man: document the new .pcrsig/.pcrpkey sections for unified kernel images

1 files changed, 110 insertions, 5 deletions

diff --git a/man/systemd-stub.xml b/man/systemd-stub.xml
index 1e9bb5d631..2479d9f5fa 100644
--- a/man/systemd-stub.xml
+++ b/man/systemd-stub.xml
@@ -68,6 +68,14 @@
 
       <listitem><para>A boot splash (in Windows <filename>.BMP</filename> format) to show on screen before
       invoking the kernel will be looked for in the <literal>.splash</literal> PE section.</para></listitem>
+
+      <listitem><para>A set of cryptographic signatures for expected TPM2 PCR values when this kernel is
+      booted, in JSON format, in the <literal>.pcrsig</literal> section. This is useful for implementing TPM2
+      policies that bind disk encryption and similar to kernels that are signed by a specific
+      key.</para></listitem>
+
+      <listitem><para>A public key in PEM format matching this TPM2 PCR signature data in the
+      <literal>.pcrpkey</literal> section.</para></listitem>
     </itemizedlist>
 
     <para>If UEFI SecureBoot is enabled and the <literal>.cmdline</literal> section is present in the executed
@@ -81,8 +89,25 @@
     DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
     <literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para>
 
-    <para>The contents of these six PE sections are measured into TPM PCR 11, that is otherwise not
-    used. Thus, it can be pre-calculated without too much effort.</para>
+    <para>The contents of seven of these eight PE sections are measured into TPM PCR 11, that is otherwise
+    not used. Thus, it can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section
+    is not included in this PCR measurement, since it's supposed to contain signatures for the expected
+    results for these measurements, i.e. of the outputs of the measurement operation, and thus cannot also be
+    input to it.</para>
+
+    <para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> are present in a unified kernel
+    image their contents are passed to the booted kernel in an synthetic initrd cpio archive that places them in the
+    <filename>/.extra/tpm2-pcr-signature.json</filename> and
+    <filename>/.extra/tpm2-pcr-public-key.pem</filename> files. Typically, a
+    <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
+    ensures they are copied into <filename>/run/systemd/tpm2-pcr-signature.json</filename> and
+    <filename>/run/systemd/tpm2-pcr-public-key.pem</filename> where they remain accessible even after the
+    system transitions out of the initrd environment into the host file system. Tools such
+    <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+    <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+    and <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+    will automatically use files present under these paths to unlock protected resources (encrypted storage
+    or credentials) or bind encryption to booted kernels.</para>
   </refsect1>
 
   <refsect1>
@@ -166,12 +191,12 @@
           </row>
 
           <row>
-            <entry>Boot splash (embedded in the unified PE binary)</entry>
+            <entry>Core kernel code (embedded in unified PE binary)</entry>
             <entry>4 + 11</entry>
           </row>
 
           <row>
-            <entry>Core kernel code (embedded in unified PE binary)</entry>
+            <entry>OS release information (embedded in the unified PE binary)</entry>
             <entry>4 + 11</entry>
           </row>
 
@@ -191,6 +216,21 @@
           </row>
 
           <row>
+            <entry>Boot splash (embedded in the unified PE binary)</entry>
+            <entry>4 + 11</entry>
+          </row>
+
+          <row>
+            <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry>
+            <entry>4 + 9</entry>
+          </row>
+
+          <row>
+            <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry>
+            <entry>4 + 9 + 11</entry>
+          </row>
+
+          <row>
             <entry>Credentials (synthesized initrd from companion files)</entry>
             <entry>9 + 12</entry>
           </row>
@@ -280,6 +320,66 @@
   </refsect1>
 
   <refsect1>
+    <title>initrd Resources</title>
+
+    <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
+    the initial file system hierarchy in the initrd execution environment:</para>
+
+    <variablelist>
+      <varlistentry>
+        <term><filename>/</filename></term>
+
+        <listitem><para>The main initrd from the <literal>.initrd</literal> PE section of the unified kernel image.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><filename>/.extra/credentials/*.cred</filename></term>
+        <listitem><para>Credential files (suffix <literal>.cred</literal>) that are placed next to the
+        unified kernel image (as described above) are copied into the
+        <filename>/.extra/credentials/</filename> directory in the initrd execution
+        environment.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><filename>/.extra/global_credentials/*.cred</filename></term>
+        <listitem><para>Similar, credential files in the <filename>/loader/credentials/</filename> directory
+        in the file system the unified kernel image is placed in are copied into the
+        <filename>/.extra/global_credentials/</filename> directory in the initrd execution
+        environment.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><filename>/.extra/sysext/*.raw</filename></term>
+        <listitem><para>System extension image files (suffix <literal>.raw</literal>) that are placed next to
+        the unified kernel image (as described above) are copied into the
+        <filename>/.extra/sysext/</filename> directory in the initrd execution environment.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><filename>/.extra/tpm2-pcr-signature.json</filename></term>
+        <listitem><para>The TPM2 PCR signature JSON object included in the <literal>.pcrsig</literal> PE
+        section of the unified kernel image is copied into the
+        <filename>/.extra/tpm2-pcr-signature.json</filename> file in the initrd execution
+        environment.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><filename>/.extra/tpm2-pcr-pkey.pem</filename></term>
+        <listitem><para>The PEM public key included in the <literal>.pcrpkey</literal> PE section of the
+        unified kernel image is copied into the <filename>/.extra/tpm2-pcr-public-key.pem</filename> file in
+        the initrd execution environment.</para></listitem>
+      </varlistentry>
+    </variablelist>
+
+    <para>Note that all these files are located in the <literal>tmpfs</literal> file system the kernel sets
+    up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
+    environment into the host file system. If these resources shall be kept around over this transition they
+    need to be copied to a place that survives the transition first, for example via a suitable
+    <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
+    default, this is done for the TPM2 PCR signature and public key files.</para>
+  </refsect1>
+
+  <refsect1>
     <title>Assembling Kernel Images</title>
 
     <para>In order to assemble an UEFI PE kernel image from various components as described above, use an
@@ -313,6 +413,10 @@
     <para>This expects a pair of X.509 private key and certificate as parameters and then signs the UEFI PE
     executable we generated above for UEFI SecureBoot and generates a signed UEFI PE executable as
     result.</para>
+
+    <para>See
+    <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
+    an example involving the <literal>.pcrsig</literal> and <literal>.pcrpkey</literal> sections.</para>
   </refsect1>
 
   <refsect1>
@@ -325,7 +429,8 @@
       <ulink url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink>,
       <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
       <citerefentry project='man-pages'><refentrytitle>objcopy</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
-      <citerefentry project='archlinux'><refentrytitle>sbsign</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+      <citerefentry project='archlinux'><refentrytitle>sbsign</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry>
     </para>
   </refsect1>
 </refentry>