[RFC] Turn lockdown into an LSM

[Matthew Garrett]

Hi James,

This is a quick attempt to integrate lockdown into the existing LSM
framework. It adds a new lockdown security hook and an LSM that defines
the existing coarse-grained policy, and also adds a new
DEFINE_EARLY_LSM() definition in order to permit lockdown (and
potentially other modules) to be initialised at the top of kernel init
in order to allow policy to be imposed on stuff that happens in
setup_arch(). The goal here is to allow policy to be devolved to other
LSMs on systems that have a secure mechanism for loading LSM policy
early in boot, allowing creation of arbitrarily complicated policies
without interfering with the common-case coarse-grained approach.

This should probably be extended so a uapi-exposed constant is passed to
the hook in order to make it easier to write policy in other LSMs, but
does this broadly look like you were imagining?