Re: [PATCH 4/4] module, KEYS: Make use of platform keyring for signature verification

On Mar 28, 2022, at 4:15 AM, joeyli [off-list ref] wrote:

Hi Mimi,

Sorry for bother you for this old topic.

On Tue, Feb 15, 2022 at 09:47:30PM +0100, Michal Suchánek wrote:

quoted

Hello,

On Tue, Feb 15, 2022 at 03:08:18PM -0500, Mimi Zohar wrote:

quoted

[Cc'ing Eric Snowberg]

Hi Michal,

On Tue, 2022-02-15 at 20:39 +0100, Michal Suchanek wrote:

quoted

Commit 278311e417be ("kexec, KEYS: Make use of platform keyring for signature verify")
adds support for use of platform keyring in kexec verification but
support for modules is missing.

Add support for verification of modules with keys from platform keyring
as well.

Permission for loading the pre-OS keys onto the "platform" keyring and
using them is limited to verifying the kexec kernel image, nothing
else.

Why is the platform keyring limited to kexec, and nothing else?

It should either be used for everything or for nothing. You have the
option to compile it in and then it should be used, and the option to
not compile it in and then it cannot be used.

There are two basic use cases:

(1) there is a vendor key which is very hard to use so you sign
something small and simple like shim with the vendor key, and sign your
kernel and modules with your own key that's typically enrolled with shim
MOK, and built into the kernel.

(2) you import your key into the firmware, and possibly disable the
vendor key. You can load the kernel directly without shim, and then your
signing key is typically in the platform keyring and built into the
kernel.

In the second use case, if user can enroll their own key to db either before
or after hardware shipping. And they don't need shim because they removed
Microsoft or OEM/ODM keys.  Why kernel can not provide a Kconfig option to
them for trusting db keys for verifying kernel module, or for IMA (using CA
in db)?

In the above use case for distro, partner doesn't need to re-compiler distro
kernel. They just need to re-sign distro kernel and modules. Which means
that the partner trusted distro. Then the partner's key in db can be used to
verify kernel image and also kernel module without shim involve.

If shim is used, the new machine keyring can be used to solve this problem. 
This pull request [1] allows additional certificates to be loaded into the MOKList 
without going through MokManager.  Have the end-user/partner create a 
shim_certificate.efi containing their key. Then sign it with their DB key.  When 
shim boots, it will validate shim_certificate.efi against the DB key and load the 
key contained within it into the MOKList.  Now both module and kernel validation 
can be performed with this key, since it is contained within the machine keyring.

[1] https://github.com/rhboot/shim/pull/446

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