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 <!-- Let's keep this in the canonical order we also measure the sections by, i.e. as in
56 src/fundamental/uki.h's UnifiedSection enum -->
58 <listitem><para>The ELF Linux kernel images will be looked for in the
<literal>.linux
</literal> PE
59 section of the executed image.
</para></listitem>
61 <listitem><para>OS release information, i.e. the
62 <citerefentry><refentrytitle>os-release
</refentrytitle><manvolnum>5</manvolnum></citerefentry> file of
63 the OS the kernel belongs to, in the
<literal>.osrel
</literal> PE section.
</para></listitem>
65 <listitem><para>The kernel command line to pass to the invoked kernel will be looked for in the
66 <literal>.cmdline
</literal> PE section.
</para></listitem>
68 <listitem><para>The initrd will be loaded from the
<literal>.initrd
</literal> PE
69 section.
</para></listitem>
71 <listitem><para>A boot splash (in Windows
<filename>.BMP
</filename> format) to show on screen before
72 invoking the kernel will be looked for in the
<literal>.splash
</literal> PE section.
</para></listitem>
74 <listitem><para>A compiled binary DeviceTree will be looked for in the
<literal>.dtb
</literal> PE
75 section.
</para></listitem>
77 <listitem><para>Kernel version information, i.e. the output of
<command>uname -r
</command> for the
78 kernel included in the UKI, in the
<literal>.uname
</literal> PE section.
</para></listitem>
80 <listitem><para><ulink url=
"https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT
</ulink> revocation
81 metadata, in the
<literal>.sbat
</literal> PE section.
</para></listitem>
83 <listitem><para>A set of cryptographic signatures for expected TPM2 PCR values when this kernel is
84 booted, in JSON format, in the
<literal>.pcrsig
</literal> section. This is useful for implementing TPM2
85 policies that bind disk encryption and similar to kernels that are signed by a specific
86 key.
</para></listitem>
88 <listitem><para>A public key in PEM format matching this TPM2 PCR signature data in the
89 <literal>.pcrpkey
</literal> section.
</para></listitem>
92 <para>If UEFI SecureBoot is enabled and the
<literal>.cmdline
</literal> section is present in the executed
93 image, any attempts to override the kernel command line by passing one as invocation parameters to the
94 EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
95 SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
96 is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR
12 (if a TPM is
99 <para>If a DeviceTree is embedded in the
<literal>.dtb
</literal> section, it replaces an existing
100 DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
101 <literal>EFI_DT_FIXUP_PROTOCOL
</literal> for hardware specific fixups to the DeviceTree.
</para>
103 <para>The contents of seven of these eight PE sections are measured into TPM PCR
11, that is otherwise
104 not used. Thus, it can be pre-calculated without too much effort. The
<literal>.pcrsig
</literal> section
105 is not included in this PCR measurement, since it's supposed to contain signatures for the expected
106 results for these measurements, i.e. of the outputs of the measurement operation, and thus cannot also be
109 <para>When
<literal>.pcrsig
</literal> and/or
<literal>.pcrpkey
</literal> are present in a unified kernel
110 image their contents are passed to the booted kernel in an synthetic initrd cpio archive that places them in the
111 <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. A
<command>cpio
</command>
139 archive is generated from all files found that way, placing them in the
140 <filename>/.extra/credentials/
</filename> directory of the initrd file hierarchy. The main initrd may
141 then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
142 authenticated credentials for use with
<varname>LoadCredentialEncrypted=
</varname> in the UEFI System
144 <citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
146 <citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
148 details on encrypted credentials. The generated
<command>cpio
</command> archive is measured into TPM
149 PCR
12 (if a TPM is present).
</para></listitem>
151 <listitem><para>Similarly, files
<filename><replaceable>foo
</replaceable>.efi.extra.d/*.raw
</filename>
152 are packed up in a
<command>cpio
</command> archive and placed in the
<filename>/.extra/sysext/
</filename>
153 directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension
154 images to the initrd. See
155 <citerefentry><refentrytitle>systemd-sysext
</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
156 details on system extension images. The generated
<command>cpio
</command> archive containing these
157 system extension images is measured into TPM PCR
13 (if a TPM is present).
</para></listitem>
159 <listitem><para>Similarly, files
160 <filename><replaceable>foo
</replaceable>.efi.extra.d/*.addon.efi
</filename>
161 are loaded and verified as PE binaries, and a
<literal>.cmdline
</literal> section is parsed from them.
162 In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
163 Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a
164 a
<literal>.uname
</literal> section, the addon will be rejected if they do not exactly match. It is
165 recommended to always add a
<literal>.sbat
</literal> section to all signed addons, so that they may be
166 revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
167 <citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will
168 add a SBAT policy by default if none is passed when building addons. For more information on SBAT see
169 <ulink url=
"https://github.com/rhboot/shim/blob/main/SBAT.md">Shim's documentation
</ulink>.
170 Addons are supposed to be used to pass additional kernel command line parameters or Devicetree blobs,
171 regardless of the kernel image being booted, for example to allow platform vendors to ship
172 platform-specific configuration. The loaded command line addon files are sorted, loaded, and measured
173 into TPM PCR
12 (if a TPM is present) and appended to the kernel command line. UKI command line options
174 are listed first, then options from addons in
<filename>/loader/addons/*.addon.efi
</filename>, and
175 finally UKI-specific addons. Device tree blobs are loaded and measured following the same algorithm.
176 Addons are always loaded in the same order based on the filename, so that, given the same set of
177 addons, the same set of measurements can be expected in PCR12. However, note that the filename is not
178 protected by the PE signature, and as such an attacker with write access to the ESP could potentially
179 rename these files to change the order in which they are loaded, in a way that could alter the
180 functionality of the kernel, as some options might be order dependent. If you sign such addons, you
181 should pay attention to the PCR12 values and make use of an attestation service so that improper use
182 of your signed addons can be detected and dealt with using one of the aforementioned revocation
183 mechanisms.
</para></listitem>
185 <listitem><para>Files
<filename>/loader/credentials/*.cred
</filename> are packed up in a
186 <command>cpio
</command> archive and placed in the
<filename>/.extra/global_credentials/
</filename>
187 directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
188 the initrd, regardless of the kernel being booted. The generated
<command>cpio
</command> archive is
189 measured into TPM PCR
12 (if a TPM is present).
</para></listitem>
191 <listitem><para>Additionally, files
<filename>/loader/addons/*.addon.efi
</filename> are loaded and
192 verified as PE binaries, and
<literal>.cmdline
</literal> and/or
<literal>.dtb
</literal> sections are
193 parsed from them. This is supposed to be used to pass additional command line parameters or Devicetree
194 blobs to the kernel, regardless of the kernel being booted.
</para></listitem>
197 <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
198 images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
199 further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
200 by
<command>systemd-creds encrypt -T
</command> (see
201 <citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
202 details); in case of the system extension images by using signed Verity images.
</para>
206 <title>TPM PCR Notes
</title>
208 <para>Note that when a unified kernel using
<command>systemd-stub
</command> is invoked the firmware will
209 measure it as a whole to TPM PCR
4, covering all embedded resources, such as the stub code itself, the
210 core kernel, the embedded initrd and kernel command line (see above for a full list).
</para>
212 <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR
9. This means
213 every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
214 measured to PCR
4, PCR
9 and PCR
11; the initrd synthesized from credentials will be measured to both PCR
215 9 and PCR
12; the initrd synthesized from system extensions will be measured to both PCR
4 and PCR
216 9. Let's summarize the OS resources and the PCRs they are measured to:
</para>
219 <title>OS Resource PCR Summary
</title>
221 <tgroup cols='
2' align='left' colsep='
1' rowsep='
1'
>
222 <colspec colname=
"pcr" />
223 <colspec colname=
"definition" />
227 <entry>OS Resource
</entry>
228 <entry>Measurement PCR
</entry>
234 <entry><command>systemd-stub
</command> code (the entry point of the unified PE binary)
</entry>
239 <entry>Core kernel code (embedded in unified PE binary)
</entry>
240 <entry>4 +
11</entry>
244 <entry>OS release information (embedded in the unified PE binary)
</entry>
245 <entry>4 +
11</entry>
249 <entry>Main initrd (embedded in unified PE binary)
</entry>
250 <entry>4 +
9 +
11</entry>
254 <entry>Default kernel command line (embedded in unified PE binary)
</entry>
255 <entry>4 +
11</entry>
259 <entry>Overridden kernel command line
</entry>
264 <entry>Boot splash (embedded in the unified PE binary)
</entry>
265 <entry>4 +
11</entry>
269 <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)
</entry>
274 <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)
</entry>
275 <entry>4 +
9 +
11</entry>
279 <entry>Credentials (synthesized initrd from companion files)
</entry>
280 <entry>9 +
12</entry>
284 <entry>System Extensions (synthesized initrd from companion files)
</entry>
285 <entry>9 +
13</entry>
293 <title>EFI Variables
</title>
295 <para>The following EFI variables are defined, set and read by
<command>systemd-stub
</command>, under the
296 vendor UUID
<literal>4a67b082-
0a4c-
41cf-b6c7-
440b29bb8c4f
</literal>, for communication between the boot
297 stub and the OS:
</para>
299 <variablelist class='efi-variables'
>
301 <term><varname>LoaderDevicePartUUID
</varname></term>
303 <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
304 from.
<citerefentry><refentrytitle>systemd-gpt-auto-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
305 uses this information to automatically find the disk booted from, in order to discover various other
306 partitions on the same disk automatically.
</para>
308 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
312 <term><varname>LoaderFirmwareInfo
</varname></term>
313 <term><varname>LoaderFirmwareType
</varname></term>
315 <listitem><para>Brief firmware information. Use
316 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
319 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
323 <term><varname>LoaderImageIdentifier
</varname></term>
325 <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
327 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
330 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
334 <term><varname>StubInfo
</varname></term>
336 <listitem><para>Brief stub information. Use
337 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
340 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
344 <term><varname>StubPcrKernelImage
</varname></term>
346 <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
347 database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
348 <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
349 unset otherwise.
</para>
351 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
355 <term><varname>StubPcrKernelParameters
</varname></term>
357 <listitem><para>The PCR register index the kernel command line and credentials are measured into,
358 formatted as decimal ASCII string (e.g.
<literal>12</literal>). This variable is set if a measurement
359 was successfully completed, and remains unset otherwise.
</para>
361 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
365 <term><varname>StubPcrInitRDSysExts
</varname></term>
367 <listitem><para>The PCR register index the systemd extensions for the initrd, which are picked up
368 from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
369 <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
370 unset otherwise.
</para>
372 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
376 <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
377 them if they aren't set already. Some of these variables are defined by the
<ulink
378 url=
"https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface
</ulink>.
</para>
382 <title>initrd Resources
</title>
384 <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
385 the initial file system hierarchy in the initrd execution environment:
</para>
389 <term><filename>/
</filename></term>
391 <listitem><para>The main initrd from the
<literal>.initrd
</literal> PE section of the unified kernel image.
</para>
393 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
397 <term><filename>/.extra/credentials/*.cred
</filename></term>
398 <listitem><para>Credential files (suffix
<literal>.cred
</literal>) that are placed next to the
399 unified kernel image (as described above) are copied into the
400 <filename>/.extra/credentials/
</filename> directory in the initrd execution
403 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
407 <term><filename>/.extra/global_credentials/*.cred
</filename></term>
408 <listitem><para>Similar, credential files in the
<filename>/loader/credentials/
</filename> directory
409 in the file system the unified kernel image is placed in are copied into the
410 <filename>/.extra/global_credentials/
</filename> directory in the initrd execution
413 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
417 <term><filename>/.extra/sysext/*.raw
</filename></term>
418 <listitem><para>System extension image files (suffix
<literal>.raw
</literal>) that are placed next to
419 the unified kernel image (as described above) are copied into the
420 <filename>/.extra/sysext/
</filename> directory in the initrd execution environment.
</para>
422 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
426 <term><filename>/.extra/tpm2-pcr-signature.json
</filename></term>
427 <listitem><para>The TPM2 PCR signature JSON object included in the
<literal>.pcrsig
</literal> PE
428 section of the unified kernel image is copied into the
429 <filename>/.extra/tpm2-pcr-signature.json
</filename> file in the initrd execution
432 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
436 <term><filename>/.extra/tpm2-pcr-pkey.pem
</filename></term>
437 <listitem><para>The PEM public key included in the
<literal>.pcrpkey
</literal> PE section of the
438 unified kernel image is copied into the
<filename>/.extra/tpm2-pcr-public-key.pem
</filename> file in
439 the initrd execution environment.
</para>
441 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
445 <para>Note that all these files are located in the
<literal>tmpfs
</literal> file system the kernel sets
446 up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
447 environment into the host file system. If these resources shall be kept around over this transition they
448 need to be copied to a place that survives the transition first, for example via a suitable
449 <citerefentry><refentrytitle>tmpfiles.d
</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
450 default, this is done for the TPM2 PCR signature and public key files.
</para>
454 <title>SMBIOS Type
11 Strings
</title>
456 <para><command>systemd-stub
</command> can be configured using SMBIOS Type
11 strings. Applicable strings
457 consist of a name, followed by
<literal>=
</literal>, followed by the value.
458 <command>systemd-stub
</command> will search the table for a string with a specific name, and if found,
459 use its value. The following strings are read:
</para>
463 <term><varname>io.systemd.stub.kernel-cmdline-extra
</varname></term>
464 <listitem><para>If set, the value of this string is added to the list of kernel command line
465 arguments that are measured in PCR12 and passed to the kernel.
</para>
467 <xi:include href=
"version-info.xml" xpointer=
"v254"/></listitem>
473 <title>Assembling Kernel Images
</title>
475 <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
476 <citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
</para>
480 <title>See Also
</title>
482 <citerefentry><refentrytitle>systemd-boot
</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
483 <citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
484 <citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
485 <citerefentry><refentrytitle>systemd-sysext
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
486 <ulink url=
"https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification
</ulink>,
487 <ulink url=
"https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface
</ulink>,
488 <citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
489 <citerefentry><refentrytitle>systemd-measure
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
490 <ulink url=
"https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd
</ulink>