Re: [PATCH v3] tpm: Document UEFI event log quirks

[Randy Dunlap]

On 7/12/19 5:49 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@xxxxxxxxxxxxxxx>
> ---
> v3: Add a section and use bullet list for references. Remove (invalid)
>     author info.
> v2: Fixed one type, adjusted the last paragraph and added the file

is that         typo  or type?

(one more below)

>     to index.rst
>  Documentation/security/tpm/index.rst         |  1 +
>  Documentation/security/tpm/tpm_event_log.rst | 55 ++++++++++++++++++++
>  2 files changed, 56 insertions(+)
>  create mode 100644 Documentation/security/tpm/tpm_event_log.rst
> 
> diff --git a/Documentation/security/tpm/index.rst b/Documentation/security/tpm/index.rst
> index 15783668644f..9e0815cb1e7f 100644
> --- a/Documentation/security/tpm/index.rst
> +++ b/Documentation/security/tpm/index.rst
> @@ -4,5 +4,6 @@ Trusted Platform Module documentation
>  
>  .. toctree::
>  
> +   tpm_event_log
>     tpm_ftpm_tee
>     tpm_vtpm_proxy
> diff --git a/Documentation/security/tpm/tpm_event_log.rst b/Documentation/security/tpm/tpm_event_log.rst
> new file mode 100644
> index 000000000000..068eeb659bb9
> --- /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 platformâs 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,

[again:]                                                    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
> 


-- 
~Randy