selinux: convert policy read-write lock to RCU

Convert the policy read-write lock to RCU.  This is significantly
simplified by the earlier work to encapsulate the policy data
structures and refactor the policy load and boolean setting logic.
Move the latest_granting sequence number into the selinux_policy
structure so that it can be updated atomically with the policy.
Since removing the policy rwlock and moving latest_granting reduces
the selinux_ss structure to nothing more than a wrapper around the
selinux_policy pointer, get rid of the extra layer of indirection.

At present this change merely passes a hardcoded 1 to
rcu_dereference_check() in the cases where we know we do not need to
take rcu_read_lock(), with the preceding comment explaining why.
Alternatively we could pass fsi->mutex down from selinuxfs and
apply a lockdep check on it instead.

Based in part on earlier attempts to convert the policy rwlock
to RCU by Kaigai Kohei [1] and by Peter Enderborg [2].

[1] https://lore.kernel.org/selinux/6e2f9128-e191-ebb3-0e87-74bfccb0767f@tycho.nsa.gov/
[2] https://lore.kernel.org/selinux/20180530141104.28569-1-peter.enderborg@sony.com/

Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Reviewed-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

Author: stephensmalley

security/selinux/hooks.c

@@ -7235,7 +7235,6 @@ static __init int selinux_init(void)
 	memset(&selinux_state, 0, sizeof(selinux_state));
 	enforcing_set(&selinux_state, selinux_enforcing_boot);
 	selinux_state.checkreqprot = selinux_checkreqprot_boot;
-	selinux_ss_init(&selinux_state.ss);
 	selinux_avc_init(&selinux_state.avc);
 	mutex_init(&selinux_state.status_lock);
 

security/selinux/include/security.h

@@ -13,6 +13,7 @@
 #include <linux/dcache.h>
 #include <linux/magic.h>
 #include <linux/types.h>
+#include <linux/rcupdate.h>
 #include <linux/refcount.h>
 #include <linux/workqueue.h>
 #include "flask.h"
@@ -84,7 +85,6 @@ extern int selinux_enabled_boot;
 #define POLICYDB_BOUNDS_MAXDEPTH	4
 
 struct selinux_avc;
-struct selinux_ss;
 struct selinux_policy;
 
 struct selinux_state {
@@ -102,10 +102,9 @@ struct selinux_state {
 	struct mutex status_lock;
 
 	struct selinux_avc *avc;
-	struct selinux_ss *ss;
+	struct selinux_policy __rcu *policy;
 } __randomize_layout;
 
-void selinux_ss_init(struct selinux_ss **ss);
 void selinux_avc_init(struct selinux_avc **avc);
 
 extern struct selinux_state selinux_state;