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

Luis R. Rodriguez
2015-05-21T22:26:33Z

On Tue, May 19, 2015 at 05:41:37PM -0700, Andy Lutomirski wrote:
> On Tue, May 19, 2015 at 5:39 PM, Luis R. Rodriguez <mcgrof@suse.com> wrote:
> > 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.
>
> Isn't stacking backwards for this, though? The semantics we'd want is
> accept if any verifiers accept, not accept if all verifiers accept,
> right?

That can be added, and if stacking is not yet merged perhaps Casey can
consider it?

Luis