systemd/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 (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>
      <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>OS release information, i.e. the
      <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file of
      the OS the kernel belongs to, in the <literal>.osrel</literal> PE section.</para></listitem>

      <listitem><para>The initrd will be loaded from 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>

      <listitem><para>A set of cryptographic signatures for expected TPM2 PCR values when this kernel is
      booted, in JSON format, in the <literal>.pcrsig</literal> section. 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 public key in PEM format matching this TPM2 PCR signature data in the
      <literal>.pcrpkey</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 seven of these eight PE sections are measured into TPM PCR 11, that is otherwise
    not used. Thus, it can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section
    is not included in this PCR measurement, since it's supposed to contain signatures for the expected
    results for these measurements, i.e. of the outputs of the measurement operation, and thus cannot also be
    input to it.</para>

    <para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> 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>
  </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 13 (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>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></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>

      <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></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></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></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></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></listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>/.extra/global_credentials/*.cred</filename></term>
        <listitem><para>Similar, 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></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></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></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></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>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>Note that these PE section offsets are example values and a properly assembled image must not
    contain any overlapping sections (this includes already existing sections inside the stub before
    assembly) or boot may fail.</para>

    <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>

    <para>See
    <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
    an example involving the <literal>.pcrsig</literal> and <literal>.pcrpkey</literal> sections.</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>,
      <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    </para>
  </refsect1>
</refentry>