linux-kernel - Re: [PATCH 4/8] MODSIGN: Provide a utility to append a PKCS#7 signature to a module [ver #4]

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Date:	Wed, 20 May 2015 09:00:05 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Howells <dhowells@...hat.com>
Cc:	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>,
	Luis Rodriguez <mcgrof@...e.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 4/8] MODSIGN: Provide a utility to append a PKCS#7
 signature to a module [ver #4]

On Wed, May 20, 2015 at 6:14 AM, David Howells <dhowells@...hat.com> wrote:
> Andy Lutomirski <luto@...nel.org> wrote:
>
>> >   enum pkey_id_type {
>> >     PKEY_ID_PGP,            /* OpenPGP generated key ID */
>> >     PKEY_ID_X509,           /* X.509 arbitrary subjectKeyIdentifier */
>> > +   PKEY_ID_PKCS7,          /* Signature in PKCS#7 message */
>> >     PKEY_ID_TYPE__LAST
>> >   };
>> >
>>
>> I don't understand these comments.  "OpenPGP generated key ID" refers to the
>> name of a key.  "X.509 arbitrary subjectKeyIdentifier" also refers to a name
>> of a key.
>
> OpenPGP was how we did things originally.  We then switched to X.509 because
> we had to take account of UEFI.  These values are implicit parts of the kernel
> ABI.
>
>> "Signature in PKCS#7 message" refers to a signature style.  This seems
>> inconsistent.
>
> Not precisely.  The format of the descriptor is immutable given the particular
> magic number.  You set the ID type to that and all the other fields bar one to
> zero and you put the signature and all the metadata in the PKCS#7 blob which
> you place directly prior to the descriptor (the length of the blob is the one
> thing you do need to specify).  Effectively, it's an override.

Is there a document anywhere in the kernel tree that defines the
actual format?  I suspect that this will confuse most people who read
the code right now.

>
>> Also, I think we're really going to want signatures that specify their
>> purpose, e.g. "module named xyz" or "firmware file named abc" or "kexec
>> image".  Let's get this right the first time rather than needing yet another
>> type in the very near future.
>
> If this is so, then this _must_ also apply to your hash list idea.

Definitely.

>
>> Finally, why are we using PKCS#7 for this?  Can't everything except kexec
>> images use raw signatures in some trivial non-ASN.1-ified format? A raw
>> signature can reference a UEFI-sourced key just fine.
>
> We have PKCS#7 already in the kernel.  It's a standard.  We can add attributes
> of our own devising to extend it if necessary (say your typing idea referenced
> above).
>
>> It could be as simple as:
>>
>> 4 bytes of signature type
>> (length of pubkey identifier, pubkey identifier)
>> 4 bytes of purpose
>> (length of purpose-specific data, purpose-specific data)
>
> Let's not create yet another unextendable non-standard standard.

It doesn't really have to be a standard at all.

Actually, I don't see why we are even trying to make the module
signature format compatible across kernel versions.  The module
payload is completely incompatible across kernel versions already.
Firmware is a different story, of course.

Also, I'll personally take some simple ad-hoc thing over PKCS#7 any
day.  I've tried reading the PKCS stuff.  90% is completely
inapplicable to anything the kernel (or the Internet in general, for
that matter) will ever do, and the other 10% is very poorly designed.

Heck, moving to NaCl format might be a good idea except for the
NIST/FIPS compliance part.

--Andy
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