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>
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/*.addon.efi
</filename></para>
32 <para><filename><replaceable>ESP
</replaceable>/.../
<replaceable>foo
</replaceable>.efi.extra.d/*.cred
</filename></para>
33 <para><filename><replaceable>ESP
</replaceable>/.../
<replaceable>foo
</replaceable>.efi.extra.d/*.raw
</filename></para>
34 <para><filename><replaceable>ESP
</replaceable>/loader/addons/*.addon.efi
</filename></para>
35 <para><filename><replaceable>ESP
</replaceable>/loader/credentials/*.cred
</filename></para>
39 <title>Description
</title>
41 <para><command>systemd-stub
</command> (stored in per-architecture files
42 <filename>linuxx64.efi.stub
</filename>,
<filename>linuxia32.efi.stub
</filename>,
43 <filename>linuxaa64.efi.stub
</filename> on disk) is a simple UEFI boot stub. An UEFI boot stub is
44 attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware
45 environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux
46 kernel is executable as regular UEFI binary, and is able to do various preparations before switching the
47 system into the Linux world.
</para>
49 <para>The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary
50 itself. This allows combining various resources inside a single PE binary image (usually called
"Unified
51 Kernel Image", or
"UKI" for short), which may then be signed via UEFI SecureBoot as a whole, covering all
52 individual resources at once. Specifically it may include:
</para>
55 <listitem><para>The ELF Linux kernel images will be looked for in the
<literal>.linux
</literal> PE
56 section of the executed image.
</para></listitem>
58 <listitem><para>OS release information, i.e. the
59 <citerefentry><refentrytitle>os-release
</refentrytitle><manvolnum>5</manvolnum></citerefentry> file of
60 the OS the kernel belongs to, in the
<literal>.osrel
</literal> PE section.
</para></listitem>
62 <listitem><para>Kernel version information, i.e. the output of
<command>uname -r
</command> for the
63 kernel included in the UKI, in the
<literal>.uname
</literal> PE section.
</para></listitem>
65 <listitem><para>The initrd will be loaded from the
<literal>.initrd
</literal> PE section.
68 <listitem><para>A compiled binary DeviceTree will be looked for in the
<literal>.dtb
</literal> PE
69 section.
</para></listitem>
71 <listitem><para>Kernel version information, i.e. the output of
<command>uname -r
</command> for the
72 kernel included in the UKI, in the
<literal>.uname
</literal> PE section.
</para></listitem>
74 <listitem><para>The kernel command line to pass to the invoked kernel will be looked for in the
75 <literal>.cmdline
</literal> PE section.
</para></listitem>
77 <listitem><para>A boot splash (in Windows
<filename>.BMP
</filename> format) to show on screen before
78 invoking the kernel will be looked for in the
<literal>.splash
</literal> PE section.
</para></listitem>
80 <listitem><para>A set of cryptographic signatures for expected TPM2 PCR values when this kernel is
81 booted, in JSON format, in the
<literal>.pcrsig
</literal> section. This is useful for implementing TPM2
82 policies that bind disk encryption and similar to kernels that are signed by a specific
83 key.
</para></listitem>
85 <listitem><para>A public key in PEM format matching this TPM2 PCR signature data in the
86 <literal>.pcrpkey
</literal> section.
</para></listitem>
89 <para>If UEFI SecureBoot is enabled and the
<literal>.cmdline
</literal> section is present in the executed
90 image, any attempts to override the kernel command line by passing one as invocation parameters to the
91 EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
92 SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
93 is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR
12 (if a TPM is
96 <para>If a DeviceTree is embedded in the
<literal>.dtb
</literal> section, it replaces an existing
97 DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
98 <literal>EFI_DT_FIXUP_PROTOCOL
</literal> for hardware specific fixups to the DeviceTree.
</para>
100 <para>The contents of seven of these eight PE sections are measured into TPM PCR
11, that is otherwise
101 not used. Thus, it can be pre-calculated without too much effort. The
<literal>.pcrsig
</literal> section
102 is not included in this PCR measurement, since it's supposed to contain signatures for the expected
103 results for these measurements, i.e. of the outputs of the measurement operation, and thus cannot also be
106 <para>When
<literal>.pcrsig
</literal> and/or
<literal>.pcrpkey
</literal> are present in a unified kernel
107 image their contents are passed to the booted kernel in an synthetic initrd cpio archive that places them in the
108 <filename>/.extra/tpm2-pcr-signature.json
</filename> and
109 <filename>/.extra/tpm2-pcr-public-key.pem
</filename> files. Typically, a
110 <citerefentry><refentrytitle>tmpfiles.d
</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
111 ensures they are copied into
<filename>/run/systemd/tpm2-pcr-signature.json
</filename> and
112 <filename>/run/systemd/tpm2-pcr-public-key.pem
</filename> where they remain accessible even after the
113 system transitions out of the initrd environment into the host file system. Tools such
114 <citerefentry><refentrytitle>systemd-cryptsetup@.service
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
115 <citerefentry><refentrytitle>systemd-cryptenroll
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
116 and
<citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
117 will automatically use files present under these paths to unlock protected resources (encrypted storage
118 or credentials) or bind encryption to booted kernels.
</para>
122 <title>Companion Files
</title>
124 <para>The
<command>systemd-stub
</command> UEFI boot stub automatically collects two types of auxiliary
125 companion files optionally placed in drop-in directories on the same partition as the EFI binary,
126 dynamically generates
<command>cpio
</command> initrd archives from them, and passes them to the kernel.
130 <listitem><para>For a kernel binary called
<filename><replaceable>foo
</replaceable>.efi
</filename>, it
131 will look for files with the
<filename>.cred
</filename> suffix in a directory named
132 <filename><replaceable>foo
</replaceable>.efi.extra.d/
</filename> next to it. A
<command>cpio
</command>
133 archive is generated from all files found that way, placing them in the
134 <filename>/.extra/credentials/
</filename> directory of the initrd file hierarchy. The main initrd may
135 then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
136 authenticated credentials for use with
<varname>LoadCredentialEncrypted=
</varname> in the UEFI System
138 <citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
140 <citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
142 details on encrypted credentials. The generated
<command>cpio
</command> archive is measured into TPM
143 PCR
12 (if a TPM is present).
</para></listitem>
145 <listitem><para>Similarly, files
<filename><replaceable>foo
</replaceable>.efi.extra.d/*.raw
</filename>
146 are packed up in a
<command>cpio
</command> archive and placed in the
<filename>/.extra/sysext/
</filename>
147 directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension
148 images to the initrd. See
149 <citerefentry><refentrytitle>systemd-sysext
</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
150 details on system extension images. The generated
<command>cpio
</command> archive containing these
151 system extension images is measured into TPM PCR
13 (if a TPM is present).
</para></listitem>
153 <listitem><para>Similarly, files
154 <filename><replaceable>foo
</replaceable>.efi.extra.d/*.addon.efi
</filename>
155 are loaded and verified as PE binaries, and a
<literal>.cmdline
</literal> section is parsed from them.
156 In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
157 Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a
158 a
<literal>.uname
</literal> section, the addon will be rejected if they do not exactly match. It is
159 recommended to always add a
<literal>.sbat
</literal> section to all signed addons, so that they may be
160 revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
161 <citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will
162 add a SBAT policy by default if none is passed when building addons. For more information on SBAT see
163 <ulink url=
"https://github.com/rhboot/shim/blob/main/SBAT.md">Shim's documentation
</ulink>.
164 Addons are supposed to be used to pass additional kernel command line parameters, regardless of the
165 kernel image being booted, for example to allow platform vendors to ship platform-specific
166 configuration. The loaded command line addon files are sorted, loaded, measured into TPM PCR
12 (if a
167 TPM is present) and appended to the kernel command line. UKI command line options are listed first,
168 then options from addons in
<filename>/loader/addons/*.addon.efi
</filename> are appended next, and
169 finally UKI-specific addons are appended last. Addons are always loaded in the same order based on the
170 filename, so that, given the same set of addons, the same set of measurements can be expected in
171 PCR12, however note that the filename is not protected by the PE signature, and as such an attacker
172 with write access to the ESP could potentially rename these files to change the order in which they
173 are loaded, in a way that could alter the functionality of the kernel, as some options might be order
174 dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of an
175 attestation service so that improper use of your signed addons can be detected and dealt with using
176 one of the aforementioned revocation mechanisms.
</para></listitem>
178 <listitem><para>Files
<filename>/loader/credentials/*.cred
</filename> are packed up in a
179 <command>cpio
</command> archive and placed in the
<filename>/.extra/global_credentials/
</filename>
180 directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
181 the initrd, regardless of the kernel being booted. The generated
<command>cpio
</command> archive is
182 measured into TPM PCR
12 (if a TPM is present).
</para></listitem>
184 <listitem><para>Additionally, files
<filename>/loader/addons/*.addon.efi
</filename> are loaded and
185 verified as PE binaries, and a
<literal>.cmdline
</literal> section is parsed from them. This is
186 supposed to be used to pass additional command line parameters to the kernel, regardless of the kernel
187 being booted.
</para></listitem>
190 <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
191 images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
192 further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
193 by
<command>systemd-creds encrypt -T
</command> (see
194 <citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
195 details); in case of the system extension images by using signed Verity images.
</para>
199 <title>TPM PCR Notes
</title>
201 <para>Note that when a unified kernel using
<command>systemd-stub
</command> is invoked the firmware will
202 measure it as a whole to TPM PCR
4, covering all embedded resources, such as the stub code itself, the
203 core kernel, the embedded initrd and kernel command line (see above for a full list).
</para>
205 <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR
9. This means
206 every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
207 measured to PCR
4, PCR
9 and PCR
11; the initrd synthesized from credentials will be measured to both PCR
208 9 and PCR
12; the initrd synthesized from system extensions will be measured to both PCR
4 and PCR
209 9. Let's summarize the OS resources and the PCRs they are measured to:
</para>
212 <title>OS Resource PCR Summary
</title>
214 <tgroup cols='
2' align='left' colsep='
1' rowsep='
1'
>
215 <colspec colname=
"pcr" />
216 <colspec colname=
"definition" />
220 <entry>OS Resource
</entry>
221 <entry>Measurement PCR
</entry>
227 <entry><command>systemd-stub
</command> code (the entry point of the unified PE binary)
</entry>
232 <entry>Core kernel code (embedded in unified PE binary)
</entry>
233 <entry>4 +
11</entry>
237 <entry>OS release information (embedded in the unified PE binary)
</entry>
238 <entry>4 +
11</entry>
242 <entry>Main initrd (embedded in unified PE binary)
</entry>
243 <entry>4 +
9 +
11</entry>
247 <entry>Default kernel command line (embedded in unified PE binary)
</entry>
248 <entry>4 +
11</entry>
252 <entry>Overridden kernel command line
</entry>
257 <entry>Boot splash (embedded in the unified PE binary)
</entry>
258 <entry>4 +
11</entry>
262 <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)
</entry>
267 <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)
</entry>
268 <entry>4 +
9 +
11</entry>
272 <entry>Credentials (synthesized initrd from companion files)
</entry>
273 <entry>9 +
12</entry>
277 <entry>System Extensions (synthesized initrd from companion files)
</entry>
278 <entry>9 +
13</entry>
286 <title>EFI Variables
</title>
288 <para>The following EFI variables are defined, set and read by
<command>systemd-stub
</command>, under the
289 vendor UUID
<literal>4a67b082-
0a4c-
41cf-b6c7-
440b29bb8c4f
</literal>, for communication between the boot
290 stub and the OS:
</para>
292 <variablelist class='efi-variables'
>
294 <term><varname>LoaderDevicePartUUID
</varname></term>
296 <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
297 from.
<citerefentry><refentrytitle>systemd-gpt-auto-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
298 uses this information to automatically find the disk booted from, in order to discover various other
299 partitions on the same disk automatically.
</para>
301 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
305 <term><varname>LoaderFirmwareInfo
</varname></term>
306 <term><varname>LoaderFirmwareType
</varname></term>
308 <listitem><para>Brief firmware information. Use
309 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
312 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
316 <term><varname>LoaderImageIdentifier
</varname></term>
318 <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
320 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
323 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
327 <term><varname>StubInfo
</varname></term>
329 <listitem><para>Brief stub information. Use
330 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
333 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
337 <term><varname>StubPcrKernelImage
</varname></term>
339 <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
340 database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
341 <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
342 unset otherwise.
</para>
344 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
348 <term><varname>StubPcrKernelParameters
</varname></term>
350 <listitem><para>The PCR register index the kernel command line and credentials are measured into,
351 formatted as decimal ASCII string (e.g.
<literal>12</literal>). This variable is set if a measurement
352 was successfully completed, and remains unset otherwise.
</para>
354 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
358 <term><varname>StubPcrInitRDSysExts
</varname></term>
360 <listitem><para>The PCR register index the systemd extensions for the initrd, which are picked up
361 from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
362 <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
363 unset otherwise.
</para>
365 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
369 <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
370 them if they aren't set already. Some of these variables are defined by the
<ulink
371 url=
"https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface
</ulink>.
</para>
375 <title>initrd Resources
</title>
377 <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
378 the initial file system hierarchy in the initrd execution environment:
</para>
382 <term><filename>/
</filename></term>
384 <listitem><para>The main initrd from the
<literal>.initrd
</literal> PE section of the unified kernel image.
</para>
386 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
390 <term><filename>/.extra/credentials/*.cred
</filename></term>
391 <listitem><para>Credential files (suffix
<literal>.cred
</literal>) that are placed next to the
392 unified kernel image (as described above) are copied into the
393 <filename>/.extra/credentials/
</filename> directory in the initrd execution
396 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
400 <term><filename>/.extra/global_credentials/*.cred
</filename></term>
401 <listitem><para>Similar, credential files in the
<filename>/loader/credentials/
</filename> directory
402 in the file system the unified kernel image is placed in are copied into the
403 <filename>/.extra/global_credentials/
</filename> directory in the initrd execution
406 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
410 <term><filename>/.extra/sysext/*.raw
</filename></term>
411 <listitem><para>System extension image files (suffix
<literal>.raw
</literal>) that are placed next to
412 the unified kernel image (as described above) are copied into the
413 <filename>/.extra/sysext/
</filename> directory in the initrd execution environment.
</para>
415 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
419 <term><filename>/.extra/tpm2-pcr-signature.json
</filename></term>
420 <listitem><para>The TPM2 PCR signature JSON object included in the
<literal>.pcrsig
</literal> PE
421 section of the unified kernel image is copied into the
422 <filename>/.extra/tpm2-pcr-signature.json
</filename> file in the initrd execution
425 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
429 <term><filename>/.extra/tpm2-pcr-pkey.pem
</filename></term>
430 <listitem><para>The PEM public key included in the
<literal>.pcrpkey
</literal> PE section of the
431 unified kernel image is copied into the
<filename>/.extra/tpm2-pcr-public-key.pem
</filename> file in
432 the initrd execution environment.
</para>
434 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
438 <para>Note that all these files are located in the
<literal>tmpfs
</literal> file system the kernel sets
439 up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
440 environment into the host file system. If these resources shall be kept around over this transition they
441 need to be copied to a place that survives the transition first, for example via a suitable
442 <citerefentry><refentrytitle>tmpfiles.d
</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
443 default, this is done for the TPM2 PCR signature and public key files.
</para>
447 <title>SMBIOS Type
11 Strings
</title>
449 <para><command>systemd-stub
</command> can be configured using SMBIOS Type
11 strings. Applicable strings
450 consist of a name, followed by
<literal>=
</literal>, followed by the value.
451 <command>systemd-stub
</command> will search the table for a string with a specific name, and if found,
452 use its value. The following strings are read:
</para>
456 <term><varname>io.systemd.stub.kernel-cmdline-extra
</varname></term>
457 <listitem><para>If set, the value of this string is added to the list of kernel command line
458 arguments that are measured in PCR12 and passed to the kernel.
</para>
460 <xi:include href=
"version-info.xml" xpointer=
"v254"/></listitem>
466 <title>Assembling Kernel Images
</title>
468 <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
469 <citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
</para>
473 <title>See Also
</title>
475 <citerefentry><refentrytitle>systemd-boot
</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
476 <citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
477 <citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
478 <citerefentry><refentrytitle>systemd-sysext
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
479 <ulink url=
"https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification
</ulink>,
480 <ulink url=
"https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface
</ulink>,
481 <citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
482 <citerefentry><refentrytitle>systemd-measure
</refentrytitle><manvolnum>1</manvolnum></citerefentry>