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