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

From: Xing, Cedric
Date: Wed Feb 14 2024 - 03:55:00 EST

On 2/13/2024 8:05 AM, James Bottomley wrote:
On Mon, 2024-02-12 at 23:36 -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,
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.

The TCG crypto agile log format is

index (32 bit),
event tag (32 bit),
digests array,
sized event entry (up to 4GB)

So an IMA log entry can definitely be transformed into this format
(providing someone agrees to the tag or set of tags). The slight
problem would be that none of the current IMA tools would understand
it, but that could be solved over time (the kernel could use the TCG
format internally but transform to the IMA format for the current
securityfs IMA log).

Hi James,

As Mikko mentioned in his reply, TCG has defined the "Canonical Event Log Format" (aka. CEL) [1], while systemd-pcr* services use a subset of CEL format in their user space log.

I skimmed through the CEL spec today. Comparing to TCG2 log, CEL follows the same design (i.e., each event has a type field that determines the structure of the event data) but separates the encoding from the information model.

IMHO, CEL only works for applications defined in its information model (currently UEFI BIOS and IMA) but wouldn't work for any other applications like systemd. The systemd source code has documented the difference [2] between their log format and CEL.

One problem of CEL is its "content_type", which contains numeric values assigned by the spec. systemd doesn't have a numeric "content_type" assigned so has to use a string value - "systemd", which can only be encoded in JSON but not in TLV or CBOR. Technically, the systemd log is NOT CEL even though they claim that's a subset of it.

Another problem of CEL is that NOT every byte of an event is hashed/extended. CEL spec has defined for each "content_type" the subset of bytes to hash, so a verifier must understand ALL content types to be able to verify the integrity of a log. In other words, the integrity of a "systemd" log can never be verified by a CEL conformant verifier.

So I wouldn't recommend CEL to be the log format here.

We are looking for, as I believe, is a format that can accommodate all applications and allow application-agnostic verifiers. For every event, the kernel only needs to know what to store in the log, and what to hash/extend and to which measurement registers, but isn't concerned by the semantics of the event. If reusing CEL terms, what needs to be defined here is the "encoding" (so that every application can store/extend "something" in a log that every verifier knows how to replay); while every application should be allowed to define its own "information model".




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?

That is funny :-D

I can't agree more, so "no log" I think is always an option.

For the new TEE measurement log, I don't think it has to be
collocated with the BIOS log, because TEEs are never TPMs.

This depends. Logs are separable by PCRs. As in every entry for the
same PCR could be in a separate, correctly ordered, log. However, you
can't have separate logs that both use the same PCR because they won't

We can have separate logs for the same PCR as long as there's a way to order those logs. A simple way is to record the current PCR value at the beginning of every log, then a verifier can always replay the log to get the PCR value at exit, and use that value to match the next log.

Anyway, those details are unimportant. What I intended to say was that those logs don't have to be in the same format.