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

Luis R. Rodriguez
2015-05-19T22:11:36Z

On Tue, May 19, 2015 at 01:59:36PM -0700, Andy Lutomirski 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*.

Indeed, that's not the only issue. For instance I recently found out that
system_trusted_keyring includes the keys under the UEFI key db, even though I
cannot see how that is done. This does ultimately mean that if MS or vendors
who have embedded keys on the UEFI key db get compromised their compromised key
can be used to sign arbitrary modules / firmware themselves. As bad as that is
I'm assured we can clear out the UEFI key db, so paranoid folks can do
that. I however do not think that this issue on trusting the keys in
the UEFI key db was readily known, or was it?.

> 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.

Agreed fully and I'm glad you bring this up. As-is with today's firmware_class
API we cannot allow custom keys if they are desired, but if we do want this we
can easily make this happen. Just let me know. With the last set of patches I
had developed my goal was to allow customizations by the callers on the key
requirements. I had this planned out because regulatory.bin used its own simple
RSA key with no x509 juju magic. I also envisioned it being easier for Kyle for
instance to use his own PGP key to sign linux-firmware files to start off with
than some complex x509 thing. Based on discussions with David, Seth, and Kyle
though it seems we were going to be happy with trusting Kyle's key for regulatory.bin,
since that will be done Kyle might as well sign all linux-firmware files and folks
who trust that can use it. If we want to override it should be easy to later.

For now then I'd be happy for us to start off with Kyle's key and
since I am adding extensions to the firmware_class API to customize
firmware signing requirements it should be fairly trivial to enable
device drivers to say they want their own firmware to be signed with
a specific key if they wanted that. I think that if we want this we
should evolve to this though as I think the complexity should be
optional.

> 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".

Sure.

> 2. Why on earth does the magic signing script reference things like
> commonName? Please keep X.509 silliness as far from the kernel as
> possible.

I asked the same thing. I'm hinted that Red Hat originally came up with PGP
support for module signing and then someone asked for x509 to be used. I was
not aware of this so originally in my approach for firmware I was trying to
target regular PGP keys. In my last review of the firmware patches I asked
to see what folks preferred and it seems that the issue with supporting
PGP for firmware and PKCS#7 for modules would be that we'd have different
entry paths to support. That means complexity, etc. Another point raised
was that we can convert PGP keys to x509 with gpgpsm, so if we really
needed that we could go about it that way.

I ended up being convinced that the simplicity over sharing code path for both
module signing and firmware signing with the flexibility to be paranoid and
using one own's PGP key and doing away with all possible evil-doers certs and
practices would suffice those who do not even trust their own parents. That is
of course unless using x509 stuff alone could jeopardize PGP keys... that'd
be a big problem, of course.

> 3. PKCS#1 v1.5, really? PKCS#1 v1.5 is known to be insecure unless
> very cafefully validated. For example:
>
> https://www.imperialviolet.org/2014/09/26/pkcs1.html
>
> Could we please consider using a signature scheme with a security proof?

I'm fine with going with some other alternative, now what do you have in mind?
I'm in hopes David can chime in here as I'm just relying on what new thing we
end up replacing for module signing.

Ideally we want to have the option to pick and choose or just stick to one
solution which we decide we'll vet for carefully for now.

> 4. As hashed to death in another thread:

"Hashed to death" is strong for something just discussed yesterday :)

> http://lkml.kernel.org/g/555A88FB.7000809@kernel.org
>
> I think that the verifier should be a dynamically loadable thing.

Did you just volunteeer?

> For an initial firmware signature verification scheme, I think that
> using digital signatures is fine, though

OK.

Luis