mirror of https://github.com/Qortal/Brooklyn
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
55 lines
2.2 KiB
55 lines
2.2 KiB
.. 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
|
|
|