kexec_load() + reboot is intended to be equivalent to a system reboot.
However kexec_load() can load arbitrary data as the target kernel,
leading to execution of arbitrary code, even though it's effectively in
a new context. Rather than being equivalent to a system reboot, it's
also equivalent to kldload(), which loads arbitrary code into the
running kernel. Since kldload() is blocked at securelevel 1, also block
kexec_load().
Reported by: markj
Fixes: e02c57ff3 ("kern: Introduce kexec system feature (MI)")
MFC after: 2 weeks
Sponsored by: Hewlett Packard Enterprise