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

[AKASHI, Takahiro]

On Wed, Nov 08, 2017 at 08:46:26PM +0100, Luis R. Rodriguez wrote:
> 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.

My point is quite simple:
my_deviceA_init() {
        err = request_firmware(&fw, "deviceA"); <--- (a)
        if (err)
                goto err_request;

        err = verify_firmware(fw);  <--- (b)
        if (err)
                goto err_verify;

        load_fw_to_deviceA(fw);     <--- (c)
        ...
}

As legacy device drivers does not have (b), there is no chance to
prevent loading a firmware at (c) for locked-down kernel.

If you allow me to bring in yet another function, say
request_firmware_signable(), which should be used in place of (a)
for all verification-aware drivers, that would be fine.
In this case, all the invocation of request_firmware() in legacy code
could be forced to fail in locked-down kernel.

But I think that "signable" should be allowed to be combined with other
features of request_firmware variants like _(no)wait or _direct.

-Takahiro AKASHI

> 
> > > 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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