tracing: Add locked_down checks to the open calls of files created for tracefs

Added various checks on open tracefs calls to see if tracefs is in lockdown
mode, and if so, to return -EPERM.

Note, the event format files (which are basically standard on all machines)
as well as the enabled_functions file (which shows what is currently being
traced) are not lockde down. Perhaps they should be, but it seems counter
intuitive to lockdown information to help you know if the system has been
modified.

Link: http://lkml.kernel.org/r/CAHk-=wj7fGPKUspr579Cii-w_y60PtRaiDgKuxVtBAMK0VNNkA@mail.gmail.com

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_trigger.c

@@ -5,6 +5,7 @@
  * Copyright (C) 2013 Tom Zanussi <tom.zanussi@linux.intel.com>
  */
 
+#include <linux/security.h>
 #include <linux/module.h>
 #include <linux/ctype.h>
 #include <linux/mutex.h>
@@ -173,7 +174,11 @@ static const struct seq_operations event_triggers_seq_ops = {
 
 static int event_trigger_regex_open(struct inode *inode, struct file *file)
 {
-	int ret = 0;
+	int ret;
+
+	ret = security_locked_down(LOCKDOWN_TRACEFS);
+	if (ret)
+		return ret;
 
 	mutex_lock(&event_mutex);
 
@@ -292,6 +297,7 @@ event_trigger_write(struct file *filp, const char __user *ubuf,
 static int
 event_trigger_open(struct inode *inode, struct file *filp)
 {
+	/* Checks for tracefs lockdown */
 	return event_trigger_regex_open(inode, filp);
 }