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authorJarkko Sakkinen <jarkko.sakkinen@linux.intel.com>2019-07-12 18:44:32 +0300
committerJonathan Corbet <corbet@lwn.net>2019-07-31 13:37:29 -0600
commit2ef5a7f1482c21579d556f31a605a7ca32f251e7 (patch)
tree6897054fd25bf3862ae2d0efc2df3edeffb4f066
parente15d5a53ea3239e537d3a0c4bdfce8a4ea4e0da6 (diff)
downloadlwn-2ef5a7f1482c21579d556f31a605a7ca32f251e7.tar.gz
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tpm: Document UEFI event log quirks
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> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
-rw-r--r--Documentation/security/tpm/index.rst1
-rw-r--r--Documentation/security/tpm/tpm_event_log.rst55
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diff --git a/Documentation/security/tpm/index.rst b/Documentation/security/tpm/index.rst
index 3296533e54cf..2bae8e8ca4bc 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_vtpm_proxy
xen-tpmfront
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