Re: [PATCH V4] efi/tpm: Fix the issue where the CC platforms event log header can't be correctly identified

[Sathyanarayanan Kuppuswamy]

On 7/11/25 7:00 AM, James Bottomley wrote:
> On Tue, 2025-07-08 at 15:54 +0800, yangge1116@xxxxxxx wrote:
> > From: Ge Yang <yangge1116@xxxxxxx>
> >
> > Since commit d228814b1913 ("efi/libstub: Add get_event_log() support
> > for CC platforms") reuses TPM2 support code for the CC platforms,
> > when launching a TDX virtual machine with coco measurement enabled,
> > the following error log is generated:
> >
> > [Firmware Bug]: Failed to parse event in TPM Final Events Log
> >
> > Call Trace:
> > efi_config_parse_tables()
> >    efi_tpm_eventlog_init()
> >      tpm2_calc_event_log_size()
> >        __calc_tpm2_event_size()
> >
> > The pcr_idx value in the Intel TDX log header is 1, causing the
> > function __calc_tpm2_event_size() to fail to recognize the log
> > header, ultimately leading to the "Failed to parse event in TPM Final
> > Events Log" error.
> >
> > According to UEFI Specification 2.10, Section 38.4.1: For TDX, TPM
> > PCR 0 maps to MRTD, so the log header uses TPM PCR 1 instead. To
> > successfully parse the TDX event log header, the check for a pcr_idx
> > value of 0 must be skipped.
> >
> > According to Table 6 in Section 10.2.1 of the TCG PC Client
> > Specification, the index field does not require the PCR index to be
> > fixed at zero. Therefore, skipping the check for a pcr_idx value of
> > 0 for CC platforms is safe.
> This is wrong: the spec does not allow a header EV_ACTION to be
> recorded with anything other than pcrIndex == 0.
>
> However, the fact that Intel, who practically wrote the TPM spec, can
> get this wrong shows that others can too.  So the best way to fix this
> is to remove the pcrIndex check for the first event.  There's no danger
> of this causing problems because we check for the TCG_SPECID_SIG
> signature as the next thing.  That means you don't need to thread
> knowledge of whether this is a CC environment and we're pre-emptively
> ready for any other spec violators who misread the spec in the same way
> Intel did.


I agree with James Bottomley's suggestion to remove the pcr_index check
without adding any replacement checks.

This check was originally introduced in the following commit to handle a
case where certain Dell platforms provided an event log without a valid
header:

commit 7dfc06a0f25b593a9f51992f540c0f80a57f3629
Author: Fabian Vogt <fvogt@xxxxxxx>
Date:   Mon Jun 15 09:16:36 2020 +0200

    efi/tpm: Verify event log header before parsing

At first, I was concerned that the pcr_index check might still be important for
this fix. However, after re-reading the commit and reviewing the intent, it appears
that relying on the event_type and digest checks should be sufficient for validating
the event log header.




> Regards,
>
> James
--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer