Re: [PATCH 3/6] firmware: differentiate between signed regulatory.db and other firmware

From: Mimi Zohar
Date: Wed May 09 2018 - 22:01:18 EST

On Wed, 2018-05-09 at 23:48 +0000, Luis R. Rodriguez wrote:
> On Wed, May 09, 2018 at 06:06:57PM -0400, Mimi Zohar wrote:
> > On Wed, 2018-05-09 at 21:22 +0000, Luis R. Rodriguez wrote:
> > >
> > > OK, its still not clear to what it will do. If it does not touch the firmware
> > > loader code, and it just sets and configures IMA to do file signature checking
> > > on its own, then yes I think both mechanisms would be doing the similar work.
> > >
> > > Wouldn't IMA do file signature checks then for all files? Or it would just
> > > enable this for whatever files userspace wishes to cover?
> >
> > Enabling CONFIG_IMA_APPRAISE_FIRMWARE would enforce firmware
> > signatures on all directly loaded firmware and fail any method of
> > loading firmware that the signature couldn't be verified.
> Ah, so a generic firmware signing mechanism via IMA. Sounds very sensible to me.
> Specially in light of the fact that its what we recommend folks to consider
> if they need to address firmware signing for devices which do not have the
> ability to do hardware firmware signing verification on their own.
> > > One of the things with READING_FIRMWARE_REGULATORY_DB is to also use and trust
> > > the wireless-regdgb maintainer's key for this file, could IMA be configured to
> > > do that?
> >
> > IMA has its own trusted keyring. ÂSo either the maintainer's key would
> > need to be added to the IMA keyring,
> I see so we'd need this documented somehow.
> > or IMA-appraisal would need to use the regdb keyring.
> Can you describe this a bit more, for those not too familiar with IMA, in terms
> of what would be involved in the kernel? Or is this all userspace configuration
> stuff?

I think it's a bit premature to be discussing how IMA could add the
builtin regulatory key to its keyring or use the regdb keyring, as
IMA-appraisal doesn't (yet) support detached signatures.

The other option would be to include the regulatory.db signature in
the package. ÂFor rpm, the file signature is included in the RPM
header. ÂMultiple attempts have been made to have Debian packages
include file signatures. ÂThis is the most recent attempt -Âhttps://li

> > > > Yes, writing regdb as a micro/mini LSM sounds reasonable. ÂThe LSM
> > > > would differentiate between other firmware and the regulatory.db based
> > > > on the firmware's pathname.
> > >
> > > If that is the only way then it would be silly to do the mini LSM as all
> > > calls would have to have the check. A special LSM hook for just the
> > > regulatory db also doesn't make much sense.
> >
> > All calls to request_firmware() are already going through this LSM
> > hook. ÂI should have said, it would be based on both READING_FIRMWARE
> > and the firmware's pathname.
> Yes, but it would still be a strcmp() computation added for all
> READING_FIRMWARE. In that sense, the current arrangement is only open coding the
> signature verification for the regulatory.db file. One way to avoid this would
> be to add an LSM specific to the regulatory db

Casey already commented on this suggestion.


> and have the
> security_check_regulatory_db() do what it needs per LSM, but that would mean
> setting a precedent for open possibly open coded future firmware verification
> call. Its not too crazy to consider if an end goal may be avoid further open
> coded firmware signature verification hacks.
> > > > Making regdb an LSM would have the same issues as currently - deciding
> > > > if regdb, IMA-appraisal, or both verify the regdb's signature.
> > >
> > > Its unclear to me why they can't co-exist yet and not have to touch
> > > the firmware_loader code at all.
> >
> > With the changes discussed above, they will co-exist. ÂOther than the
> > Kconfig changes, I don't think it will touch the firmware_loader code.
> Great.
> Luis