man/systemd-stub.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   5
   6 <refentry id="systemd-stub" conditional='HAVE_GNU_EFI'
   7     xmlns:xi="http://www.w3.org/2001/XInclude">
   8   <refentryinfo>
   9     <title>systemd-stub</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>systemd-stub</refentrytitle>
  15     <manvolnum>7</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>systemd-stub</refname>
  20     <refname>sd-stub</refname>
  21     <refname>linuxx64.efi.stub</refname>
  22     <refname>linuxia32.efi.stub</refname>
  23     <refname>linuxaa64.efi.stub</refname>
  24     <refpurpose>A simple UEFI kernel boot stub</refpurpose>
  25   </refnamediv>
  26
  27   <refsynopsisdiv>
  28     <para><filename>/usr/lib/systemd/boot/efi/linuxx64.efi.stub</filename></para>
  29     <para><filename>/usr/lib/systemd/boot/efi/linuxia32.efi.stub</filename></para>
  30     <para><filename>/usr/lib/systemd/boot/efi/linuxaa64.efi.stub</filename></para>
  31     <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.cred</filename></para>
  32     <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.raw</filename></para>
  33     <para><filename><replaceable>ESP</replaceable>/loader/credentials/*.cred</filename></para>
  34   </refsynopsisdiv>
  35
  36   <refsect1>
  37     <title>Description</title>
  38
  39     <para><command>systemd-stub</command> (stored in per-architecture files
  40     <filename>linuxx64.efi.stub</filename>, <filename>linuxia32.efi.stub</filename>,
  41     <filename>linuxaa64.efi.stub</filename> on disk) is a simple UEFI boot stub. An UEFI boot stub is
  42     attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware
  43     environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux
  44     kernel is executable as regular UEFI binary, and is able to do various preparations before switching the
  45     system into the Linux world.</para>
  46
  47     <para>The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary
  48     itself. This allows combining various resources inside a single PE binary image, which may then be signed
  49     via UEFI SecureBoot as a whole, covering all individual resources at once. Specifically it may
  50     include:</para>
  51
  52     <itemizedlist>
  53       <listitem><para>The ELF Linux kernel images will be looked for in the <literal>.linux</literal> PE
  54       section of the executed image.</para></listitem>
  55
  56       <listitem><para>The initial RAM disk (initrd) will be looked for in the <literal>.initrd</literal> PE
  57       section.</para></listitem>
  58
  59       <listitem><para>A compiled binary DeviceTree will be looked for in the <literal>.dtb</literal> PE
  60       section.</para></listitem>
  61
  62       <listitem><para>The kernel command line to pass to the invoked kernel will be looked for in the
  63       <literal>.cmdline</literal> PE section.</para></listitem>
  64
  65       <listitem><para>A boot splash (in Windows <filename>.BMP</filename> format) to show on screen before
  66       invoking the kernel will be looked for in the <literal>.splash</literal> PE section.</para></listitem>
  67     </itemizedlist>
  68
  69     <para>If UEFI SecureBoot is enabled and the <literal>.cmdline</literal> section is present in the executed
  70     image, any attempts to override the kernel command line by passing one as invocation parameters to the
  71     EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
  72     SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
  73     is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 8 (if a TPM is
  74     present).</para>
  75
  76     <para>If a DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing
  77     DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
  78     <literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para>
  79   </refsect1>
  80
  81   <refsect1>
  82     <title>Companion Files</title>
  83
  84     <para>The <command>systemd-stub</command> UEFI boot stub automatically collects two types of auxiliary
  85     companion files optionally placed in drop-in directories on the same partition as the EFI binary,
  86     dynamically generates <command>cpio</command> initrd archives from them, and passes them to the kernel.
  87     Specifically:</para>
  88
  89     <itemizedlist>
  90       <listitem><para>For a kernel binary called <filename><replaceable>foo</replaceable>.efi</filename>, it
  91       will look for files with the <filename>.cred</filename> suffix in a directory named
  92       <filename><replaceable>foo</replaceable>.efi.extra.d/</filename> next to it. A <command>cpio</command>
  93       archive is generated from all files found that way, placing them in the
  94       <filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may
  95       then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
  96       authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System
  97       Partition. See
  98       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  99       and
 100       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 101       for
 102       details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM
 103       PCR 4 (if a TPM is present).</para></listitem>
 104
 105       <listitem><para>Similarly, files <filename><replaceable>foo</replaceable>.efi.extra.d/*.raw</filename>
 106       are packed up in a <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename>
 107       directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension
 108       images to the initrd. See
 109       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
 110       details on system extension images. The generated <command>cpio</command> archive containing these
 111       system extension images is measured into TPM PCR 8 (if a TPM is present).</para></listitem>
 112
 113       <listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
 114       <command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
 115       directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
 116       the initrd, regardless of the kernel being booted. The generated <command>cpio</command> archive is
 117       measured into TPM PCR 4 (if a TPM is present)</para></listitem>
 118     </itemizedlist>
 119
 120     <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
 121     images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
 122     further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
 123     by <command>systemd-creds encrypt -T</command> (see
 124     <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 125     details); in case of the system extension images by using signed Verity images.</para>
 126   </refsect1>
 127
 128   <refsect1>
 129     <title>EFI Variables</title>
 130
 131     <para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
 132     vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
 133     stub and the OS:</para>
 134
 135     <variablelist class='efi-variables'>
 136       <varlistentry>
 137         <term><varname>LoaderDevicePartUUID</varname></term>
 138
 139         <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
 140         from. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 141         uses this information to automatically find the disk booted from, in order to discover various other
 142         partitions on the same disk automatically.</para></listitem>
 143       </varlistentry>
 144
 145       <varlistentry>
 146         <term><varname>LoaderFirmwareInfo</varname></term>
 147         <term><varname>LoaderFirmwareType</varname></term>
 148
 149         <listitem><para>Brief firmware information. Use
 150         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
 151         data.</para></listitem>
 152       </varlistentry>
 153
 154       <varlistentry>
 155         <term><varname>LoaderImageIdentifier</varname></term>
 156
 157         <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
 158         directory. Use
 159         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 160         this data.</para></listitem>
 161       </varlistentry>
 162
 163       <varlistentry>
 164         <term><varname>StubInfo</varname></term>
 165
 166         <listitem><para>Brief stub information. Use
 167         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 168         this data.</para></listitem>
 169       </varlistentry>
 170     </variablelist>
 171
 172     <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
 173     them if they aren't set already. Some of these variables are defined by the <ulink
 174     url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
 175   </refsect1>
 176
 177   <refsect1>
 178     <title>Assembling Kernel Images</title>
 179
 180     <para>In order to assemble an UEFI PE kernel image from various components as described above, use an
 181     <citerefentry project='man-pages'><refentrytitle>objcopy</refentrytitle><manvolnum>1</manvolnum></citerefentry> command line
 182     like this:</para>
 183
 184     <programlisting>objcopy \
 185     --add-section .osrel=os-release --change-section-vma .osrel=0x20000 \
 186     --add-section .cmdline=cmdline.txt --change-section-vma .cmdline=0x30000 \
 187     --add-section .dtb=devicetree.dtb --change-section-vma .dtb=0x40000 \
 188     --add-section .splash=splash.bmp --change-section-vma .splash=0x100000 \
 189     --add-section .linux=vmlinux --change-section-vma .linux=0x2000000 \
 190     --add-section .initrd=initrd.cpio --change-section-vma .initrd=0x3000000 \
 191     /usr/lib/systemd/boot/efi/linuxx64.efi.stub \
 192     foo-unsigned.efi</programlisting>
 193
 194     <para>This generates one PE executable file <filename>foo-unsigned.efi</filename> from the six individual
 195     files for OS release information, kernel command line, boot splash image, kernel image, main initrd and
 196     UEFI boot stub.</para>
 197
 198     <para>To then sign the resulting image for UEFI SecureBoot use an
 199     <citerefentry project='archlinux'><refentrytitle>sbsign</refentrytitle><manvolnum>1</manvolnum></citerefentry> command like
 200     the following:</para>
 201
 202     <programlisting>sbsign \
 203     --key mykey.pem \
 204     --cert mykey.crt \
 205     --output foo.efi \
 206     foo-unsigned.efi</programlisting>
 207
 208     <para>This expects a pair of X.509 private key and certificate as parameters and then signs the UEFI PE
 209     executable we generated above for UEFI SecureBoot and generates a signed UEFI PE executable as
 210     result.</para>
 211   </refsect1>
 212
 213   <refsect1>
 214     <title>See Also</title>
 215     <para>
 216       <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 217       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 218       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 219       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 220       <ulink url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink>,
 221       <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
 222       <citerefentry project='man-pages'><refentrytitle>objcopy</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 223       <citerefentry project='archlinux'><refentrytitle>sbsign</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 224     </para>
 225   </refsect1>
 226 </refentry>