Re: [PATCH 0/8] MODSIGN: Use PKCS#7 for module signatures [ver #4]

From: Petko Manolov <petkan@mip-labs.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: David Howells <dhowells@redhat.com>,
	Andy Lutomirski <luto@kernel.org>,
	Rusty Russell <rusty@rustcorp.com.au>,
	Michal Marek <mmarek@suse.cz>,
	Matthew Garrett <mjg59@srcf.ucam.org>,
	keyrings@linux-nfs.org,
	Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
	Luis Rodriguez <mcgrof@suse.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Seth Forshee <seth.forshee@canonical.com>,
	LSM List <linux-security-module@vger.kernel.org>,
	David Woodhouse <dwmw2@infradead.org>
Subject: Re: [PATCH 0/8] MODSIGN: Use PKCS#7 for module signatures [ver #4]
Date: Wed, 20 May 2015 19:55:06 +0300	[thread overview]
Message-ID: <20150520165506.GE10473@localhost> (raw)
In-Reply-To: <CALCETrV0zb-Sm=L1CG6z9uQc1W+Wh2KTpAx5vUm8pcn+-qmsFg@mail.gmail.com>

On 15-05-20 09:41:21, Andy Lutomirski wrote:
> On Wed, May 20, 2015 at 9:21 AM, Petko Manolov <petkan@mip-labs.com> wrote:
> > On 15-05-20 08:56:21, Andy Lutomirski wrote:
> >>
> >> Would it make more sense to permit X.509 chains to be loaded into the keyring
> >> instead if we actually need that feature?  IOW, let userspace (or early
> >> initramfs stuff) extend our keyring trust to intermediate certs that validly
> >> chain to already-trusted things?  I think that a reasonable design goal would
> >> be that everything overcomplicated that's involved should be optional, and
> >> moving toward embedding PKCS#7 signatures in the modules themselves does the
> >> other direction?
> >
> > This is similar to what i am doing right now - create CA hierarchy so we can
> > have something like:
> >
> >                                +-> KeyB
> >                                |
> > RootCA --->  CertA ---> CertB ---> CertC ---> KeyC
> >                     |
> >                     +-> CertA' ---> KeyA"
> >
> > The RootCA may be the one whose private key was used to sign the modules and all
> > downstream certificates are either directly signed by it or one of the others.
> > Not all of the infrastructure is in the mainline kernel, but this can easily be
> > rectified.
> 
> Right.  I guess that I can imagine some uses for this, but I don't see
> why those intermediate certs would be embedded with the signatures
> being verified as opposed to being loaded beforehand.

They aren't.  They shouldn't.

I think module signing/verification should be simple but cryptographically sound 
process.  Over-designing it is dumb, but i find it useful when there is 
compatibility (at least at key format level) with other systems, namely IMA and 
EVM.

> > Now, as Mimi pointed out this scheme is flawed and should be used with care 
> > if at all.  Revoking certificates is always a PITA.  Being valid for one 
> > year only adds to the fun.
> >
> 
> Valid for only one year is worse than that.  We might be verifying the 
> signature on our clock driver :) I think that, at best, we could reject 
> certificates that expired before the running kernel was built.

Nah, nothing as complex as that.  It is so easy to play with the system clock.  
In the general case, and if needed, the user would either use self signed 
certificate or one signed by whoever the user trusts - Fedora, Debian, LF or 
even NSA.  :)

		Petko