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

Mimi Zohar
2021-09-09T13:17:04Z

On Wed, 2021-09-08 at 16:25 -0600, Eric Snowberg wrote:
> > 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.

Instead of making the reviewer look up the failed attempts, please
summarize why they failed (e.g. all preboot firmware keys were
trusted), and then continue, like below, with how this attempt differs.

>
> This series introduces a new Linux kernel keyring containing the Machine
> Owner Keys (MOK) called .machine.

Other archs will also want to allow loading "end-users" key. Please
prefix this paragraph with something like "On UEFI based systems".

> It also adds a new MOK variable to shim.

Replace "adds" with "defines".

> 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.

Thank you for crediting me for limiting loading only the CA keys stored
in the MOK db onto the "machine" keyring, but the limitation should be
better integrated in the paragraph.

> 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

To differentiate this "signing_key" from others, perhaps name the file
"machine_sigining_key" or "local_signing_key".

thanks,

Mimi

>
> 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