There are some weird quirks when it comes to UEFI event log. Provide a brief introduction to TPM event log mechanism and describe the quirks and how they can be sorted out. Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx> --- Documentation/security/tpm/tpm-eventlog.rst | 53 +++++++++++++++++++++ 1 file changed, 53 insertions(+) create mode 100644 Documentation/security/tpm/tpm-eventlog.rst diff --git a/Documentation/security/tpm/tpm-eventlog.rst b/Documentation/security/tpm/tpm-eventlog.rst new file mode 100644 index 000000000000..2ca8042bdb17 --- /dev/null +++ b/Documentation/security/tpm/tpm-eventlog.rst @@ -0,0 +1,53 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============= +TPM Event Log +============= + +| Authors: +| Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx> + +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. Unfortanely, +the events generated by ExitBootServices() do end up to 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 stub gets to run. Thus, it needs to copy the final +events table preboot size to the custom configuration table so that +kernel offset it later on. + +[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 -- 2.20.1
