x86, kaslr: fix module lock ordering problem

There was a potential lock ordering problem with the module kASLR patch
("x86, kaslr: randomize module base load address"). This patch removes
the usage of the module_mutex and creates a new mutex to protect the
module base address offset value.

Chain exists of:
  text_mutex --> kprobe_insn_slots.mutex --> module_mutex

[    0.515561]  Possible unsafe locking scenario:
[    0.515561]
[    0.515561]        CPU0                    CPU1
[    0.515561]        ----                    ----
[    0.515561]   lock(module_mutex);
[    0.515561]                                lock(kprobe_insn_slots.mutex);
[    0.515561]                                lock(module_mutex);
[    0.515561]   lock(text_mutex);
[    0.515561]
[    0.515561]  *** DEADLOCK ***

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andy Honig <ahonig@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/module.c

@@ -48,6 +48,9 @@ do {							\
 static unsigned long module_load_offset;
 static int randomize_modules = 1;
 
+/* Mutex protects the module_load_offset. */
+static DEFINE_MUTEX(module_kaslr_mutex);
+
 static int __init parse_nokaslr(char *p)
 {
 	randomize_modules = 0;
@@ -58,7 +61,7 @@ early_param("nokaslr", parse_nokaslr);
 static unsigned long int get_module_load_offset(void)
 {
 	if (randomize_modules) {
-		mutex_lock(&module_mutex);
+		mutex_lock(&module_kaslr_mutex);
 		/*
 		 * Calculate the module_load_offset the first time this
 		 * code is called. Once calculated it stays the same until
@@ -67,7 +70,7 @@ static unsigned long int get_module_load_offset(void)
 		if (module_load_offset == 0)
 			module_load_offset =
 				(get_random_int() % 1024 + 1) * PAGE_SIZE;
-		mutex_unlock(&module_mutex);
+		mutex_unlock(&module_kaslr_mutex);
 	}
 	return module_load_offset;
 }