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 (usually called "Unified
  49     Kernel Image", or "UKI" for short), which may then be signed via UEFI SecureBoot as a whole, covering all
  50     individual resources at once. Specifically it may 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>OS release information, i.e. the
  57       <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file of
  58       the OS the kernel belongs to, in the <literal>.osrel</literal> PE section.</para></listitem>
  59
  60       <listitem><para>The initrd will be loaded from the <literal>.initrd</literal> PE section.
  61       </para></listitem>
  62
  63       <listitem><para>A compiled binary DeviceTree will be looked for in the <literal>.dtb</literal> PE
  64       section.</para></listitem>
  65
  66       <listitem><para>The kernel command line to pass to the invoked kernel will be looked for in the
  67       <literal>.cmdline</literal> PE section.</para></listitem>
  68
  69       <listitem><para>A boot splash (in Windows <filename>.BMP</filename> format) to show on screen before
  70       invoking the kernel will be looked for in the <literal>.splash</literal> PE section.</para></listitem>
  71
  72       <listitem><para>A set of cryptographic signatures for expected TPM2 PCR values when this kernel is
  73       booted, in JSON format, in the <literal>.pcrsig</literal> section. This is useful for implementing TPM2
  74       policies that bind disk encryption and similar to kernels that are signed by a specific
  75       key.</para></listitem>
  76
  77       <listitem><para>A public key in PEM format matching this TPM2 PCR signature data in the
  78       <literal>.pcrpkey</literal> section.</para></listitem>
  79     </itemizedlist>
  80
  81     <para>If UEFI SecureBoot is enabled and the <literal>.cmdline</literal> section is present in the executed
  82     image, any attempts to override the kernel command line by passing one as invocation parameters to the
  83     EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
  84     SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
  85     is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is
  86     present).</para>
  87
  88     <para>If a DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing
  89     DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
  90     <literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para>
  91
  92     <para>The contents of seven of these eight PE sections are measured into TPM PCR 11, that is otherwise
  93     not used. Thus, it can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section
  94     is not included in this PCR measurement, since it's supposed to contain signatures for the expected
  95     results for these measurements, i.e. of the outputs of the measurement operation, and thus cannot also be
  96     input to it.</para>
  97
  98     <para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> are present in a unified kernel
  99     image their contents are passed to the booted kernel in an synthetic initrd cpio archive that places them in the
 100     <filename>/.extra/tpm2-pcr-signature.json</filename> and
 101     <filename>/.extra/tpm2-pcr-public-key.pem</filename> files. Typically, a
 102     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
 103     ensures they are copied into <filename>/run/systemd/tpm2-pcr-signature.json</filename> and
 104     <filename>/run/systemd/tpm2-pcr-public-key.pem</filename> where they remain accessible even after the
 105     system transitions out of the initrd environment into the host file system. Tools such
 106     <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 107     <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 108     and <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 109     will automatically use files present under these paths to unlock protected resources (encrypted storage
 110     or credentials) or bind encryption to booted kernels.</para>
 111   </refsect1>
 112
 113   <refsect1>
 114     <title>Companion Files</title>
 115
 116     <para>The <command>systemd-stub</command> UEFI boot stub automatically collects two types of auxiliary
 117     companion files optionally placed in drop-in directories on the same partition as the EFI binary,
 118     dynamically generates <command>cpio</command> initrd archives from them, and passes them to the kernel.
 119     Specifically:</para>
 120
 121     <itemizedlist>
 122       <listitem><para>For a kernel binary called <filename><replaceable>foo</replaceable>.efi</filename>, it
 123       will look for files with the <filename>.cred</filename> suffix in a directory named
 124       <filename><replaceable>foo</replaceable>.efi.extra.d/</filename> next to it. A <command>cpio</command>
 125       archive is generated from all files found that way, placing them in the
 126       <filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may
 127       then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
 128       authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System
 129       Partition. See
 130       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 131       and
 132       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 133       for
 134       details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM
 135       PCR 12 (if a TPM is present).</para></listitem>
 136
 137       <listitem><para>Similarly, files <filename><replaceable>foo</replaceable>.efi.extra.d/*.raw</filename>
 138       are packed up in a <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename>
 139       directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension
 140       images to the initrd. See
 141       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
 142       details on system extension images. The generated <command>cpio</command> archive containing these
 143       system extension images is measured into TPM PCR 13 (if a TPM is present).</para></listitem>
 144
 145       <listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
 146       <command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
 147       directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
 148       the initrd, regardless of the kernel being booted. The generated <command>cpio</command> archive is
 149       measured into TPM PCR 12 (if a TPM is present)</para></listitem>
 150     </itemizedlist>
 151
 152     <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
 153     images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
 154     further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
 155     by <command>systemd-creds encrypt -T</command> (see
 156     <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 157     details); in case of the system extension images by using signed Verity images.</para>
 158   </refsect1>
 159
 160   <refsect1>
 161     <title>TPM PCR Notes</title>
 162
 163     <para>Note that when a unified kernel using <command>systemd-stub</command> is invoked the firmware will
 164     measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the
 165     core kernel, the embedded initrd and kernel command line (see above for a full list).</para>
 166
 167     <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
 168     every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
 169     measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials will be measured to both PCR
 170     9 and PCR 12; the initrd synthesized from system extensions will be measured to both PCR 4 and PCR
 171     9. Let's summarize the OS resources and the PCRs they are measured to:</para>
 172
 173     <table>
 174       <title>OS Resource PCR Summary</title>
 175
 176       <tgroup cols='2' align='left' colsep='1' rowsep='1'>
 177         <colspec colname="pcr" />
 178         <colspec colname="definition" />
 179
 180         <thead>
 181           <row>
 182             <entry>OS Resource</entry>
 183             <entry>Measurement PCR</entry>
 184           </row>
 185         </thead>
 186
 187         <tbody>
 188           <row>
 189             <entry><command>systemd-stub</command> code (the entry point of the unified PE binary)</entry>
 190             <entry>4</entry>
 191           </row>
 192
 193           <row>
 194             <entry>Core kernel code (embedded in unified PE binary)</entry>
 195             <entry>4 + 11</entry>
 196           </row>
 197
 198           <row>
 199             <entry>OS release information (embedded in the unified PE binary)</entry>
 200             <entry>4 + 11</entry>
 201           </row>
 202
 203           <row>
 204             <entry>Main initrd (embedded in unified PE binary)</entry>
 205             <entry>4 + 9 + 11</entry>
 206           </row>
 207
 208           <row>
 209             <entry>Default kernel command line (embedded in unified PE binary)</entry>
 210             <entry>4 + 11</entry>
 211           </row>
 212
 213           <row>
 214             <entry>Overridden kernel command line</entry>
 215             <entry>12</entry>
 216           </row>
 217
 218           <row>
 219             <entry>Boot splash (embedded in the unified PE binary)</entry>
 220             <entry>4 + 11</entry>
 221           </row>
 222
 223           <row>
 224             <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry>
 225             <entry>4 + 9</entry>
 226           </row>
 227
 228           <row>
 229             <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry>
 230             <entry>4 + 9 + 11</entry>
 231           </row>
 232
 233           <row>
 234             <entry>Credentials (synthesized initrd from companion files)</entry>
 235             <entry>9 + 12</entry>
 236           </row>
 237
 238           <row>
 239             <entry>System Extensions (synthesized initrd from companion files)</entry>
 240             <entry>9 + 13</entry>
 241           </row>
 242         </tbody>
 243       </tgroup>
 244     </table>
 245   </refsect1>
 246
 247   <refsect1>
 248     <title>EFI Variables</title>
 249
 250     <para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
 251     vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
 252     stub and the OS:</para>
 253
 254     <variablelist class='efi-variables'>
 255       <varlistentry>
 256         <term><varname>LoaderDevicePartUUID</varname></term>
 257
 258         <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
 259         from. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 260         uses this information to automatically find the disk booted from, in order to discover various other
 261         partitions on the same disk automatically.</para></listitem>
 262       </varlistentry>
 263
 264       <varlistentry>
 265         <term><varname>LoaderFirmwareInfo</varname></term>
 266         <term><varname>LoaderFirmwareType</varname></term>
 267
 268         <listitem><para>Brief firmware information. Use
 269         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
 270         data.</para></listitem>
 271       </varlistentry>
 272
 273       <varlistentry>
 274         <term><varname>LoaderImageIdentifier</varname></term>
 275
 276         <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
 277         directory. Use
 278         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 279         this data.</para></listitem>
 280       </varlistentry>
 281
 282       <varlistentry>
 283         <term><varname>StubInfo</varname></term>
 284
 285         <listitem><para>Brief stub information. Use
 286         <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
 287         this data.</para></listitem>
 288       </varlistentry>
 289
 290       <varlistentry>
 291         <term><varname>StubPcrKernelImage</varname></term>
 292
 293         <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
 294         database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
 295         <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
 296         unset otherwise.</para></listitem>
 297       </varlistentry>
 298
 299       <varlistentry>
 300         <term><varname>StubPcrKernelParameters</varname></term>
 301
 302         <listitem><para>The PCR register index the kernel command line and credentials are measured into,
 303         formatted as decimal ASCII string (e.g. <literal>12</literal>). This variable is set if a measurement
 304         was successfully completed, and remains unset otherwise.</para></listitem>
 305       </varlistentry>
 306
 307       <varlistentry>
 308         <term><varname>StubPcrInitRDSysExts</varname></term>
 309
 310         <listitem><para>The PCR register index the systemd extensions for the initrd, which are picked up
 311         from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
 312         <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
 313         unset otherwise.</para></listitem>
 314       </varlistentry>
 315     </variablelist>
 316
 317     <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
 318     them if they aren't set already. Some of these variables are defined by the <ulink
 319     url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
 320   </refsect1>
 321
 322   <refsect1>
 323     <title>initrd Resources</title>
 324
 325     <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
 326     the initial file system hierarchy in the initrd execution environment:</para>
 327
 328     <variablelist>
 329       <varlistentry>
 330         <term><filename>/</filename></term>
 331
 332         <listitem><para>The main initrd from the <literal>.initrd</literal> PE section of the unified kernel image.</para></listitem>
 333       </varlistentry>
 334
 335       <varlistentry>
 336         <term><filename>/.extra/credentials/*.cred</filename></term>
 337         <listitem><para>Credential files (suffix <literal>.cred</literal>) that are placed next to the
 338         unified kernel image (as described above) are copied into the
 339         <filename>/.extra/credentials/</filename> directory in the initrd execution
 340         environment.</para></listitem>
 341       </varlistentry>
 342
 343       <varlistentry>
 344         <term><filename>/.extra/global_credentials/*.cred</filename></term>
 345         <listitem><para>Similar, credential files in the <filename>/loader/credentials/</filename> directory
 346         in the file system the unified kernel image is placed in are copied into the
 347         <filename>/.extra/global_credentials/</filename> directory in the initrd execution
 348         environment.</para></listitem>
 349       </varlistentry>
 350
 351       <varlistentry>
 352         <term><filename>/.extra/sysext/*.raw</filename></term>
 353         <listitem><para>System extension image files (suffix <literal>.raw</literal>) that are placed next to
 354         the unified kernel image (as described above) are copied into the
 355         <filename>/.extra/sysext/</filename> directory in the initrd execution environment.</para></listitem>
 356       </varlistentry>
 357
 358       <varlistentry>
 359         <term><filename>/.extra/tpm2-pcr-signature.json</filename></term>
 360         <listitem><para>The TPM2 PCR signature JSON object included in the <literal>.pcrsig</literal> PE
 361         section of the unified kernel image is copied into the
 362         <filename>/.extra/tpm2-pcr-signature.json</filename> file in the initrd execution
 363         environment.</para></listitem>
 364       </varlistentry>
 365
 366       <varlistentry>
 367         <term><filename>/.extra/tpm2-pcr-pkey.pem</filename></term>
 368         <listitem><para>The PEM public key included in the <literal>.pcrpkey</literal> PE section of the
 369         unified kernel image is copied into the <filename>/.extra/tpm2-pcr-public-key.pem</filename> file in
 370         the initrd execution environment.</para></listitem>
 371       </varlistentry>
 372     </variablelist>
 373
 374     <para>Note that all these files are located in the <literal>tmpfs</literal> file system the kernel sets
 375     up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
 376     environment into the host file system. If these resources shall be kept around over this transition they
 377     need to be copied to a place that survives the transition first, for example via a suitable
 378     <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
 379     default, this is done for the TPM2 PCR signature and public key files.</para>
 380   </refsect1>
 381
 382   <refsect1>
 383     <title>Assembling Kernel Images</title>
 384
 385     <para>In order to assemble an UEFI PE kernel image from various components as described above, use an
 386     <citerefentry project='man-pages'><refentrytitle>objcopy</refentrytitle><manvolnum>1</manvolnum></citerefentry> command line
 387     like this:</para>
 388
 389     <programlisting>objcopy \
 390     --add-section .osrel=os-release --change-section-vma .osrel=0x20000 \
 391     --add-section .cmdline=cmdline.txt --change-section-vma .cmdline=0x30000 \
 392     --add-section .dtb=devicetree.dtb --change-section-vma .dtb=0x40000 \
 393     --add-section .splash=splash.bmp --change-section-vma .splash=0x100000 \
 394     --add-section .linux=vmlinux --change-section-vma .linux=0x2000000 \
 395     --add-section .initrd=initrd.cpio --change-section-vma .initrd=0x3000000 \
 396     /usr/lib/systemd/boot/efi/linuxx64.efi.stub \
 397     foo-unsigned.efi</programlisting>
 398
 399     <para>This generates one PE executable file <filename>foo-unsigned.efi</filename> from the six individual
 400     files for OS release information, kernel command line, boot splash image, kernel image, main initrd and
 401     UEFI boot stub.</para>
 402
 403     <para>To then sign the resulting image for UEFI SecureBoot use an
 404     <citerefentry project='archlinux'><refentrytitle>sbsign</refentrytitle><manvolnum>1</manvolnum></citerefentry> command like
 405     the following:</para>
 406
 407     <programlisting>sbsign \
 408     --key mykey.pem \
 409     --cert mykey.crt \
 410     --output foo.efi \
 411     foo-unsigned.efi</programlisting>
 412
 413     <para>This expects a pair of X.509 private key and certificate as parameters and then signs the UEFI PE
 414     executable we generated above for UEFI SecureBoot and generates a signed UEFI PE executable as
 415     result.</para>
 416
 417     <para>See
 418     <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 419     an example involving the <literal>.pcrsig</literal> and <literal>.pcrpkey</literal> sections.</para>
 420   </refsect1>
 421
 422   <refsect1>
 423     <title>See Also</title>
 424     <para>
 425       <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 426       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 427       <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 428       <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 429       <ulink url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink>,
 430       <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
 431       <citerefentry project='man-pages'><refentrytitle>objcopy</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 432       <citerefentry project='archlinux'><refentrytitle>sbsign</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 433       <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 434     </para>
 435   </refsect1>
 436 </refentry>