[v4] tpm: Document UEFI event log quirks
diff mbox series

Message ID 20190712154439.10642-1-jarkko.sakkinen@linux.intel.com
State New
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  • [v4] tpm: Document UEFI event log quirks
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Commit Message

Jarkko Sakkinen July 12, 2019, 3:44 p.m. UTC
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>
---
v4: - Unfortanely -> Unfortunately
v3: - Add a section for refs and use a bullet list to enumerate them.
    - Remove an invalid author info.
v2: - Fix one typo.
    - Refine the last paragraph to better explain how the two halves
      of the event log are concatenated.
 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

Comments

Jonathan Corbet July 31, 2019, 7:39 p.m. UTC | #1
On Fri, 12 Jul 2019 18:44:32 +0300
Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> 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>
> ---
> v4: - Unfortanely -> Unfortunately
> v3: - Add a section for refs and use a bullet list to enumerate them.
>     - Remove an invalid author info.
> v2: - Fix one typo.
>     - Refine the last paragraph to better explain how the two halves
>       of the event log are concatenated.
>  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

I've applied this, thanks.  Before I could do so, though, I had to edit
the headers, which read:

> Content-Type: text/plain; charset=y

"git am" *really* doesn't like "charset=y".  I think this is something
that git send-email likes to do occasionally, don't know why...

Thanks,

jon
Jarkko Sakkinen Aug. 1, 2019, 5:43 p.m. UTC | #2
On Wed, Jul 31, 2019 at 01:39:48PM -0600, Jonathan Corbet wrote:
> On Fri, 12 Jul 2019 18:44:32 +0300
> Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> 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>
> > ---
> > v4: - Unfortanely -> Unfortunately
> > v3: - Add a section for refs and use a bullet list to enumerate them.
> >     - Remove an invalid author info.
> > v2: - Fix one typo.
> >     - Refine the last paragraph to better explain how the two halves
> >       of the event log are concatenated.
> >  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
> 
> I've applied this, thanks.  Before I could do so, though, I had to edit
> the headers, which read:
> 
> > Content-Type: text/plain; charset=y
> 
> "git am" *really* doesn't like "charset=y".  I think this is something
> that git send-email likes to do occasionally, don't know why...

Thank you!

/Jarkko

Patch
diff mbox series

diff --git a/Documentation/security/tpm/index.rst b/Documentation/security/tpm/index.rst
index af77a7bbb070..db566350bcd5 100644
--- a/Documentation/security/tpm/index.rst
+++ b/Documentation/security/tpm/index.rst
@@ -4,4 +4,5 @@  Trusted Platform Module documentation
 
 .. toctree::
 
+   tpm_event_log
    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..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