path: root/man/systemd-stub.xml

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<refentry id="systemd-stub" conditional='HAVE_GNU_EFI'
    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/*.cred</filename></para>
    <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.raw</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, which may then be signed
    via UEFI SecureBoot as a whole, covering all individual resources at once. Specifically it may
    include:</para>

    <itemizedlist>
      <listitem><para>The ELF Linux kernel images will be looked for in the <literal>.linux</literal> PE
      section of the executed image.</para></listitem>

      <listitem><para>The initial RAM disk (initrd) will be looked for in the <literal>.initrd</literal> PE
      section.</para></listitem>

      <listitem><para>A compiled binary DeviceTree will be looked for in the <literal>.dtb</literal> PE
      section.</para></listitem>

      <listitem><para>The kernel command line to pass to the invoked kernel will be looked for in the
      <literal>.cmdline</literal> PE section.</para></listitem>

      <listitem><para>A boot splash (in Windows <filename>.BMP</filename> format) to show on screen before
      invoking the kernel will be looked for in the <literal>.splash</literal> PE 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>
  </refsect1>

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

    <para>The <command>systemd-stub</command> UEFI boot stub automatically collects two 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. 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 4 (if a TPM is present).</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>
    </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>TPM2 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 twice: the initrd embedded in the kernel image will be measured to
    both PCR 4 and PCR 9; the initrd synthesized from credentials will be measured to both PCR 12 and PCR 9;
    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>Boot splash (embedded in the unified PE binary)</entry>
            <entry>4</entry>
          </row>

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

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

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

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

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

          <row>
            <entry>System Extensions (synthesized initrd from companion files)</entry>
            <entry>4 + 9</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></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></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></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></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>Assembling Kernel Images</title>

    <para>In order to assemble an UEFI PE kernel image from various components as described above, use an
    <citerefentry project='man-pages'><refentrytitle>objcopy</refentrytitle><manvolnum>1</manvolnum></citerefentry> command line
    like this:</para>

    <programlisting>objcopy \
    --add-section .osrel=os-release --change-section-vma .osrel=0x20000 \
    --add-section .cmdline=cmdline.txt --change-section-vma .cmdline=0x30000 \
    --add-section .dtb=devicetree.dtb --change-section-vma .dtb=0x40000 \
    --add-section .splash=splash.bmp --change-section-vma .splash=0x100000 \
    --add-section .linux=vmlinux --change-section-vma .linux=0x2000000 \
    --add-section .initrd=initrd.cpio --change-section-vma .initrd=0x3000000 \
    /usr/lib/systemd/boot/efi/linuxx64.efi.stub \
    foo-unsigned.efi</programlisting>

    <para>This generates one PE executable file <filename>foo-unsigned.efi</filename> from the six individual
    files for OS release information, kernel command line, boot splash image, kernel image, main initrd and
    UEFI boot stub.</para>

    <para>To then sign the resulting image for UEFI SecureBoot use an
    <citerefentry project='archlinux'><refentrytitle>sbsign</refentrytitle><manvolnum>1</manvolnum></citerefentry> command like
    the following:</para>

    <programlisting>sbsign \
    --key mykey.pem \
    --cert mykey.crt \
    --output foo.efi \
    foo-unsigned.efi</programlisting>

    <para>This expects a pair of X.509 private key and certificate as parameters and then signs the UEFI PE
    executable we generated above for UEFI SecureBoot and generates a signed UEFI PE executable as
    result.</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://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink>,
      <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
      <citerefentry project='man-pages'><refentrytitle>objcopy</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry project='archlinux'><refentrytitle>sbsign</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    </para>
  </refsect1>
</refentry>