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SubjectRe: [PATCH] tpm: Document UEFI event log quirks
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On 7/3/19 9:11 AM, Jarkko Sakkinen wrote:
> 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@linux.intel.com>
> ---
> 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@linux.vnet.ibm.com>
> +
> +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,

Unfortunately,

> +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.

? kernel can 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
>


--
~Randy

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