security: Add a static lockdown policy LSM

While existing LSMs can be extended to handle lockdown policy,
distributions generally want to be able to apply a straightforward
static policy. This patch adds a simple LSM that can be configured to
reject either integrity or all lockdown queries, and can be configured
at runtime (through securityfs), boot time (via a kernel parameter) or
build time (via a kconfig option). Based on initial code by David
Howells.

Signed-off-by: Matthew Garrett <mjg59@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

include/linux/security.h

@@ -97,6 +97,9 @@ enum lsm_event {
  * potentially a moving target. It is easy to misuse this information
  * in a way that could break userspace. Please be careful not to do
  * so.
+ *
+ * If you add to this, remember to extend lockdown_reasons in
+ * security/lockdown/lockdown.c.
  */
 enum lockdown_reason {
 	LOCKDOWN_NONE,