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- .. 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
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