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

[Andy Lutomirski <luto@amacapital.net>]

On Tue, May 19, 2015 at 11:50 AM, David Howells <dhowells@redhat.com> wrote:
> Andy Lutomirski <luto@amacapital.net> wrote:
>
>> No, in the hash tree variant, it really is 32 bytes.  No one ever
>> needs the full list once the build is done.
>
> Yes, you do.  You have to check the hash on the hash list or you can't trust
> it.
>

No, you don't :)  See below.

On Tue, May 19, 2015 at 11:44 AM, David Howells <dhowells@redhat.com> wrote:
> Andy Lutomirski <luto@amacapital.net> wrote:
>
>> The actual runtime code needed to implement a hash tree solution is
>> maybe twenty lines.  The bzImage will be smaller,
>
> But the initramfs image will be bigger because it will have to carry the
> entire module hash list just in case any particular module needs to get loaded
> from the initramfs.  You have to carry the entire hash set so that you can
> hash it and compare against the one hash in the vmlinux file.
>

No.  Here's one way it could work:

Suppose you have a depth-k tree (i.e. up to 2^k modules).  We'll
compute a 32-byte value Tree(d, i) for each d from 0 to k and each i
from 0 to 2^d-1.  First you assign each module an index starting at
zero (with the maximum index less than 2^k).  Then you hash each
module.

To generate the leaves (i.e. nodes at depth k), you compute, for each
i, Tree(k, i) = H(k, i, H(module payload)).  For leaves that don't
correspond to modules, you use some placeholder.

For the ith node at lower depth, compute Tree(d, i) = H(k-1, i,
Tree(d+1, 2*i), Tree(d+1, 2*i+1)).

The proof associated with module i is Tree(k, i^1), Tree(k-1,
(i>>1)^1), Tree(k-2, (i>>2)^1), etc, up through depth 1.  Tree(0, 0)
is built into the kernel.

Variants of this scheme are possible.  Don't emulate Amazon or
Bittorrent here, though -- they both managed to screw up the crypto.

> And that doesn't include the issue of hashing the firmware blobs you might
> need.

As before, that's true.  To verify firmware, either you need to hash
it, use a termporary signing key, or use a long-term signing key.
Choose your poison.  I still prefer a hash over a temporary signing
key.

>
>> With your proposal, I need to trust that whoever built the actual
>> running kernel image really did throw away the key.  If they didn't,
>> then under whatever threat model requires that I enable module
>> verification, I'm screwed -- the bad guy has the private key.
>
> Each private key is used for one single kernel, so if they steal one, you can
> blacklist it if you have the capability (eg. UEFI) and change your kernel.
>

How do you know it was stolen?

--Andy