          </row>

          <row>
            <entry>Selected profile unless zero</entry>
            <entry>12</entry>
          </row>
        </tbody>
      </tgroup>
    </table>
  </refsect1>

  <refsect1>
    <title>EFI Variables</title>

    <para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
    vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
    stub and the OS:</para>

    <variablelist class='efi-variables'>
      <varlistentry>
        <term><varname>LoaderDevicePartUUID</varname></term>

        <listitem><para>Contains the partition UUID of the partition the boot loader has been started from on
        the current boot (usually a EFI System Partition). If already set by the boot loader, this will
        remain untouched by <command>systemd-stub</command>. If not set yet, this will be set to the
        partition UUID of the partition the unified kernel is started from, in order to support systems that
        directly boot into a unified kernel image, bypassing any boot loader.
        <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        uses this information to automatically find the disk booted from, in order to discover various other
        partitions on the same disk automatically.</para>

        <xi:include href="version-info.xml" xpointer="v224"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LoaderFirmwareInfo</varname></term>
        <term><varname>LoaderFirmwareType</varname></term>

        <listitem><para>Brief firmware information. Use
        <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
        data.</para>

        <xi:include href="version-info.xml" xpointer="v250"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LoaderImageIdentifier</varname></term>

        <listitem><para>The file system path to the EFI executable of the boot loader for the current boot,
        relative to the partition's root directory (i.e. relative to the partition indicated by
        <varname>LoaderDevicePartUUID</varname>, see above). If not set yet, this will be set to the file
        system path of the EFI executable of the booted unified kernel, in order to support systems that
        directly boot into a unified kernel image, bypassing any boot loader.  Use
        <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
        this data.</para>

        <xi:include href="version-info.xml" xpointer="v237"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>StubDevicePartUUID</varname></term>
        <term><varname>StubImageIdentifier</varname></term>

        <listitem><para>Similar to <varname>LoaderDevicePartUUID</varname> and
        <varname>StubImageIdentifier</varname>, but indicates the location of the unified kernel image EFI
        binary rather than the location of the boot loader binary, regardless if booted via a boot loader
        or not.</para>

        <xi:include href="version-info.xml" xpointer="v257"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>StubInfo</varname></term>

        <listitem><para>Brief stub information. Use
        <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
        this data.</para>

        <xi:include href="version-info.xml" xpointer="v250"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>StubPcrKernelImage</varname></term>

        <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
        database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
        <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
        unset otherwise.</para>

        <xi:include href="version-info.xml" xpointer="v252"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>StubPcrKernelParameters</varname></term>

        <listitem><para>The PCR register index the kernel command line and credentials are measured into,
        formatted as decimal ASCII string (e.g. <literal>12</literal>). This variable is set if a measurement
        was successfully completed, and remains unset otherwise.</para>

        <xi:include href="version-info.xml" xpointer="v252"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>StubPcrInitRDSysExts</varname></term>

        <listitem><para>The PCR register index the system extensions for the initrd, which are picked up from
        the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
        <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
        unset otherwise.</para>

        <xi:include href="version-info.xml" xpointer="v252"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>StubPcrInitRDConfExts</varname></term>

        <listitem><para>The PCR register index the configuration extensions for the initrd, which are picked
        up from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
        <literal>12</literal>). This variable is set if a measurement was successfully completed, and remains
        unset otherwise.</para>

        <xi:include href="version-info.xml" xpointer="v255"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>StubProfile</varname></term>

        <listitem><para>The numeric index of the selected profile, without the <literal>@</literal>,
        formatted as decimal string. Set both on single-profile and multi-profile UKIs. (In the former case
        this variable will be set to <literal>0</literal> unconditionally.)</para>

        <xi:include href="version-info.xml" xpointer="v257"/></listitem>
      </varlistentry>
    </variablelist>

    <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
    them if they aren't set already. Some of these variables are defined by the <ulink
    url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
  </refsect1>

  <refsect1>
    <title>initrd Resources</title>

    <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
    the initial file system hierarchy in the initrd execution environment:</para>

    <variablelist>
      <varlistentry>
        <term><filename>/</filename></term>

        <listitem><para>The main initrd from the <literal>.initrd</literal> PE section of the unified kernel
        image.</para>

        <xi:include href="version-info.xml" xpointer="v252"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>/.extra/credentials/*.cred</filename></term>
        <listitem><para>Credential files (suffix <literal>.cred</literal>) that are placed next to the
        unified kernel image (as described above) are copied into the
        <filename>/.extra/credentials/</filename> directory in the initrd execution
        environment.</para>

        <xi:include href="version-info.xml" xpointer="v252"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>/.extra/global_credentials/*.cred</filename></term>
        <listitem><para>Similarly, credential files in the <filename>/loader/credentials/</filename>
        directory in the file system the unified kernel image is placed in are copied into the
        <filename>/.extra/global_credentials/</filename> directory in the initrd execution
        environment.</para>

        <xi:include href="version-info.xml" xpointer="v252"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>/.extra/sysext/*.sysext.raw</filename></term>
        <listitem><para>System extension image files (suffix <literal>.sysext.raw</literal>) that are placed
        next to the unified kernel image (as described above) are copied into the
        <filename>/.extra/sysext/</filename> directory in the initrd execution environment.</para>

        <xi:include href="version-info.xml" xpointer="v252"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>/.extra/confext/*.confext.raw</filename></term>
        <listitem><para>Configuration extension image files (suffix <literal>.confext.raw</literal>) that are
        placed next to the unified kernel image (as described above) are copied into the
        <filename>/.extra/confext/</filename> directory in the initrd execution environment.</para>

        <xi:include href="version-info.xml" xpointer="v255"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>/.extra/tpm2-pcr-signature.json</filename></term>
        <listitem><para>The TPM2 PCR signature JSON object included in the <literal>.pcrsig</literal> PE
        section of the unified kernel image is copied into the
        <filename>/.extra/tpm2-pcr-signature.json</filename> file in the initrd execution environment.</para>

        <xi:include href="version-info.xml" xpointer="v252"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>/.extra/tpm2-pcr-public-key.pem</filename></term>
        <listitem><para>The PEM public key included in the <literal>.pcrpkey</literal> PE section of the
        unified kernel image is copied into the <filename>/.extra/tpm2-pcr-public-key.pem</filename> file in
        the initrd execution environment.</para>

        <xi:include href="version-info.xml" xpointer="v252"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>/.extra/profile</filename></term>
        <term><filename>/.extra/os-release</filename></term>
        <listitem><para>The contents of the <literal>.profile</literal> and <literal>.osrel</literal>
        sections of the selected profile, if any.</para>

        <xi:include href="version-info.xml" xpointer="v257"/></listitem>
      </varlistentry>
    </variablelist>

    <para>Note that all these files are located in the <literal>tmpfs</literal> file system the kernel sets
    up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
    environment into the host file system. If these resources shall be kept around over this transition they
    need to be copied to a place that survives the transition first, for example via a suitable
    <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
    default, this is done for the TPM2 PCR signature and public key files.</para>
  </refsect1>

  <refsect1>
    <title>SMBIOS Type 11 Strings</title>

    <para><command>systemd-stub</command> can be configured using SMBIOS Type 11 strings. Applicable strings
    consist of a name, followed by <literal>=</literal>, followed by the value. Unless
    <command>systemd-stub</command> detects it is running inside a confidential computing environment,
    <command>systemd-stub</command> will search the table for a string with a specific name, and if found,
    use its value. The following strings are read:</para>

    <variablelist>
      <varlistentry>
        <term><varname>io.systemd.stub.kernel-cmdline-extra</varname></term>
        <listitem><para>If set, the value of this string is added to the list of kernel command line
        arguments that are measured in PCR12 and passed to the kernel.</para>

        <xi:include href="version-info.xml" xpointer="v254"/></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Assembling Kernel Images</title>

    <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
    <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para><simplelist type="inline">
      <member><citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink></member>
      <member><ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink></member>
      <member><citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink></member>
    </simplelist></para>
  </refsect1>
</refentry>