Re: [PATCH] tpm: Require that all digests are present in TCG_PCR_EVENT2 structures

Ard Biesheuvel
2020-06-16T09:08:53Z

(cc Matthew and Peter)

On Tue, 16 Jun 2020 at 01:28, Tyler Hicks <tyhicks@linux.microsoft.com> wrote:
>
> Require that the TCG_PCR_EVENT2.digests.count value strictly matches the
> value of TCG_EfiSpecIdEvent.numberOfAlgorithms in the event field of the
> TCG_PCClientPCREvent event log header. Also require that
> TCG_EfiSpecIdEvent.numberOfAlgorithms is non-zero.
>
> The TCG PC Client Platform Firmware Profile Specification section 9.1
> (Family "2.0", Level 00 Revision 1.04) states:
>
> For each Hash algorithm enumerated in the TCG_PCClientPCREvent entry,
> there SHALL be a corresponding digest in all TCG_PCR_EVENT2 structures.
> Note: This includes EV_NO_ACTION events which do not extend the PCR.
>
> Section 9.4.5.1 provides this description of
> TCG_EfiSpecIdEvent.numberOfAlgorithms:
>
> The number of Hash algorithms in the digestSizes field. This field MUST
> be set to a value of 0x01 or greater.
>
> Enforce these restrictions, as required by the above specification, in
> order to better identify and ignore invalid sequences of bytes at the
> end of an otherwise valid TPM2 event log. Firmware doesn't always have
> the means necessary to inform the kernel of the actual event log size so
> the kernel's event log parsing code should be stringent when parsing the
> event log for resiliency against firmware bugs. This is true, for
> example, when firmware passes the event log to the kernel via a reserved
> memory region described in device tree.
>

When does this happen? Do we have code in mainline that does this?

> Prior to this patch, a single bit set in the offset corresponding to
> either the TCG_PCR_EVENT2.eventType or TCG_PCR_EVENT2.eventSize fields,
> after the last valid event log entry, could confuse the parser into
> thinking that an additional entry is present in the event log. This
> patch raises the bar on how difficult it is for stale memory to confuse
> the kernel's event log parser but there's still a reliance on firmware
> to properly initialize the remainder of the memory region reserved for
> the event log as the parser cannot be expected to detect a stale but
> otherwise properly formatted firmware event log entry.
>
> Fixes: fd5c78694f3f ("tpm: fix handling of the TPM 2.0 event logs")
> Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
> ---

I am all for stringent checks, but this could potentially break
measured boot on systems that are working fine today, right?

> include/linux/tpm_eventlog.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/tpm_eventlog.h b/include/linux/tpm_eventlog.h
> index 4f8c90c93c29..d83eb9fd5614 100644
> --- a/include/linux/tpm_eventlog.h
> +++ b/include/linux/tpm_eventlog.h
> @@ -201,7 +201,7 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
> efispecid = (struct tcg_efi_specid_event_head *)event_header->event;
>
> /* Check if event is malformed. */
> - if (count > efispecid->num_algs) {
> + if (!efispecid->num_algs || count != efispecid->num_algs) {
> size = 0;
> goto out;
> }
> --
> 2.25.1
>