Re: [RFD] linux-firmware key arrangement for firmware signing

[Andy Lutomirski]

On Tue, May 19, 2015 at 4:30 PM, Julian Calaby <julian.calaby@xxxxxxxxx> wrote:
> Hi All,
>
> On Wed, May 20, 2015 at 6:59 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> [added cc's from the other thread]
>>
>> On 05/19/2015 01:02 PM, Luis R. Rodriguez wrote:
>>>
>>> David Howells has posted v4 of his series of supporting PKCS#7 for module
>>> signing. I'm in my v3 series now on RFCs for firmware PKCS#7 support, and
>>> after
>>> some review and patch shuffling I think this is ready for patch form.  My
>>> own
>>> series however depend on quite a bit of other pending changes, one series
>>> which
>>> will go through Rusty's tree, another series of fixes on firmware_class
>>> which
>>> should go through Greg's tree. I'll wait until all this and David's own
>>> patches
>>> get merged before posting firmware PKCS#7 support. Before all this though
>>> in
>>> preparation for fw signing one thing we should start to talk about more
>>> broadly
>>> however is how linux-firmware binary file signing would work in practice
>>> and
>>> what we need, and make sure folks are OK with all this.
>>>
>>> First, firmware signing will be completely optional as with module
>>> signing.
>>>
>>
>> ...
>>
>>> Other than this last nitpick, any other concerns or recommendations ?
>>
>>
>> A couple.  Some of these are general concerns with the existing
>> infrastructure, but #1 is a specific problem that gets much worse if we add
>> firmware signing.  Feel free to ignore 2-4.
>>
>> 1. We should get the signature semantics right.  I think that, for modules,
>> we currently sign literally the module payload.  For modules, in my
>> semi-amateurish crypto universe [1], this is fine *as long as the key in
>> question is used for no other purpose*.  For firmware, it's dangerous, since
>> it would be vulnerable to substitution attacks in which the adversary
>> convinces us to interpret one firmware file as firmware for another device
>> or purpose entirely.
>>
>> We should be signing something that's semantically equivalent to "This is a
>> valid module: xyz", "This is a valid 'regulatory.bin': xyz", or "This is a
>> valid kexec image: xyz".
>
> Something that occurred to me (as a complete bystander) was: would it
> make sense to have keys able to be restricted to particular "types" of
> signable data? I.e. the key that can sign a valid regulatory.bin file
> cannot be used to sign a module or a kexec image. - This could remove
> the need to have multiple keyrings. (Also, UEFI keys unless otherwise
> tagged could be restricted to only signing bootloaders or kernels)

Seems sensible to me.

FWIW, I'm starting to think that UEFI-based validation of kexec images
should be totally separate.  It uses a nasty PE format with a hideous
PKCS #7 formatted signature.  Maybe that should be a completely
separate piece of code.

>
> Also, are multiple signatures a sensible thing? E.g. regulatory.bin
> gets signed by Seth, then Kyle, then $DISTRO and any one key is enough
> to validate it.

That might further complicate matters.

--Andy
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