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diff --git a/Documentation/security/tpm/tpm_event_log.rst b/Documentation/security/tpm/tpm_event_log.rst new file mode 100644 index 000000000000..f00f7a1d5e92 --- /dev/null +++ b/Documentation/security/tpm/tpm_event_log.rst @@ -0,0 +1,55 @@ +.. 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 |