systemd-pcrphase.service, systemd-pcrphase-sysinit.service, systemd-pcrphase-initrd.service, systemd-pcrphase — Measure boot phase into TPM2 PCR 11
systemd-pcrphase-initrd.service are system services that measure specific strings
into TPM2 PCR 11 during boot at various milestones of the boot process.
These services require systemd-stub(7) to be used in a unified kernel image (UKI) setup. They execute no operation when invoked 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 certain literal strings indicating various phases of the boot process into TPM2 PCR 11. During a regular boot process the following strings are extended into PCR 11.
enter-initrd" is extended into PCR 11 early when the initrd
initializes, before activating system extension images for the initrd. It is supposed to act as 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 string is extended at start of
leave-initrd" is extended into PCR 11 when the initrd is about to
transition into the host file system, i.e. when it achieved its purpose. It is supposed to act as
barrier between kernel/initrd code and host OS code. (This string is extended at stop of
sysinit" is extended into PCR 11 when basic system initialization is
complete (which includes local file systems have been mounted), and the system begins starting regular
system services. (This string is extended at start of
ready" is extended into PCR 11 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 is
supposed to act as barrier between the time where unprivileged regular users are still prohibited to
log in and where they are allowed to log in. (This string is extended at start of
shutdown" is extended into PCR 11 when system shutdown begins. It is
supposed to act as barrier between the time the system is fully up and running and where it is about to
shut down. (This string is extended at stop of
final" is extended into PCR 11 at the end of system shutdown. It is
supposed to act as barrier between the time the service manager still runs and when it transitions into
the final boot phase where service management is not available anymore. (This string is extended at
During a regular system lifecycle, the strings "
leave-initrd" → "
sysinit" → "
shutdown" → "
final" are extended into PCR 11, one after the
Specific phases of the boot process may be referenced via the series of strings measured, separated
by colons (the "boot path"). For example, the boot path for the regular system runtime is
enter-initrd:leave-initrd:sysinit:ready", while the one for the initrd is just
enter-initrd". The boot path for the 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 boot 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.
pre-calculate expected PCR 11 values for specific boot phases (via the
/usr/lib/systemd/system-pcrphase executable may also be invoked from the
command line, where it expects the word to extend into PCR 11, as well as the following switches:
Takes the PCR banks to extend the specified word into. If not specified the tool automatically determines all enabled PCR banks and measures the word into all of them.
Controls which TPM2 device to use. Expects a device node path referring to the TPM2
/dev/tpmrm0). Alternatively the special value "
may be specified, in order to automatically determine the device node of a suitable TPM2 device (of
which there must be exactly one). The special value "
list" may be used to enumerate
all suitable TPM2 devices currently discovered.