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

2 files changed, 148 insertions, 26 deletions

diff --git a/man/systemd-measure.xml b/man/systemd-measure.xml
index 0fc0d0e87d..69ac348184 100644
--- a/man/systemd-measure.xml
+++ b/man/systemd-measure.xml
@@ -37,12 +37,12 @@
     <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry> is
     booted up. It accepts paths to the ELF kernel image file, initial ram disk image file, devicetree file,
     kernel command line file,
-    <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file, and
-    boot splash file that make up the unified kernel image, and determines the PCR values expected to be in
-    place after booting the image. Calculation starts with a zero-initialized PCR 11, and is executed in a
-    fashion compatible with what <filename>systemd-stub</filename> does at boot. The result may optionally be
-    signed cryptographically, to allow TPM2 policies that can only be unlocked if a certain set of kernels is
-    booted, for which such a PCR signature can be provided.</para>
+    <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file, boot
+    splash file, and TPM2 PCR PEM public key file that make up the unified kernel image, and determines the
+    PCR values expected to be in place after booting the image. Calculation starts with a zero-initialized
+    PCR 11, and is executed in a fashion compatible with what <filename>systemd-stub</filename> does at
+    boot. The result may optionally be signed cryptographically, to allow TPM2 policies that can only be
+    unlocked if a certain set of kernels is booted, for which such a PCR signature can be provided.</para>
   </refsect1>
 
   <refsect1>
@@ -66,9 +66,9 @@
         <listitem><para>Pre-calculate the expected values seen in PCR register 11 after boot-up of a unified
         kernel image consisting of the components specified with <option>--linux=</option>,
         <option>--osrel=</option>, <option>--cmdline=</option>, <option>--initrd=</option>,
-        <option>--splash=</option>, <option>--dtb=</option>, see below. Only <option>--linux=</option> is
-        mandatory. (Alternatively, specify <option>--current</option> to use the current values of PCR
-        register 11 instead.)</para></listitem>
+        <option>--splash=</option>, <option>--dtb=</option>, <option>--pcrpkey=</option> see below. Only
+        <option>--linux=</option> is mandatory. (Alternatively, specify <option>--current</option> to use the
+        current values of PCR register 11 instead.)</para></listitem>
       </varlistentry>
 
       <varlistentry>
@@ -104,6 +104,7 @@
         <term><option>--initrd=PATH</option></term>
         <term><option>--splash=PATH</option></term>
         <term><option>--dtb=PATH</option></term>
+        <term><option>--pcrpkey=PATH</option></term>
 
         <listitem><para>When used with the <command>calculate</command> or <command>sign</command> verb,
         configures the files to read the unified kernel image components from. Each option corresponds with
@@ -135,7 +136,14 @@
         <term><option>--public-key=PATH</option></term>
 
         <listitem><para>These switches take paths to a pair of PEM encoded RSA key files, for use with
-        the <command>sign</command> command.</para></listitem>
+        the <command>sign</command> command.</para>
+
+        <para>Note the difference between the <option>--pcrpkey=</option> and <option>--public-key=</option>
+        switches. The former selects the data to include in the <literal>.pcrpkey</literal> PE section of the
+        unified kernel image, the latter picks the public key of the key pair used to sign the resulting PCR
+        11 values. The former is the key that the booted system will likely use to lock disk and credential
+        encryption to, the latter is the key used for unlocking such resources again. Hence, typically the
+        same PEM key should be supplied in both cases.</para></listitem>
       </varlistentry>
 
       <varlistentry>
@@ -185,19 +193,11 @@
     </example>
 
     <example>
-      <title>Generate a private/public key pair, and a unified kernel image, and a TPM PCR 11 signature for it</title>
+      <title>Generate a private/public key pair, and a unified kernel image, and a TPM PCR 11 signature for
+      it, and embed the signature and the public key in the image</title>
 
       <programlisting># openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out tpm2-pcr-private.pem
 # openssl rsa -pubout -in tpm2-pcr-private.pem -out tpm2-pcr-public.pem
-# objcopy \
-    --add-section .linux=vmlinux --change-section-vma .linux=0x2000000 \
-    --add-section .osrel=os-release.txt --change-section-vma .osrel=0x20000 \
-    --add-section .cmdline=cmdline.txt --change-section-vma .cmdline=0x30000 \
-    --add-section .initrd=initrd.cpio --change-section-vma .initrd=0x3000000 \
-    --add-section .splash=splash.bmp --change-section-vma .splash=0x100000 \
-    --add-section .dtb=devicetree.dtb --change-section-vma .dtb=0x40000 \
-    /usr/lib/systemd/boot/efi/linuxx64.efi.stub \
-    foo.efi
 # systemd-measure sign \
      --linux=vmlinux \
      --osrel=os-release.txt \
@@ -205,10 +205,22 @@
      --initrd=initrd.cpio \
      --splash=splash.bmp \
      --dtb=devicetree.dtb \
+     --pcrpkey=tpm2-pcr-public.pem \
      --bank=sha1 \
      --bank=sha256 \
      --private-key=tpm2-pcr-private.pem \
-     --public-key=tpm2-pcr-public.pem > tpm2-pcr-signature.json</programlisting>
+     --public-key=tpm2-pcr-public.pem > tpm2-pcr-signature.json
+# objcopy \
+    --add-section .linux=vmlinux --change-section-vma .linux=0x2000000 \
+    --add-section .osrel=os-release.txt --change-section-vma .osrel=0x20000 \
+    --add-section .cmdline=cmdline.txt --change-section-vma .cmdline=0x30000 \
+    --add-section .initrd=initrd.cpio --change-section-vma .initrd=0x3000000 \
+    --add-section .splash=splash.bmp --change-section-vma .splash=0x100000 \
+    --add-section .dtb=devicetree.dtb --change-section-vma .dtb=0x40000 \
+    --add-section .pcrsig=tpm2-pcr-signature.json --change-section-vma .splash=0x80000 \
+    --add-section .pcrpkey=tpm2-pcr-public.pem --change-section-vma .splash=0x90000 \
+    /usr/lib/systemd/boot/efi/linuxx64.efi.stub \
+    foo.efi</programlisting>
 
      <para>Later on, enroll the signed PCR policy on a LUKS volume:</para>
 
@@ -217,6 +229,11 @@
      <para>And then unlock the device with the signature:</para>
 
      <programlisting># /usr/lib/systemd/systemd-cryptsetup attach myvolume /dev/sda5 - tpm2-device=auto,tpm2-signature=/path/to/tpm2-pcr-signature.json</programlisting>
+
+     <para>Note that when the generated unified kernel image <filename>foo.efi</filename> is booted the
+     signature and public key files will be placed at locations <command>systemd-cryptenroll</command> and
+     <command>systemd-cryptsetup</command> will look for anyway, and thus these paths do not actually need to
+     be specified.</para>
     </example>
   </refsect1>
 
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>