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 -->
6 <refentry id=
"systemd-stub" conditional='ENABLE_BOOTLOADER'
7 xmlns:
xi=
"http://www.w3.org/2001/XInclude">
9 <title>systemd-stub
</title>
10 <productname>systemd
</productname>
14 <refentrytitle>systemd-stub
</refentrytitle>
15 <manvolnum>7</manvolnum>
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>
29 <member><filename>/usr/lib/systemd/boot/efi/linuxx64.efi.stub
</filename></member>
30 <member><filename>/usr/lib/systemd/boot/efi/linuxia32.efi.stub
</filename></member>
31 <member><filename>/usr/lib/systemd/boot/efi/linuxaa64.efi.stub
</filename></member>
32 <member><filename><replaceable>ESP
</replaceable>/.../
<replaceable>foo
</replaceable>.efi.extra.d/*.addon.efi
</filename></member>
33 <member><filename><replaceable>ESP
</replaceable>/.../
<replaceable>foo
</replaceable>.efi.extra.d/*.cred
</filename></member>
34 <member><filename><replaceable>ESP
</replaceable>/.../
<replaceable>foo
</replaceable>.efi.extra.d/*.raw
</filename></member>
35 <member><filename><replaceable>ESP
</replaceable>/loader/addons/*.addon.efi
</filename></member>
36 <member><filename><replaceable>ESP
</replaceable>/loader/credentials/*.cred
</filename></member>
41 <title>Description
</title>
43 <para><command>systemd-stub
</command> (stored in per-architecture files
44 <filename>linuxx64.efi.stub
</filename>,
<filename>linuxia32.efi.stub
</filename>,
45 <filename>linuxaa64.efi.stub
</filename> on disk) is a simple UEFI boot stub. An UEFI boot stub is
46 attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware
47 environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux
48 kernel is executable as regular UEFI binary, and is able to do various preparations before switching the
49 system into the Linux world.
</para>
51 <para>The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary
52 itself. This allows combining various resources inside a single PE binary image (usually called
"Unified
53 Kernel Image", or
"UKI" for short), which may then be signed via UEFI SecureBoot as a whole, covering all
54 individual resources at once. Specifically it may include:
</para>
57 <!-- Let's keep this in the canonical order we also measure the sections by, i.e. as in
58 src/fundamental/uki.h's UnifiedSection enum -->
60 <listitem><para>A
<literal>.linux
</literal> section with the ELF Linux kernel image.
</para></listitem>
62 <listitem><para>An
<literal>.osrel
</literal> section with OS release information, i.e. the contents of
63 the
<citerefentry><refentrytitle>os-release
</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
64 of the OS the kernel belongs to.
</para></listitem>
66 <listitem><para>A
<literal>.cmdline
</literal> section with the kernel command line to pass to the
67 invoked kernel.
</para></listitem>
69 <listitem><para>An
<literal>.initrd
</literal> section with the initrd.
</para></listitem>
71 <listitem><para>A
<literal>.splash
</literal> section with an image (in the Windows
72 <filename>.BMP
</filename> format) to show on screen before invoking the kernel.
</para></listitem>
74 <listitem><para>A
<literal>.dtb
</literal> section with a compiled binary DeviceTree.
</para></listitem>
76 <listitem><para>A
<literal>.uname
</literal> section with the kernel version information, i.e. the
77 output of
<command>uname -r
</command> for the kernel included in the
<literal>.linux
</literal>
78 section.
</para></listitem>
80 <listitem><para>An
<literal>.sbat
</literal> section with
81 <ulink url=
"https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT
</ulink> revocation
82 metadata.
</para></listitem>
84 <listitem><para>A
<literal>.pcrsig
</literal> section with a set of cryptographic signatures for the
85 expected TPM2 PCR values after the kernel has been booted, in JSON format. This is useful for
86 implementing TPM2 policies that bind disk encryption and similar to kernels that are signed by a
87 specific key.
</para></listitem>
89 <listitem><para>A
<literal>.pcrpkey
</literal> section with a public key in the PEM format matching the
90 signature data in the the
<literal>.pcrsig
</literal> section.
</para></listitem>
93 <para>If UEFI SecureBoot is enabled and the
<literal>.cmdline
</literal> section is present in the executed
94 image, any attempts to override the kernel command line by passing one as invocation parameters to the
95 EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
96 SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
97 is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR
12 (if a TPM is
100 <para>If a DeviceTree is embedded in the
<literal>.dtb
</literal> section, it replaces an existing
101 DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
102 <literal>EFI_DT_FIXUP_PROTOCOL
</literal> for hardware specific fixups to the DeviceTree.
</para>
104 <para>The contents of eight of these nine sections are measured into TPM PCR
11. It is otherwise not used
105 and thus the result can be pre-calculated without too much effort. The
<literal>.pcrsig
</literal> section
106 is not included in this PCR measurement, since it is supposed to contain signatures for the output of the
107 measurement operation, and thus cannot also be input to it.
</para>
109 <para>When
<literal>.pcrsig
</literal> and/or
<literal>.pcrpkey
</literal> sections are present in a
110 unified kernel image their contents are passed to the booted kernel in an synthetic initrd cpio archive
111 that places them in the
<filename>/.extra/tpm2-pcr-signature.json
</filename> and
112 <filename>/.extra/tpm2-pcr-public-key.pem
</filename> files. Typically, a
113 <citerefentry><refentrytitle>tmpfiles.d
</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
114 ensures they are copied into
<filename>/run/systemd/tpm2-pcr-signature.json
</filename> and
115 <filename>/run/systemd/tpm2-pcr-public-key.pem
</filename> where they remain accessible even after the
116 system transitions out of the initrd environment into the host file system. Tools such
117 <citerefentry><refentrytitle>systemd-cryptsetup@.service
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
118 <citerefentry><refentrytitle>systemd-cryptenroll
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
119 and
<citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
120 will automatically use files present under these paths to unlock protected resources (encrypted storage
121 or credentials) or bind encryption to booted kernels.
</para>
123 <para>For further details about the UKI concept, see the
<ulink
124 url=
"https://uapi-group.org/specifications/specs/unified_kernel_image/">UKI specification
</ulink>.
</para>
128 <title>Companion Files
</title>
130 <para>The
<command>systemd-stub
</command> UEFI boot stub automatically collects three types of auxiliary
131 companion files optionally placed in drop-in directories on the same partition as the EFI binary,
132 dynamically generates
<command>cpio
</command> initrd archives from them, and passes them to the kernel.
136 <listitem><para>For a kernel binary called
<filename><replaceable>foo
</replaceable>.efi
</filename>, it
137 will look for files with the
<filename>.cred
</filename> suffix in a directory named
138 <filename><replaceable>foo
</replaceable>.efi.extra.d/
</filename> next to it. If the kernel binary
139 uses a counter for the purpose of
140 <ulink url=
"https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot Assessment
</ulink>, this
141 counter will be ignored. For example,
<filename><replaceable>foo
</replaceable>+
3-
0.efi
</filename>
142 will look in directory
<filename><replaceable>foo
</replaceable>.efi.extra.d/
</filename>.
143 A
<command>cpio
</command>
144 archive is generated from all files found that way, placing them in the
145 <filename>/.extra/credentials/
</filename> directory of the initrd file hierarchy. The main initrd may
146 then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
147 authenticated credentials for use with
<varname>LoadCredentialEncrypted=
</varname> in the UEFI System
149 <citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
151 <citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
153 details on encrypted credentials. The generated
<command>cpio
</command> archive is measured into TPM
154 PCR
12 (if a TPM is present).
</para></listitem>
156 <listitem><para>Similarly, files
<filename><replaceable>foo
</replaceable>.efi.extra.d/*.raw
</filename>
157 are packed up in a
<command>cpio
</command> archive and placed in the
<filename>/.extra/sysext/
</filename>
158 directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension
159 images to the initrd. See
160 <citerefentry><refentrytitle>systemd-sysext
</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
161 details on system extension images. The generated
<command>cpio
</command> archive containing these
162 system extension images is measured into TPM PCR
13 (if a TPM is present).
</para></listitem>
164 <listitem><para>Similarly, files
165 <filename><replaceable>foo
</replaceable>.efi.extra.d/*.addon.efi
</filename> are loaded and verified as
166 PE binaries, and a
<literal>.cmdline
</literal> section is parsed from them. Addons are supposed to be
167 used to pass additional kernel command line parameters or Devicetree blobs, regardless of the kernel
168 image being booted, for example to allow platform vendors to ship platform-specific
169 configuration.
</para>
171 <para>In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
172 Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a a
173 <literal>.uname
</literal> section, the addon will be rejected if they do not match exactly. It is
174 recommended to always add a
<literal>.sbat
</literal> section to all signed addons, so that they may be
175 revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
176 <citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will add
177 a SBAT policy by default if none is passed when building addons. For more information on SBAT see
178 <ulink url=
"https://github.com/rhboot/shim/blob/main/SBAT.md">Shim documentation
</ulink>.
</para>
180 <para>Addon files are sorted, loaded, and measured into TPM PCR
12 (if a TPM is present) and appended
181 to the kernel command line. UKI command line options are listed first, then options from addons in
182 <filename>/loader/addons/*.addon.efi
</filename>, and finally UKI-specific addons. Device tree blobs are
183 loaded and measured following the same algorithm. Addons are always loaded in the same order based on
184 the filename, so that, given the same set of addons, the same set of measurements can be expected in
185 PCR12. However, note that the filename is not protected by the PE signature, and as such an attacker
186 with write access to the ESP could potentially rename these files to change the order in which they are
187 loaded, in a way that could alter the functionality of the kernel, as some options might be
188 order-dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of
189 an attestation service so that improper use of your signed addons can be detected and dealt with using
190 one of the aforementioned revocation mechanisms.
</para></listitem>
192 <listitem><para>Files
<filename>/loader/credentials/*.cred
</filename> are packed up in a
193 <command>cpio
</command> archive and placed in the
<filename>/.extra/global_credentials/
</filename>
194 directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
195 the initrd, regardless of the kernel being booted. The generated
<command>cpio
</command> archive is
196 measured into TPM PCR
12 (if a TPM is present).
</para></listitem>
198 <listitem><para>Additionally, files
<filename>/loader/addons/*.addon.efi
</filename> are loaded and
199 verified as PE binaries, and
<literal>.cmdline
</literal> and/or
<literal>.dtb
</literal> sections are
200 parsed from them. This is supposed to be used to pass additional command line parameters or Devicetree
201 blobs to the kernel, regardless of the kernel being booted.
</para></listitem>
204 <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
205 images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
206 further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
207 by
<command>systemd-creds encrypt -T
</command> (see
208 <citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
209 details); in case of the system extension images by using signed Verity images.
</para>
213 <title>TPM PCR Notes
</title>
215 <para>Note that when a unified kernel using
<command>systemd-stub
</command> is invoked the firmware will
216 measure it as a whole to TPM PCR
4, covering all embedded resources, such as the stub code itself, the
217 core kernel, the embedded initrd and kernel command line (see above for a full list).
</para>
219 <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR
9. This means
220 every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
221 measured to PCR
4, PCR
9 and PCR
11; the initrd synthesized from credentials will be measured to both PCR
222 9 and PCR
12; the initrd synthesized from system extensions will be measured to both PCR
4 and PCR
223 9. Let's summarize the OS resources and the PCRs they are measured to:
</para>
226 <title>OS Resource PCR Summary
</title>
228 <tgroup cols='
2' align='left' colsep='
1' rowsep='
1'
>
229 <colspec colname=
"pcr" />
230 <colspec colname=
"definition" />
234 <entry>OS Resource
</entry>
235 <entry>Measurement PCR
</entry>
241 <entry><command>systemd-stub
</command> code (the entry point of the unified PE binary)
</entry>
246 <entry>Core kernel code (embedded in unified PE binary)
</entry>
247 <entry>4 +
11</entry>
251 <entry>OS release information (embedded in the unified PE binary)
</entry>
252 <entry>4 +
11</entry>
256 <entry>Main initrd (embedded in unified PE binary)
</entry>
257 <entry>4 +
9 +
11</entry>
261 <entry>Default kernel command line (embedded in unified PE binary)
</entry>
262 <entry>4 +
11</entry>
266 <entry>Overridden kernel command line
</entry>
271 <entry>Boot splash (embedded in the unified PE binary)
</entry>
272 <entry>4 +
11</entry>
276 <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)
</entry>
281 <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)
</entry>
282 <entry>4 +
9 +
11</entry>
286 <entry>Credentials (synthesized initrd from companion files)
</entry>
287 <entry>9 +
12</entry>
291 <entry>System Extensions (synthesized initrd from companion files)
</entry>
292 <entry>9 +
13</entry>
300 <title>EFI Variables
</title>
302 <para>The following EFI variables are defined, set and read by
<command>systemd-stub
</command>, under the
303 vendor UUID
<literal>4a67b082-
0a4c-
41cf-b6c7-
440b29bb8c4f
</literal>, for communication between the boot
304 stub and the OS:
</para>
306 <variablelist class='efi-variables'
>
308 <term><varname>LoaderDevicePartUUID
</varname></term>
310 <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
311 from.
<citerefentry><refentrytitle>systemd-gpt-auto-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
312 uses this information to automatically find the disk booted from, in order to discover various other
313 partitions on the same disk automatically.
</para>
315 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
319 <term><varname>LoaderFirmwareInfo
</varname></term>
320 <term><varname>LoaderFirmwareType
</varname></term>
322 <listitem><para>Brief firmware information. Use
323 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
326 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
330 <term><varname>LoaderImageIdentifier
</varname></term>
332 <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
334 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
337 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
341 <term><varname>StubInfo
</varname></term>
343 <listitem><para>Brief stub information. Use
344 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
347 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
351 <term><varname>StubPcrKernelImage
</varname></term>
353 <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
354 database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
355 <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
356 unset otherwise.
</para>
358 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
362 <term><varname>StubPcrKernelParameters
</varname></term>
364 <listitem><para>The PCR register index the kernel command line and credentials are measured into,
365 formatted as decimal ASCII string (e.g.
<literal>12</literal>). This variable is set if a measurement
366 was successfully completed, and remains unset otherwise.
</para>
368 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
372 <term><varname>StubPcrInitRDSysExts
</varname></term>
374 <listitem><para>The PCR register index the systemd extensions for the initrd, which are picked up
375 from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
376 <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
377 unset otherwise.
</para>
379 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
383 <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
384 them if they aren't set already. Some of these variables are defined by the
<ulink
385 url=
"https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface
</ulink>.
</para>
389 <title>initrd Resources
</title>
391 <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
392 the initial file system hierarchy in the initrd execution environment:
</para>
396 <term><filename>/
</filename></term>
398 <listitem><para>The main initrd from the
<literal>.initrd
</literal> PE section of the unified kernel
401 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
405 <term><filename>/.extra/credentials/*.cred
</filename></term>
406 <listitem><para>Credential files (suffix
<literal>.cred
</literal>) that are placed next to the
407 unified kernel image (as described above) are copied into the
408 <filename>/.extra/credentials/
</filename> directory in the initrd execution
411 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
415 <term><filename>/.extra/global_credentials/*.cred
</filename></term>
416 <listitem><para>Similarly, credential files in the
<filename>/loader/credentials/
</filename>
417 directory in the file system the unified kernel image is placed in are copied into the
418 <filename>/.extra/global_credentials/
</filename> directory in the initrd execution
421 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
425 <term><filename>/.extra/sysext/*.raw
</filename></term>
426 <listitem><para>System extension image files (suffix
<literal>.raw
</literal>) that are placed next to
427 the unified kernel image (as described above) are copied into the
428 <filename>/.extra/sysext/
</filename> directory in the initrd execution environment.
</para>
430 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
434 <term><filename>/.extra/tpm2-pcr-signature.json
</filename></term>
435 <listitem><para>The TPM2 PCR signature JSON object included in the
<literal>.pcrsig
</literal> PE
436 section of the unified kernel image is copied into the
437 <filename>/.extra/tpm2-pcr-signature.json
</filename> file in the initrd execution environment.
</para>
439 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
443 <term><filename>/.extra/tpm2-pcr-pkey.pem
</filename></term>
444 <listitem><para>The PEM public key included in the
<literal>.pcrpkey
</literal> PE section of the
445 unified kernel image is copied into the
<filename>/.extra/tpm2-pcr-public-key.pem
</filename> file in
446 the initrd execution environment.
</para>
448 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
452 <para>Note that all these files are located in the
<literal>tmpfs
</literal> file system the kernel sets
453 up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
454 environment into the host file system. If these resources shall be kept around over this transition they
455 need to be copied to a place that survives the transition first, for example via a suitable
456 <citerefentry><refentrytitle>tmpfiles.d
</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
457 default, this is done for the TPM2 PCR signature and public key files.
</para>
461 <title>SMBIOS Type
11 Strings
</title>
463 <para><command>systemd-stub
</command> can be configured using SMBIOS Type
11 strings. Applicable strings
464 consist of a name, followed by
<literal>=
</literal>, followed by the value.
465 <command>systemd-stub
</command> will search the table for a string with a specific name, and if found,
466 use its value. The following strings are read:
</para>
470 <term><varname>io.systemd.stub.kernel-cmdline-extra
</varname></term>
471 <listitem><para>If set, the value of this string is added to the list of kernel command line
472 arguments that are measured in PCR12 and passed to the kernel.
</para>
474 <xi:include href=
"version-info.xml" xpointer=
"v254"/></listitem>
480 <title>Assembling Kernel Images
</title>
482 <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
483 <citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
</para>
487 <title>See Also
</title>
488 <para><simplelist type=
"inline">
489 <member><citerefentry><refentrytitle>systemd-boot
</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
490 <member><citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
491 <member><citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
492 <member><citerefentry><refentrytitle>systemd-sysext
</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
493 <member><ulink url=
"https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification
</ulink></member>
494 <member><ulink url=
"https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface
</ulink></member>
495 <member><citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
496 <member><citerefentry><refentrytitle>systemd-measure
</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
497 <member><ulink url=
"https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd
</ulink></member>