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

From: "Luis R. Rodriguez" <mcgrof@suse.com>
To: Andy Lutomirski <luto@amacapital.net>,
	Mimi Zohar <zohar@linux.vnet.ibm.com>,
	Matthew Garrett <mjg59@srcf.ucam.org>,
	Casey Schaufler <casey@schaufler-ca.com>
Cc: Julian Calaby <julian.calaby@gmail.com>,
	Andy Lutomirski <luto@kernel.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	James Morris <james.l.morris@oracle.com>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	linux-wireless <linux-wireless@vger.kernel.org>,
	David Howells <dhowells@redhat.com>,
	Kyle McMartin <kyle@kernel.org>,
	David Woodhouse <david.woodhouse@intel.com>,
	Seth Forshee <seth.forshee@canonical.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Mimi Zohar <zohar@linux.vnet.ibm.com>,
	Konstantin Ryabitsev <mricon@kernel.org>,
	Michal Marek <mmarek@suse.cz>,
	Abelardo Ricart III <aricart@memnix.com>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	keyrings@linux-nfs.org, Borislav Petkov <bp@alien8.de>,
	Jiri Kosina <jkosina@suse.cz>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFD] linux-firmware key arrangement for firmware signing
Date: Wed, 20 May 2015 02:39:17 +0200	[thread overview]
Message-ID: <20150520003917.GV23057@wotan.suse.de> (raw)
In-Reply-To: <CALCETrX1YrdwfPJxVbzE0MQK=HChK8sDztt6MZcJiCvTEDCn7Q@mail.gmail.com>

On Tue, May 19, 2015 at 04:42:05PM -0700, Andy Lutomirski wrote:
> On Tue, May 19, 2015 at 4:30 PM, Julian Calaby <julian.calaby@gmail.com> wrote:
> > Hi All,
> >
> > On Wed, May 20, 2015 at 6:59 AM, Andy Lutomirski <luto@kernel.org> wrote:
> >> [added cc's from the other thread]
> >>
> >> On 05/19/2015 01:02 PM, Luis R. Rodriguez wrote:
> >>>
> >>> David Howells has posted v4 of his series of supporting PKCS#7 for module
> >>> signing. I'm in my v3 series now on RFCs for firmware PKCS#7 support, and
> >>> after
> >>> some review and patch shuffling I think this is ready for patch form.  My
> >>> own
> >>> series however depend on quite a bit of other pending changes, one series
> >>> which
> >>> will go through Rusty's tree, another series of fixes on firmware_class
> >>> which
> >>> should go through Greg's tree. I'll wait until all this and David's own
> >>> patches
> >>> get merged before posting firmware PKCS#7 support. Before all this though
> >>> in
> >>> preparation for fw signing one thing we should start to talk about more
> >>> broadly
> >>> however is how linux-firmware binary file signing would work in practice
> >>> and
> >>> what we need, and make sure folks are OK with all this.
> >>>
> >>> First, firmware signing will be completely optional as with module
> >>> signing.
> >>>
> >>
> >> ...
> >>
> >>> Other than this last nitpick, any other concerns or recommendations ?
> >>
> >>
> >> A couple.  Some of these are general concerns with the existing
> >> infrastructure, but #1 is a specific problem that gets much worse if we add
> >> firmware signing.  Feel free to ignore 2-4.
> >>
> >> 1. We should get the signature semantics right.  I think that, for modules,
> >> we currently sign literally the module payload.  For modules, in my
> >> semi-amateurish crypto universe [1], this is fine *as long as the key in
> >> question is used for no other purpose*.  For firmware, it's dangerous, since
> >> it would be vulnerable to substitution attacks in which the adversary
> >> convinces us to interpret one firmware file as firmware for another device
> >> or purpose entirely.
> >>
> >> We should be signing something that's semantically equivalent to "This is a
> >> valid module: xyz", "This is a valid 'regulatory.bin': xyz", or "This is a
> >> valid kexec image: xyz".
> >
> > Something that occurred to me (as a complete bystander) was: would it
> > make sense to have keys able to be restricted to particular "types" of
> > signable data? I.e. the key that can sign a valid regulatory.bin file
> > cannot be used to sign a module or a kexec image. - This could remove
> > the need to have multiple keyrings. (Also, UEFI keys unless otherwise
> > tagged could be restricted to only signing bootloaders or kernels)
> 
> Seems sensible to me.

As for having keys for fw signing be specific to fw data without a keyring,
if that is desirable I think we can devise a way to do that. For instance
if we wanted to we can have FW_SIG by default trust first keys on
system_trusted_keyring just as module signature works -- or if we wanted to
just trust, say a Kyle key. Not sure if the later is possible yet, but htat
would require some changes. Then as an evolution if we wanted to enable a
specific request fw to be mapped to a specific fw file the new APIs I was
looking to add could easily enable this provided that we first decide we
do want to trust say one key perhaps not on system_trusted_keyring for fw
signing. That'd need to be decided first.

As for the UEFI stuff -- from what I gather its too late there. We could
certainly go with something else for fw signing though, just lemme hear it
hard and clear.

> FWIW, I'm starting to think that UEFI-based validation of kexec images
> should be totally separate.  It uses a nasty PE format with a hideous
> PKCS #7 formatted signature.  Maybe that should be a completely
> separate piece of code.

LSM'ify it I guess? Again, if that's reasonable then I think we'll need
stacking and that's still not merged.

  Luis