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

From: Andy Lutomirski <luto@amacapital.net>
To: Andy Lutomirski <luto@amacapital.net>,
	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 09:41:21 -0700	[thread overview]
Message-ID: <CALCETrV0zb-Sm=L1CG6z9uQc1W+Wh2KTpAx5vUm8pcn+-qmsFg@mail.gmail.com> (raw)
In-Reply-To: <20150520162059.GC10473@localhost>

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.

>
> 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.

>
>                 Petko

-- 
Andy Lutomirski
AMA Capital Management, LLC