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


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