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

Luis R. Rodriguez
2015-05-20T00:39:25Z

On Tue, May 19, 2015 at 04:42:05PM -0700, Andy Lutomirski wrote:
> On Tue, May 19, 2015 at 4:30 PM, Julian Calaby <julian.calaby@gmail.com> wrote:
> > Hi All,
> >
> > On Wed, May 20, 2015 at 6:59 AM, Andy Lutomirski <luto@kernel.org> 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.

As for having keys for fw signing be specific to fw data without a keyring,
if that is desirable I think we can devise a way to do that. For instance
if we wanted to we can have FW_SIG by default trust first keys on
system_trusted_keyring just as module signature works -- or if we wanted to
just trust, say a Kyle key. Not sure if the later is possible yet, but htat
would require some changes. Then as an evolution if we wanted to enable a
specific request fw to be mapped to a specific fw file the new APIs I was
looking to add could easily enable this provided that we first decide we
do want to trust say one key perhaps not on system_trusted_keyring for fw
signing. That'd need to be decided first.

As for the UEFI stuff -- from what I gather its too late there. We could
certainly go with something else for fw signing though, just lemme hear it
hard and clear.

> 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.

LSM'ify it I guess? Again, if that's reasonable then I think we'll need
stacking and that's still not merged.

Luis