Re: [RFC PATCH v2 0/4] tsm: Runtime measurement registers ABI

Mikko Ylinen
2024-02-13T16:54:31Z

On Mon, Feb 12, 2024 at 11:36:27PM -0800, Xing, Cedric wrote:
> On 2/9/2024 12:58 PM, Dan Williams wrote:
> > James Bottomley wrote:
> > > Just to correct this: IMA uses its own log format, but I think this was
> > > a mistake long ago and the new log should use TCG2 format so all the
> > > tools know how to parse it.
> >
> > Is this a chance to nudge IMA towards a standard log format? In other
> > words, one of the goals alongside userspace consumers of the RTMR log
> > would be for IMA to support it as well as an alternate in-kernel backend
> > next to TPM. IMA-over-TPM continues with its current format,
> > IMA-over-RTMR internally unifies with the log format that is shared with
> > RTMR-user-ABI.
> >
> I'm not a TCG expert. As far as I know, https://trustedcomputinggroup.org/wp-content/uploads/TCG-PC-Client-Platform-Firmware-Profile-Version-1.06-Revision-52_pub-1.pdf
> defines the event types for TCG2 logs for firmware uses only. I cannot find
> a spec that defines event types for OS or applications. We may reuse the
> firmware event types for Linux but I doubt they can accommodate IMA.
>
> IMHO, we don't have to follow TCG2 format because TDX is never TPM, nor are
> any other TEEs that support runtime measurements. The existing TCG2 format
> looks to me somewhat like ASN.1 - well defined but schema is needed to
> decode. In contrast, JSON is a lot more popular than ASN.1 nowadays because
> it's human readable and doesn't require a schema. I just wonder if we should
> introduce a text based log format. We could make the log a text file, in
> which each line is an event record and the digest of the line is extended to
> the specified runtime measurement register. The content of each line could
> be free-form at the ABI level, but we can still recommend a convention for
> applications - e.g., the first word/column must be an URL for readers to
> find out the format/syntax of the rest of the line. Thoughts?

There's also the 'Canonical Event Log' format from TCG. It
covers IMA but it looks it's PC/client specific otherwise.

systemd seems to be following this format for its systemd-pcr*
services and exposing the log in JSON format under /run/log [1].

[1] https://www.freedesktop.org/software/systemd/man/latest/systemd-pcrphase.service.html

>
> > ...but be warned the above is a comment from someone who knows nothing
> > about IMA internals, just reacting to the comment.
> >
> >
> > > > I am wondering where will the event log be stored? Is it in the
> > > > log_area region of CCEL table?
> > >
> > > IMA stores its log in kernel memory and makes it visible in securityfs
> > > (in the smae place as the measured boot log). Since this interface is
> > > using configfs, that's where I'd make the log visible.
> > >
> > > Just to add a note about how UEFI works: the measured boot log is
> > > effectively copied into kernel memory because the UEFI memory it once
> > > occupied is freed after exit boot services, so no UEFI interface will
> > > suffice for the log location.
> > >
> > > I'd make the file exporting it root owned but probably readable by only
> > > the people who can also extend it (presumably enforced by group?).
> >
> > I assume EFI copying into kernel memory is ok because that log has a
> > limited number of entries. If this RTMR log gets large I assume it needs
> > some way cull entries that have been moved to storage. Maybe this is a
> > problem IMA has already solved.
>
> We don't have to, and are also not supposed to I guess, append to the log
> generated by BIOS. The kernel can start a new log, and potentially in a
> different format. I think the BIOS log is exposed via securityfs today. Am I
> correct? For the new TEE measurement log, I don't think it has to be
> collocated with the BIOS log, because TEEs are never TPMs.

--
Regards, Mikko