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

Jarkko Sakkinen
2021-09-08T16:03:28Z

On Tue, 2021-09-07 at 12:00 -0400, Eric Snowberg wrote:
> Many UEFI Linux distributions boot using shim. The UEFI shim provides
> what is called Machine Owner Keys (MOK). Shim uses both the UEFI Secure
> Boot DB and MOK keys to validate the next step in the boot chain. The
> MOK facility can be used to import user generated keys. These keys can
> be used to sign an end-user development kernel build. When Linux boots,
> pre-boot keys (both UEFI Secure Boot DB and MOK keys) get loaded in the
> Linux .platform keyring.
>
> Currently, pre-boot keys are not trusted within the Linux trust boundary
> [1]. These platform keys can only be used for kexec. If an end-user

What exactly is "trust boundary"? And what do you mean when you say that
Linux "trusts" something? AFAIK, software does not have feelings. Please,
just speak about exact things.

That's the whole point of the cover letter. It's better to not have cover
letter at all, than a confusing cover letter that reads like a white paper.
Code changes at least tell the exact story, and not speak about feelings.

> wants to use their own key within the Linux trust boundary, they must
> either compile it into the kernel themselves or use the insert-sys-cert
> script. Both options present a problem. Many end-users do not want to
> compile their own kernels. With the insert-sys-cert option, there are
> missing upstream changes [2]. Also, with the insert-sys-cert option,
> the end-user must re-sign their kernel again with their own key, and
> then insert that key into the MOK db. Another problem with
> insert-sys-cert is that only a single key can be inserted into a
> compressed kernel.

I use a pre-compiled kernel in my desktop: https://liquorix.net/. When
a new version comes up it requires a sbsign one-liner to sign it for
secure boot. I'm wondering what is the problem I'm facing because I do
not see it.

If there are something missing changes that you use as a rationale for
this large patch set, you should at least broadly explain what we are
missing. How I conclude this paragraph is that, since there is only an
xref, they are not really "that important" changes, which are missing.

> Having the ability to insert a key into the Linux trust boundary opens
> up various possibilities. The end-user can use a pre-built kernel and
> sign their own kernel modules. It also opens up the ability for an

Which both can be done by end-user as of today, or I'm misreading this.

> end-user to more easily use digital signature based IMA-appraisal. To
> get a key into the ima keyring, it must be signed by a key within the
> Linux trust boundary.

What is IMA appraisal? I just don't know it because I don't use IMA.
Again, this trust boundary is really something I do not. Looking at
code changes, you could just speak about exact assets in the kernel.

> 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.

/Jarkko