[Ksummit-discuss] [TECH TOPIC] Firmware signing

[Andy Lutomirski]

On Tue, Jul 28, 2015 at 8:10 AM, James Bottomley
<James.Bottomley at hansenpartnership.com> wrote:
> On Tue, 2015-07-28 at 14:36 +0100, David Howells wrote:
>> Patches are in the works for the provision of signatures for firmware blobs
>> for the kernel to check, thus allowing the kernel to act as gatekeeper on what
>> firmware blobs get loaded where.
>>
>> Note that it has been agreed that signatures will be in separate files to the
>> firmware blobs so as not to potentially corrupt a blob by copying it to an OS
>> that doesn't expect the signature.  Also, we don't want to modify the blob in
>> case of IP.
>>
>> We're currently using PKCS#7/CMS messages as the signature format since we
>> have a PKCS#7 parser and verifier already in the kernel for kexec.
>>
>> Patches have been proposed for inclusion in security/next that allow PKCS#11
>> to be used to supply h/w keys to the sign-file program and to the kernel build
>> process.
>>
>> There are a number of areas that could do with sorting out with regard key
>> policy:
>>
>>  (1) Should signatures produced by the manager of the linux-firmware package
>>      be allowed only?
>
> Firmware is a binary blob usually with no decompilation.  How would the
> package manager know its provenance?  The only people who know are the
> people who write the driver.  If they sign, I think we have to accept
> their signature and if not, we could have a weaker level of trust on the
> package manager based on unbroken chain of transmission from the
> firmware provider, but you'd have to trust the packaging organisation.

But IMO we really don't want trust $RANDOM_USB_WIDGET_VENDOR to supply
firmware for the GPU, for example.

>>  (8) Can we then trust that key if we load it on the basis that a driver
>>      specifies it by public key hash, even it we can't chain back from it to
>>      the system_trusted_keyring.
>
> What's the point of tying to the system root of trust?
>

Agreed.  See about.  I don't think the concept of trust should be as
simple as "we trust" or "we don't trust" -- we should trust certain
vendors for certain purposes only.

>> If we do have this discussion, it would be useful to have some or all of Luis
>> Rodriguez, David Woodhouse, Andy Lutomirski, Kyle McMartin, Seth Forshee and
>> Mimi Zohar present.
>

I'll be there.

--Andy