device_cgroup: fix RCU usage

dev_cgroup->exceptions is protected with devcgroup_mutex for writes
and RCU for reads; however, RCU usage isn't correct.

* dev_exception_clean() doesn't use RCU variant of list_del() and
  kfree().  The function can race with may_access() and may_access()
  may end up dereferencing already freed memory.  Use list_del_rcu()
  and kfree_rcu() instead.

* may_access() may be called only with RCU read locked but doesn't use
  RCU safe traversal over ->exceptions.  Use list_for_each_entry_rcu().
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Cc: stable@vger.kernel.org
Cc: Aristeu Rozanski <aris@redhat.com>
Cc: Li Zefan <lizefan@huawei.com>

security/device_cgroup.c

@@ -164,8 +164,8 @@ static void dev_exception_clean(struct dev_cgroup *dev_cgroup)
 	struct dev_exception_item *ex, *tmp;
 
 	list_for_each_entry_safe(ex, tmp, &dev_cgroup->exceptions, list) {
-		list_del(&ex->list);
-		kfree(ex);
+		list_del_rcu(&ex->list);
+		kfree_rcu(ex, rcu);
 	}
 }
 
@@ -298,7 +298,7 @@ static int may_access(struct dev_cgroup *dev_cgroup,
 	struct dev_exception_item *ex;
 	bool match = false;
 
-	list_for_each_entry(ex, &dev_cgroup->exceptions, list) {
+	list_for_each_entry_rcu(ex, &dev_cgroup->exceptions, list) {
 		if ((refex->type & DEV_BLOCK) && !(ex->type & DEV_BLOCK))
 			continue;
 		if ((refex->type & DEV_CHAR) && !(ex->type & DEV_CHAR))