Date:	Thu, 21 May 2015 15:47:49 -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 3:31 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
> On Thu, May 21, 2015 at 3:24 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Thu, May 21, 2015 at 3:16 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
>>> On Thu, May 21, 2015 at 3:06 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>>> Given that, I would say that merely shoving firmware files through the
>>>> module verifier as-is would not be okay.
>>>
>>> Replacing one dog and pony show for another is what is going on, what
>>> you describe and suggest seems best, and I welcome patches, it seems
>>> you know what you are talking about :)
>>>
>>
>> Don't hold your breath.  My plate is over-full.  I'm probably a decent
>> reviewer of crypto, though.
>
> Well as good as you are in 10 years we'll have better ones. So when
> module signature went into the kernel the real expectation should have
> been:
>
> This code looks good now but is going to be complete shit and
> breakable a few years from now.
>
> Hence my first implicit and now explicit claims on dog and pony shows.
> Best thing we can do IMHO is to just allow us to replace stupid human
> code with better human code later, and eventually hopefully better AI
> code, and so on. Since you don't have time for a real replacement
> maybe what we can do is at least document / target / agree for what
> pipe dream we want and shoot for it with time. Hopefully folks will
> find time to implement it.

I disagree.  I'm a firm believer in security proofs.  While I'm not
trained in formal crypto proofs, I can sketch out a proof of why a
system that properly tags its signatures is secure against a
reasonable threat model.  I can also show why that proof wouldn't work
for a scheme without tags, and I can demonstrate the actual weakness
in a scheme without tags.

In ten years, the only reason a scheme that I say looks good would be
because (a) I screwed up, (b) an underlying assumption is wrong, or
(c) the implementation is subtly wrong.  In particular, it won't fail
because I'm insufficiently clever.

A real professional expert would be less likely to screw up.

(For reference, I wrote an actual doctoral thesis involving crypto.)

>
> In the meantime should that block current dog and pony show trading? I
> don't think so.

Yes, since I can demonstrate the actual weakness without tags, and
crypto is notoriously hard to fix once done poorly and there's a great
history of obviously-theoretically-weak systems being meaningfully
attacked in the real world.  See, for example, every single old
SSL/TLS cipher.  (And yes, the crypto community knew what was wrong in
theory and how to fix it when the protocol was designed.  People just
didn't pay attention.)

--Andy
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