Re: [RFD] linux-firmware key arrangement for firmware signing

David Howells
2015-05-20T15:09:29Z

Seth Forshee <seth.forshee@canonical.com> wrote:

> > This begs the question on how we'd manage keys for firmware signing on
> > linux-firmare. Since the keys are x509 keys we need a CA. Based on some
> > initial discussions it would seem we'd need the Linux Foundation to create
> > a key, this would be embedded in the kernel and that key would be used to
> > sign Kyle's key. Kyle would in turn use his key for signing
> > linux-firmware files. David, Kyle, did I summarize this correctly ?
>
> I raised the question of key revocation when we discussed this on irc,
> but it wasn't answered to my satisfaction. If a key signed by the
> kernel-embedded key is compromised, how can that key be revoked so that
> it is no longer trusted?
>
> Someone mentioned UEFI blacklists, which I don't know much about, but
> not all systems have UEFI. The only reliable option that comes to mind
> for me is an in-kernel blacklist of keys which should no longer be
> trusted.

Key revocation is generally an unpleasant problem. How do you inform a system
that a key of any sort is revoked? With PGP, for instance, you might be able
to connect to the net and consult a server.

UEFI has a blacklist that can theoretically be used to prevent both usage of a
key and usage of a particular object. As I understand it, the blacklist in
UEFI is just a table of SHA256 hashes.

Relying on UEFI presents three problems, though: (1) the system admin has to
manually, as far as I'm aware, inform the BIOS; (2) the UEFI storage is
limited; and (3) not all systems have UEFI.

What you do on a non-UEFI system, I'm not sure. If the kernel isn't verified
by the loader or the system firmware then you don't have a 'fully' secure
system anyway and the blacklist may be of questionable value.

David