Re: [PATCH v5 00/12] Enroll kernel keys thru MOK

Eric Snowberg
2021-09-08T22:26:29Z

> On Sep 8, 2021, at 10:49 AM, Jarkko Sakkinen <jarkko@kernel.org> wrote:
>
> On Wed, 2021-09-08 at 19:03 +0300, Jarkko Sakkinen wrote:
>>
>>> Downstream Linux distros try to have a single signed kernel for each
>>> architecture. Each end-user may use this kernel in entirely different
>>> ways. Some downstream kernels have chosen to always trust platform keys
>>> within the Linux trust boundary for kernel module signing. These
>>> kernels have no way of using digital signature base IMA appraisal.
>>>
>>> This series introduces a new Linux kernel keyring containing the Machine
>>> Owner Keys (MOK) called .machine. It also adds a new MOK variable to shim.
>>> This variable allows the end-user to decide if they want to trust keys
>>> enrolled in the MOK within the Linux trust boundary. By default,
>>> nothing changes; MOK keys are not trusted within the Linux kernel. They
>>> are only trusted after the end-user makes the decision themselves. The
>>> end-user would set this through mokutil using a new --trust-mok option
>>> [3]. This would work similar to how the kernel uses MOK variables to
>>> enable/disable signature validation as well as use/ignore the db.
>>
>> OK, changes are described here (again speaking about trusting tho). The
>> motivation part is missing. The text before this is more like confusion
>> part. When you describe motivation to do something you should really be in
>> grass roots, e.g. "when you have this feature in the kernel, look, I can
>> do now this". It's not that hard. E.g. with an usage example it is quite
>> quick accomplish this.
>
> The code changes overally make sense but this motivotional part is the
> problem. E.g. if you do a pull request, it is completely *unusable* in
> that context. In that case I would have to write something that should
> have been the cover letter. It's 12 patches, so it is perfectly sensible
> to ask a better one.

Would this be a more appropriate cover letter that includes a better
motivation?

Back in 2013 Linus requested a feature to allow end-users to have the
ability "to add their own keys and sign modules they trust". This was his
*second* order outlined here [1]. There have been many attempts over the
years to solve this problem, all have been rejected. Many distributions
carry one of these rejected attempts. This series tries to solve this problem
with a solution that takes into account all the problems brought up in the
previous attempts.

This series introduces a new Linux kernel keyring containing the Machine
Owner Keys (MOK) called .machine. It also adds a new MOK variable to shim.
This variable allows the end-user to decide if they want to load keys
enrolled in the MOK within the Linux kernel. By default, nothing changes;
MOK keys are not loaded within the Linux kernel. They are only loaded after
the end-user makes the decision themselves. The end-user would set this
through mokutil using a new --trust-mok option [2]. This would work similar
to how the kernel uses MOK variables to enable/disable signature validation
as well as use/ignore the db. Mimi has suggested that only CA keys be loaded
into this keyring. All other certs will load into the platform keyring instead.

Secure Boot keys will never be loaded. They will always be loaded into
the platform keyring. If an end-user wanted to load one, they would
need to enroll it into the MOK.

Steps required by the end user:

Sign kernel module with user created key:
$ /usr/src/kernels/$(uname -r)/scripts/sign-file sha512 signing_key.priv \
signing_key.x509 my_module.ko

Import the key into the MOK
$ mokutil --import signing_key.x509

Setup the kernel to load MOK keys into the .machine keyring
$ mokutil —trust-mok

Then reboot, the MokManager will load and ask if you want to trust the MOK
key and enroll the MOK into the MOKList. Afterwards the signed kernel module
will load.

I have included links to both the mokutil [2] and shim [3] changes I
have made to support this new functionality.

[1] https://marc.info/?l=linux-kernel&m=136185386310140&w=2
[2] https://github.com/esnowberg/mokutil/tree/0.3.0-mokvars-v2
[3] https://github.com/esnowberg/shim/tree/mokvars-v2