man/systemd-stub.xml - systemd - Git at Google

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 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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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 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>
     </para>
   </refsect1>
 </refentry>
	<?xml version='1.0'?> <!---nxml--->
	<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
	"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
	<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

	<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 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>
	</para>
	</refsect1>
	</refentry>