1.. SPDX-License-Identifier: GPL-2.0 2 3============= 4TPM Event Log 5============= 6 7This document briefly describes what TPM log is and how it is handed 8over from the preboot firmware to the operating system. 9 10Introduction 11============ 12 13The preboot firmware maintains an event log that gets new entries every 14time something gets hashed by it to any of the PCR registers. The events 15are segregated by their type and contain the value of the hashed PCR 16register. Typically, the preboot firmware will hash the components to 17who execution is to be handed over or actions relevant to the boot 18process. 19 20The main application for this is remote attestation and the reason why 21it is useful is nicely put in the very first section of [1]: 22 23"Attestation is used to provide information about the platform���s state 24to a challenger. However, PCR contents are difficult to interpret; 25therefore, attestation is typically more useful when the PCR contents 26are accompanied by a measurement log. While not trusted on their own, 27the measurement log contains a richer set of information than do the PCR 28contents. The PCR contents are used to provide the validation of the 29measurement log." 30 31UEFI event log 32============== 33 34UEFI provided event log has a few somewhat weird quirks. 35 36Before calling ExitBootServices() Linux EFI stub copies the event log to 37a custom configuration table defined by the stub itself. Unfortunately, 38the events generated by ExitBootServices() don't end up in the table. 39 40The firmware provides so called final events configuration table to sort 41out this issue. Events gets mirrored to this table after the first time 42EFI_TCG2_PROTOCOL.GetEventLog() gets called. 43 44This introduces another problem: nothing guarantees that it is not called 45before the Linux EFI stub gets to run. Thus, it needs to calculate and save the 46final events table size while the stub is still running to the custom 47configuration table so that the TPM driver can later on skip these events when 48concatenating two halves of the event log from the custom configuration table 49and the final events table. 50 51References 52========== 53 54- [1] https://trustedcomputinggroup.org/resource/pc-client-specific-platform-firmware-profile-specification/ 55- [2] The final concatenation is done in drivers/char/tpm/eventlog/efi.c 56