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

Mimi Zohar
2015-05-19T23:38:12Z

On Wed, 2015-05-20 at 00:19 +0200, Luis R. Rodriguez wrote:
> On Tue, May 19, 2015 at 05:48:37PM -0400, Mimi Zohar wrote:
> > On Tue, 2015-05-19 at 22:02 +0200, 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.
> >
> > Commit 13752fe "security: introduce kernel_fw_from_file hook" introduced
> > a new security hook. (IMA is on this hook as well.) Have you
> > considered using this hook?
>
> Yes, the same hook is used here.
>
> > Are there other places that this hook would need to be called?
>
> Nope, it'd be called. Folks who do not want to use key signing stuff can use
> their own preferred LSM hook just as module signing has the kernel module
> signing infrastructure but also module LSM hooks. It'd be similar here for
> firmware.

When the kernel module signing signature verification was upstreamed,
Rusty was not aware of IMA-appraisal -
https://lkml.org/lkml/2013/1/22/20 In this case, not only is there a
security hook, but the IMA hook exists as well. To appraise firmware,
add a line to the IMA policy containing "appraise func=FIRMWARE_CHECK".
Similarly, to add a measurement to the measurement list, add a line to
the IMA policy containing "measure func=FIRMWAE_CHECK".

> Now that we have LSM hooks stacked on the way perhaps this is more in line with
> what Andy has envisioned for alternatives for module signature verification.
> But then again since an LSM hook already exists for both modules and firmware
> perhaps this is sufficient for what Andy envisions? That is if folks do not want
> this signing thing just disable it and add use your preferred LSM module of choice?
>
> Now granted -- if we envision this module signing infrastructure as an LSM hook
> in and of itself perhaps we should LSM'ify it. Its not right now.
>
> > > I think we need one change here, we'd need to ensure that such key could only
> > > be used for vetting firmware files, not modules loaded. The firmware_class
> > > could for instance still use all the keys in system_trusted_keyring, which
> > > would include the UEFI key db, but it does not seems reasonable to expect keys
> > > used for fw signing to also go into system_trusted_keyring to also be used for
> > > module signing.
> >
> > I agree totally! For this reason, IMA defined a separate trusted
> > keyring to be used for verifying file signatures.
>
> OK I'll add that to my TODO list here.

You'll probably want to create a new trusted firmware keyring. By
trusted, only signed keys by a key on the system_keyring can be added to
the.ima keyring. Using the "ca_keys" boot command line option a
specific key on the system keyring can be specified.

Mimi