mirror of
https://github.com/torvalds/linux.git
synced 2024-11-21 19:41:42 +00:00
2ef5a7f148
There are some weird quirks when it comes to UEFI event log. Provide a brief introduction to TPM event log mechanism and describe the quirks and how they can be sorted out. Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
56 lines
2.2 KiB
ReStructuredText
56 lines
2.2 KiB
ReStructuredText
.. 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
|