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

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Message-Id: <1525895838.3551.247.camel@linux.vnet.ibm.com>
Date:   Wed, 09 May 2018 15:57:18 -0400
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     linux-wireless <linux-wireless@...r.kernel.org>,
        Kalle Valo <kvalo@...eaurora.org>,
        Seth Forshee <seth.forshee@...onical.com>,
        Johannes Berg <johannes.berg@...el.com>,
        linux-integrity@...r.kernel.org,
        Hans de Goede <hdegoede@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Jones <pjones@...hat.com>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andres Rodriguez <andresx7@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 3/6] firmware: differentiate between signed
 regulatory.db and other firmware

On Wed, 2018-05-09 at 19:15 +0000, Luis R. Rodriguez wrote:

> > > > If both are enabled, do we require both signatures or is one enough.
> > > 
> > > Good question. Considering it as a stacked LSM (although not implemented
> > > as one), I'd say its up to who enabled the Kconfig entries. If IMA and
> > > CONFIG_CFG80211_REQUIRE_SIGNED_REGDB are enabled then both. If someone enabled
> > > IMA though, then surely I agree that enabling
> > > CONFIG_CFG80211_REQUIRE_SIGNED_REGDB is stupid and redundant, but its up to the
> > > system integrator to decide.
> > 
> > Just because IMA-appraisal is enabled in the kernel doesn't mean that
> > firmware signatures will be verified.  That is a run time policy
> > decision.
> 
> Sure, I accept this if IMA does not do signature verification. However
> signature verification seems like a stackable LSM decision, no?

IMA-appraisal can be configured to enforce file signatures.  Refer to
discussion below as to how.

> > > If we however want to make it clear that such things as
> > > CONFIG_CFG80211_REQUIRE_SIGNED_REGDB are not required when IMA is enabled we
> > > could just make the kconfig depend on !IMA or something?  Or perhaps a new
> > > kconfig for IMA which if selected it means that drivers can opt to open code
> > > *further* kernel signature verification, even though IMA already is sufficient.
> > > Perhaps CONFIG_ENABLE_IMA_OVERLAPPING, and the driver depends on it?
> > 
> > The existing CONFIG_IMA_APPRAISE is not enough.  If there was a build
> > time IMA config that translated into an IMA policy requiring firmware
> > signature verification (eg. CONFIG_IMA_APPRAISE_FIRMWARE), this could
> > be sorted out at build time.
> 
> I see makes sense.

Ok, so instead of introducing READING_FIRMWARE_REGULATORY_DB, I'll
post patches introducing CONFIG_IMA_APPRAISE_FIRMWARE, as described
above.

> 
> > > > Assigning a different id for regdb signed firmware allows LSMs and IMA
> > > > to handle regdb files differently.
> > > 
> > > That's not the main concern here, its the precedent we are setting here for
> > > any new kernel interface which open codes firmware signing on its own. What
> > > you are doing means other kernel users who open codes their own firmware
> > > signing may need to add yet-another reading ID. That doesn't either look
> > > well on code, and seems kind of silly from a coding perspective given
> > > the above, in which I clarify IMA still is doing its own appraisal on it.
> > 
> > Suppose,
> > 
> > 1. Either CONFIG_CFG80211_REQUIRE_SIGNED_REGDB or
> > "CONFIG_IMA_APPRAISE_FIRMWARE" would be configured at build.
> > 
> > 2. If CONFIG_CFG80211_REQUIRE_SIGNED_REGDB is configured, not
> > "CONFIG_IMA_APPRAISE_FIRMWARE", a custom IMA-policy rule that
> > appraises the firmware signature could be defined.  In this case, both
> > signature verification methods would be enforced.
> > 
> > then READING_FIRMWARE_REGULATORY_DB would not be needed.
> 
> True, however I'm suggesting that CONFIG_CFG80211_REQUIRE_SIGNED_REGDB
> could just be a mini subsystem stackable LSM.

Yes, writing regdb as a micro/mini LSM sounds reasonable.  The LSM
would differentiate between other firmware and the regulatory.db based
on the firmware's pathname.

Making regdb an LSM would have the same issues as currently - deciding
if regdb, IMA-appraisal, or both verify the regdb's signature.

Mimi