systemd/man/systemd-stub.xml

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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><filename>/usr/lib/systemd/boot/efi/linuxx64.efi.stub</filename></para>
    <para><filename>/usr/lib/systemd/boot/efi/linuxia32.efi.stub</filename></para>
    <para><filename>/usr/lib/systemd/boot/efi/linuxaa64.efi.stub</filename></para>
    <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename></para>
    <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.cred</filename></para>
    <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.raw</filename></para>
    <para><filename><replaceable>ESP</replaceable>/loader/addons/*.addon.efi</filename></para>
    <para><filename><replaceable>ESP</replaceable>/loader/credentials/*.cred</filename></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 (usually called "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:</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.</para></listitem>

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

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

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

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

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

    <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 eight of these nine 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.</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/*.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>

      <listitem><para>Similarly, files
      <filename><replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename> are loaded and verified as
      PE binaries, and a <literal>.cmdline</literal> section is parsed from them. Addons are supposed to be
      used to pass additional kernel command line parameters or Devicetree blobs, regardless of the kernel
      image being booted, for example to allow 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 will be rejected otherwise. Additionally, if the both the addon and the UKI contain a 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. 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 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> and/or <literal>.dtb</literal> sections are
      parsed from them. This is supposed to be used to pass additional command line parameters or Devicetree
      blobs to the kernel, regardless of the kernel 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>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).</para>

    <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
    every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
    measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials 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>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>
        </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 EFI System Partition the EFI image was run
        from. <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="v250"/></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 path of EFI executable, relative to the EFI System Partition's root
        directory. 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>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 systemd 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>
    </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/*.raw</filename></term>
        <listitem><para>System extension image files (suffix <literal>.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/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-pkey.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>
    </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.
    <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>
      <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink>,
      <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
      <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink>
    </para>
  </refsect1>
</refentry>