Firmware signing -- Re: [PATCH 00/27] security, efi: Add kernel lockdown

[Luis R. Rodriguez]

On Wed, Nov 08, 2017 at 03:15:54PM +0900, AKASHI, Takahiro wrote:
> Luis,
> 
> Thank you for this heads-up.
> 
> On Wed, Nov 08, 2017 at 12:07:00AM +0100, Luis R. Rodriguez wrote:
> > On Thu, Nov 02, 2017 at 06:10:41PM -0400, Mimi Zohar wrote:
> > > On Thu, 2017-11-02 at 22:04 +0000, David Howells wrote:
> > > > Mimi Zohar <zohar at linux.vnet.ibm.com> wrote:
> > > > 
> > > > > > Only validly signed device firmware may be loaded.
> > > > > 
> > > > > fw_get_filesystem_firmware() calls kernel_read_file_from_path() to
> > > > > read the firmware, which calls into the security hooks. Is there
> > > > > another place that validates the firmware signatures.  I'm not seeing
> > > > > which patch requires firmware to be signed?
> > > > 
> > > > Luis has a set of patches for this.  However, I'm not sure if that's going
> > > > anywhere at the moment.  Possibly I should remove this from the manpage for
> > > > the moment.
> > 
> > Remove it for now. The state of of affairs for firmware signing is complex given
> > that we first wanted to address how to properly grow the API without making
> > the API worse. This in and of itself was an effort, and that effort also
> > evaluated two different development paradigms:
> > 
> > 	o functional API
> > 	o data driven API
> > 
> > I only recently was convinced that functional API should be used, even for
> > commonly used exported symbols,
> 
> Are you?

Yes, this stemmed from the fact that even system calls can be abused through
data driven APIs, and that long term a functional API at least can make
evolutions much easier to review and bisect.

That said this is all based on *empirical hearsay*, and no formal observations.
But the ease to more easily bisect long term is enough for me to consider this 
for the firmware API given subtle regressions have been a long standing pain
on the firmware API and I would not want to make the process of bisecting
any harder.

If anyone *does* have actual efforts which compares and contrasts both, I'd
love to get them, to further back my current position, but as-is I'm already
sold on functional API driven interface.

> I haven't answered Linus' question, but my concern about functional APIs,
> as far as firmware signing goes, is that we have no way to _enforce_
> firmware signing to existing (i.e. verification-unaware) drivers if we need
> an explicit call of a function, say, verify_firmware().

Your concern seems to be that a functional driven API for firmware signing would
implicate having to support verify_firmware() for drivers which *cannot* get
signed firmware verified, is that correct?

IMHO that should just fail then, ie, a "locked down" kernel should not want to
*pass* a firmware signature if such thing could not be done.

Its no different than trying to verify a signed module on a "locked down" for
which it has no signature.

But perhaps I'm not understanding the issue well, let me know.

> > and as such I've been going back and slowly
> > grooming the firmware API with small atomic changes to first clean up the
> > complex flag mess we have.
> > 
> > Since I'm busy with that Takahiro AKASHI has taken up firmware singing effort
> > but this will depend on the above small cleanup to be done first. I was busy
> > with addressing existing bugs on the firmware API for a while, then company
> > travel / conferences so was not able to address this, but I'm back now and
> > I believe I should be able to tackle the cleanup now.
> 
> Good to hear.
> 
> > Only after this is merged can we expect a final respin of the firmware signing
> > effort.
> > 
> > > Or reflect that IMA-appraisal, if enabled, will enforce firmware being
> > > validly signed.
> > 
> > But FWICT lockdown is a built-in kernel thingy, unless lockdown implies IMA
> > it would not be the place to refer to it.
> 
> I think that the situation is the same as in module signing.

But by definition a "locked down" kernel will enforce module signing, and it
would seem logical that if modules signing is enforced, eventually a statement
about firmware signing can be added, as it relates to a kernel enforcement
effort, not a kernel-userpace framework such as IMA.

  Luis

> -Takahiro AKASHI
> 
> > It seems the documentation was proposed to help users if an error was caught.
> > That error should cover only what is being addressed in code on the kernel.
> > 
> >   Luis
> 

-- 
Luis Rodriguez, SUSE LINUX GmbH
Maxfeldstrasse 5; D-90409 Nuernberg
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