Consider signing FCOS update metadata

[jlebon]

Describe the enhancement

The FCOS Cincinnati service receives its update metadata as JSON from the Cloudfront'ed S3 bucket over TLS. Matching the security model employed for the OSTree data itself, it'd be nice if we additionally signed this metadata and added "downgrade protection" (i.e. have the service ignore signed metadata with a metadata["last-modified"] date older than its previous fetch; though this will cause "stop this rollout" events to no longer be a pure git revert).

Additional information

Came out of discussions in coreos/rpm-ostree#2819 and #749.

[bgilbert]

See also #774, which is the same issue for a different set of metadata.

[cgwalters]

Also as part of this I realized that I think just due to pure oversight, rpm-ostree has never added the ostree ref binding metadata. We could consider turning that on, but we'd need to work through the implications.

Another thing that would likely help is to add a much weaker binding like ostree.binding = "coreos" and libostree would require that that metadata stay the same across upgrades by default. That would block things like a MITM offering a signed silverblue commit as an FCOS update.

[rugk]

added the ostree ref binding metadata. We could consider turning that on, but we'd need to work through the implications.

Then do so. 😉 Is there an issue tracking this?
I’d be very interested to see this being done.