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

[Julian Calaby]

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)

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.

Thanks,

-- 
Julian Calaby

Email: julian.calaby@xxxxxxxxx
Profile: http://www.google.com/profiles/julian.calaby/
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