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

Nayna
2021-11-16T00:16:04Z

On 11/12/21 03:30, Michal Suchánek wrote:
> 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@suse.de> writes:
>>>>
>>>>> On Fri, Nov 05, 2021 at 09:55:52PM +1100, Daniel Axtens wrote:
>>>>>> Michal Suchanek <msuchanek@suse.de> 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.

Yes, S390 uses appended signature, but they also do not support
measurements.

On the other hand for arm64/x86, PECOFF works only with KEXEC_SIG. Look
at the KEXEC_IMAGE_VERIFY_SIG config dependencies in arch/arm64/Kconfig
and KEXEC_BZIMAGE_VERIFY_SIG config dependencies in arch/x86/Kconfig.
Now, if KEXEC_SIG is not enabled, then IMA appraisal policies are
enforced if secure boot is enabled, refer to
security/integrity/ima_efi.c . IMA would fail verification if kernel is
not signed with module sig appended signatures or signature verification
fails.

In short, IMA is used to enforce the existence of a policy if secure
boot is enabled. If they don't support module sig appended signatures,
by definition it fails. Thus PECOFF doesn't work with both KEXEC_SIG and
IMA_KEXEC, but only with KEXEC_SIG.

Lakshmi, do you agree with my reasoning ?

>
>> 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.
I am unsure what do you mean by "complex code" here. Can you please
elaborate ? IMA policies support for secureboot already exists and can
be used as it is without adding any extra work as in
arch/powerpc/kernel/ima_arch.c.

Also, if my analysis is right, I think I understand arm64/x86 support
for both KEXEC_SIG and IMA_KEXEC as it can support two signature formats
- PECOFF/module sig appended signature.

I am not clear from the patch descriptions on the need to add KEXEC_SIG
support on POWER when that will also be based on module sig appended
signatures like IMA_KEXEC.

Thanks & Regards,

- Nayna