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

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Date:	Thu, 21 May 2015 14:44:13 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	David Howells <dhowells@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Michal Marek <mmarek@...e.cz>,
	Matthew Garrett <mjg59@...f.ucam.org>, keyrings@...ux-nfs.org,
	Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Seth Forshee <seth.forshee@...onical.com>,
	LSM List <linux-security-module@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH 0/8] MODSIGN: Use PKCS#7 for module signatures [ver #4]

On Thu, May 21, 2015 at 2:38 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
> On Wed, May 20, 2015 at 07:21:00PM +0300, Petko Manolov 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"
>
> How exactly do you go about uploading CertB to the kernel BTW? And could fw
> signing replace that functionality? Keep in mind "fw uploading" should be
> rebranded as "system data upload", which is one of the goals I have when
> extending the firware_class module.

In PKCS#7 land, you don't.  Instead you stick CertB and CertC into the
PKCS#7 signature on the module signed by KeyC.  Then the kernel
verifies them (in theory) using the X.509/PKIX godawful verification
algorithm.  For fun, go search for the large number of errors in the
implementation of this awful algorithm in basically every TLS
implementation out there.

One option would be to add another type of verifiable thing.  We can
verify modules, and we should add firmware to the types of things that
can be signed.  We could add signing keys, too.  IOW, you could ask
the kernel to load a signing key with certain rights, and, if they key
is validly signed by some other key that has the same rights and has a
bit set saying that it can delegate those rights, then the kernel will
add that signing key to the keyring.

If the general infrastructure were there, this would be very little
additional code.

--Andy
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