On Wed, Jun 26, 2019 at 9:59 PM James Morris [off-list ref] wrote:

This is not a criticism of the patch but a related issue which I haven't
seen discussed (apologies if it has).

If signed code is loaded into ring 0, verified by the kernel, then
executed, you still lose your secure/trusted/verified boot state. If the
currently running kernel has been runtime-compromised, any signature
verification performed by the kernel cannot be trusted.

This problem is out of scope for the lockdown threat model (which
naturally cannot include a compromised kernel), but folk should be aware
that signature-verified kexec does not provide equivalent assurance to a
full reboot on a secure-boot system.

By that metric, on a secure boot system how do we determine that code
running in the firmware environment wasn't compromised before it
launched the initial signed kernel?