systemd-pcrphase.service, systemd-pcrphase-sysinit.service, and systemd-pcrphase-initrd.service are
system services that measure specific strings into TPM2 PCR 11 during boot at various milestones of the
boot process.
systemd-pcrmachine.service is a system service that measures the machine ID (see machine-id(5)) into PCR
15.
systemd-pcrfs-root.service and systemd-pcrfs@.service are services that measure file system identity
information (i.e. mount point, file system type, label and UUID, partition label and UUID) into PCR 15.
systemd-pcrfs-root.service does so for the root file system, systemd-pcrfs@.service is a template unit
that measures the file system indicated by its instance identifier instead.
These services require systemd-stub(7) to be used in a unified kernel image (UKI). They execute no
operation when the stub has not been used to invoke the kernel. The stub will measure the invoked kernel
and associated vendor resources into PCR 11 before handing control to it; once userspace is invoked these
services then will extend TPM2 PCR 11 with certain literal strings indicating phases of the boot process.
During a regular boot process PCR 11 is extended with the following strings:
1. "enter-initrd" — early when the initrd initializes, before activating system extension images for the
initrd. It acts as a barrier between the time where the kernel initializes and where the initrd
starts operating and enables system extension images, i.e. code shipped outside of the UKI. (This
extension happens when the systemd-pcrphase-initrd.service(8) service is started.)
2. "leave-initrd" — when the initrd is about to transition into the host file system. It acts as barrier
between initrd code and host OS code. (This extension happens when the systemd-pcrphase-sysinit.service(8) service is stopped.)
3. "sysinit" — when basic system initialization is complete (which includes local file systems having
been mounted), and the system begins starting regular system services. (This extension happens when
the systemd-pcrphase-sysinit.service(8) service is started.)
4. "ready" — during later boot-up, after remote file systems have been activated (i.e. after
remote-fs.target), but before users are permitted to log in (i.e. before
systemd-user-sessions.service). It acts as barrier between the time where unprivileged regular users
are still prohibited to log in and where they are allowed to log in. (This extension happens when the
systemd-pcrphase-sysinit.service(8) service is started.)
5. "shutdown" — when the system shutdown begins. It acts as barrier between the time the system is fully
up and running and where it is about to shut down. (This extension happens when the
systemd-pcrphase.service service is stopped.)
6. "final" — at the end of system shutdown. It acts as barrier between the time the service manager
still runs and when it transitions into the final shutdown phase where service management is not
available anymore. (This extension happens when the systemd-pcrphase-sysinit.service(8) service is
stopped.)
During a regular system lifecycle, PCR 11 is extended with the strings "enter-initrd", "leave-initrd",
"sysinit", "ready", "shutdown", and "final".
Specific phases of the boot process may be referenced via the series of strings measured, separated by
colons (the "phase path"). For example, the phase path for the regular system runtime is
"enter-initrd:leave-initrd:sysinit:ready", while the one for the initrd is just "enter-initrd". The phase
path for the boot phase before the initrd is an empty string; because that's hard to pass around a single
colon (":") may be used instead. Note that the aforementioned six strings are just the default strings
and individual systems might measure other strings at other times, and thus implement different and more
fine-grained boot phases to bind policy to.
By binding policy of TPM2 objects to a specific phase path it is possible to restrict access to them to
specific phases of the boot process, for example making it impossible to access the root file system's
encryption key after the system transitioned from the initrd into the host root file system.
Use systemd-measure(1) to pre-calculate expected PCR 11 values for specific boot phases (via the --phase=
switch).
systemd-pcrfs-root.service and systemd-pcrfs@.service are automatically pulled into the initial
transaction by systemd-gpt-auto-generator(8) for the root and /var/ file systems. systemd-fstab-generator(8) will do this for all mounts with the x-systemd.pcrfs mount option in /etc/fstab.
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