linux-kernel - Re: Should we automatically generate a module signing key at all?

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Message-ID: <CALCETrWvmV0Qn-qSG0ROg913Cu2va2FYbe2bihmFW4rriU0GVA@mail.gmail.com>
Date:	Fri, 22 May 2015 13:44:37 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	George Spelvin <linux@...izon.com>,
	David Howells <dhowells@...hat.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	LSM List <linux-security-module@...r.kernel.org>,
	petkan@...-labs.com, "Theodore Ts'o" <tytso@....edu>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>
Subject: Re: Should we automatically generate a module signing key at all?

On Fri, May 22, 2015 at 1:40 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, May 22, 2015 at 7:13 AM, George Spelvin <linux@...izon.com> wrote:
>>
>> 1) Create a tool to canonicalize the kernel and modules,
>>    stripping out the signatures before comparing them.  This has
>>    precedent in the way the prelink tool can un-prelink binaries
>>    so that hashes can be verified.
>
> So I'd obviously prefer this, so that we have just one model for verification.
>

In the threat model where module signatures matter in the first place
[1], this prevents reproducible builds from serving their purpose.  I
can build a kernel with a fresh signing key and throw away the private
key.  You can build a canonically identical kernel with a private key
that you keep.  A third party using mine is safe, but a third party
using yours is unsafe, even though the whole packages canonicalize to
exactly the same bytes.

[1] I still think this is a silly threat model, but many people
disagree with me.

--Andy
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