Re: [RFC PATCH 0/3] Add additional MOK vars

Mimi Zohar
2021-05-19T14:33:00Z

Hi Eric,

On Mon, 2021-05-17 at 18:57 -0400, Eric Snowberg wrote:
> This series is being sent as an RFC. I am looking for feedback; if
> adding additional MOK variables would be an acceptable solution to help
> downstream Linux distros solve some of the problems we are facing?
>
> Currently, pre-boot keys are not trusted within the Linux boundary [1].
> Pre-boot keys include UEFI Secure Boot DB keys and MOKList keys. These
> keys are loaded into the platform keyring and can only be used for kexec.
> If an end-user 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.
>
> 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
> 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.
>
> 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. In addition, most downstream kernels
> do not have an easy way for an end-user to use digital signature based
> IMA-appraisal.
>
> This series adds two new MOK variables to shim. The first variable
> allows the end-user to decide if they want to trust keys contained
> within the platform keyring within the Linux trust boundary. By default,
> nothing changes; platform keys are not trusted within the Linux kernel.
> They are only trusted after the end-user makes the decision themself.
> The end-user would set this through mokutil using a new --trust-platform
> 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.
>
> The second MOK variable allows a downstream Linux distro to make
> better use of the IMA architecture specific Secure Boot policy. This
> IMA policy is enabled whenever Secure Boot is enabled. By default, this
> new MOK variable is not defined. This causes the IMA architecture
> specific Secure Boot policy to be disabled. Since this changes the
> current behavior, it is placed behind a new Kconfig option. Kernels
> built with IMA_UEFI_ARCH_POLICY enabled would allow the end-user
> to enable this through mokutil using a new --ima-sb-enable option [3].
> This gives the downstream Linux distro the capability to offer the
> IMA architecture specific Secure Boot policy option, while giving
> the end-user the ability to decide if they want to use it.
>
> I have included links to both the mokutil [3] and shim [4] changes I
> made to support this new functionality.
>
> Thank you and looking forward to hearing your reviews.

This patch set addresses two very different issues - allowing keys on
the platform keyring to be trusted for things other than verifying the
kexec kernel image signature, overwriting the arch specific IMA secure
boot policy rules. The only common denominator is basing those
decisions on UEFI variables, which has been previously suggested and
rejected. The threat model hasn't changed.

The desire for allowing a single local CA key to be loaded onto a
trusted keyring is understandable. A local CA key can be used to sign
certificates, allowing them to be loaded onto the IMA keyring. What is
the need for multiple keys?

Making an exception for using a UEFI key for anything other than
verifying the kexec kernel image, can not be based solely on UEFI
variables, but should require some form of kernel
agreement/confirmation. If/when a safe mechanism for identifying a
single local CA key is defined, the certificate should be loaded
directly onto the secondary keyring, not linked to the platform
keyring.

The system owner can enable/disable secure boot. Disabling the arch
secure boot IMA policy rules is not needed. However, another mechanism
for enabling them would be acceptable.

thanks,

Mimi