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

[Michal Suchánek]

Hello,

On Thu, Nov 11, 2021 at 05:26:41PM -0500, Nayna wrote:
> 
> On 11/8/21 07:05, Michal Suchánek wrote:
> > Hello,
> > 
> > On Mon, Nov 08, 2021 at 09:18:56AM +1100, Daniel Axtens wrote:
> > > Michal Suchánek <msuchanek@xxxxxxx> writes:
> > > 
> > > > On Fri, Nov 05, 2021 at 09:55:52PM +1100, Daniel Axtens wrote:
> > > > > Michal Suchanek <msuchanek@xxxxxxx> 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 that is the root of the problem here. Until distributions pick
> > up IMA and properly document step by step in detail how to implement,
> > enable, and debug it the _SIG options are required for users to be able
> > to make use of signatures.
> 
> For secureboot, IMA appraisal policies are configured in kernel at boot time
> based on secureboot state of the system, refer
> arch/powerpc/kernel/ima_arch.c and security/integrity/ima/ima_efi.c. This
> doesn't require any user configuration. Yes, I agree it would be helpful to
> update kernel documentation specifying steps to sign the kernel image using
> sign-file.
> 
> > 
> > The other part is that distributions apply 'lockdown' patches that change
> > the security policy depending on secure boot status which were rejected
> > by upstream which only hook into the _SIG options, and not into the IMA_
> > options. Of course, I expect this to change when the IMA options are
> > universally available across architectures and the support picked up by
> > distributions.
> > 
> > Which brings the third point: IMA features vary across architectures,
> > and KEXEC_SIG is more common than IMA_KEXEC.
> > 
> > config/arm64/default:CONFIG_HAVE_IMA_KEXEC=y
> > config/ppc64le/default:CONFIG_HAVE_IMA_KEXEC=y
> > 
> > config/arm64/default:CONFIG_KEXEC_SIG=y
> > config/s390x/default:CONFIG_KEXEC_SIG=y
> > config/x86_64/default:CONFIG_KEXEC_SIG=y
> > 
> > KEXEC_SIG makes it much easier to get uniform features across
> > architectures.
> 
> Architectures use KEXEC_SIG vs IMA_KEXEC based on their requirement.
> IMA_KEXEC is for the kernel images signed using sign-file (appended
> signatures, not PECOFF), provides measurement along with verification, and

That's certainly not the case. S390 uses appended signatures with
KEXEC_SIG, arm64 uses PECOFF with both KEXEC_SIG and IMA_KEXEC.

> is tied to secureboot state of the system at boot time.

In distrubutions it's also the case with KEXEC_SIG, it's only upstream
where this is different. I don't know why Linux upstream has rejected
this support for KEXEC_SIG.

Anyway, sounds like the difference is that IMA provides measurement but
if you don't use it it does not makes any difference except more comlex
code.

Thanks

Michal