xref: /openbmc/linux/Documentation/security/tpm/tpm_event_log.rst (revision 2ef5a7f1)

.. SPDX-License-Identifier: GPL-2.0

=============
TPM Event Log
=============

This document briefly describes what TPM log is and how it is handed
over from the preboot firmware to the operating system.

Introduction
============

The preboot firmware maintains an event log that gets new entries every
time something gets hashed by it to any of the PCR registers. The events
are segregated by their type and contain the value of the hashed PCR
register. Typically, the preboot firmware will hash the components to
who execution is to be handed over or actions relevant to the boot
process.

The main application for this is remote attestation and the reason why
it is useful is nicely put in the very first section of [1]:

"Attestation is used to provide information about the platform’s state
to a challenger. However, PCR contents are difficult to interpret;
therefore, attestation is typically more useful when the PCR contents
are accompanied by a measurement log. While not trusted on their own,
the measurement log contains a richer set of information than do the PCR
contents. The PCR contents are used to provide the validation of the
measurement log."

UEFI event log
==============

UEFI provided event log has a few somewhat weird quirks.

Before calling ExitBootServices() Linux EFI stub copies the event log to
a custom configuration table defined by the stub itself. Unfortunately,
the events generated by ExitBootServices() don't end up in the table.

The firmware provides so called final events configuration table to sort
out this issue. Events gets mirrored to this table after the first time
EFI_TCG2_PROTOCOL.GetEventLog() gets called.

This introduces another problem: nothing guarantees that it is not called
before the Linux EFI stub gets to run. Thus, it needs to calculate and save the
final events table size while the stub is still running to the custom
configuration table so that the TPM driver can later on skip these events when
concatenating two halves of the event log from the custom configuration table
and the final events table.

References
==========

- [1] https://trustedcomputinggroup.org/resource/pc-client-specific-platform-firmware-profile-specification/
- [2] The final concatenation is done in drivers/char/tpm/eventlog/efi.c