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

Seth Forshee
2015-05-20T15:48:18Z

On Wed, May 20, 2015 at 04:08:58PM +0100, David Howells wrote:
> 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.

Distros could distribute updates to the blacklist via their usual update
mechanisms. That could be a new kernel with an updated blacklist (after
all we should expect blacklist updates to be very infrequent).

I suppose a database in the initrd which was loaded prior to loading any
firmware could work too, then perhaps new blacklists could be loaded
into a running kernel without a reboot as well. But that database should
probably be signed too, which creates a chicken-and-egg sort of problem.

> 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.

Yeah, that doesn't really sound like a good solution. Not all users are
sys admins.

> 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.

I think there's still value - compromised firmware could easily be a
vector to compromise the kernel. Just because I can't verify my system
security doesn't mean that I don't want measures in place to keep it
from being compromised.

Seth