Subject: Re: [PATCH 0/3] KEXEC_SIG with appended signature

From: Daniel Axtens <[email-masked]>
Date: Mon, 08 Nov 2021 09:18:56 +1100

Michal Suchánek <[email-masked]> writes:

> On Fri, Nov 05, 2021 at 09:55:52PM +1100, Daniel Axtens wrote:
>> Michal Suchanek <[email-masked]> writes:
>> 
>> > S390 uses appended signature for kernel but implements the check
>> > separately from module loader.
>> >
>> > Support for secure boot on powerpc with appended signature is planned -
>> > grub patches submitted upstream but not yet merged.
>> 
>> Power Non-Virtualised / OpenPower already supports secure boot via kexec
>> with signature verification via IMA. I think you have now sent a
>> follow-up series that merges some of the IMA implementation, I just
>> wanted to make sure it was clear that we actually already have support
>
> So is IMA_KEXEC and KEXEC_SIG redundant?
>
> I see some architectures have both. I also see there is a lot of overlap
> between the IMA framework and the KEXEC_SIG and MODULE_SIg.

Mimi would be much better placed than me to answer this.

The limits of my knowledge are basically that signature verification for
modules and kexec kernels can be enforced by IMA policies.

For example a secure booted powerpc kernel with module support will have
the following IMA policy set at the arch level:

"appraise func=KEXEC_KERNEL_CHECK appraise_flag=check_blacklist appraise_type=imasig|modsig",
(in arch/powerpc/kernel/ima_arch.c)

Module signature enforcement can be set with either IMA (policy like
"appraise func=MODULE_CHECK appraise_flag=check_blacklist appraise_type=imasig|modsig" )
or with CONFIG_MODULE_SIG_FORCE/module.sig_enforce=1.

Sometimes this leads to arguably unexpected interactions - for example
commit fa4f3f56ccd2 ("powerpc/ima: Fix secure boot rules in ima arch
policy"), so it might be interesting to see if we can make things easier
to understand.  I suspect another amusing interaction is that if the IMA
verification is used, the signature could be in an xattr rather than an
appended signature.

Kind regards,
Daniel