alarmtimer: Check return value of class_find_device()

alarmtimer_late_init() uses class_find_device() to find a alarm
capable rtc device. The match callback stores a pointer to the name in
the char pointer handed in from the call site. alarmtimer_late_init()
checks the char pointer for NULL, but the pointer is on the stack and
not initialized to NULL before the call. So it can have random content
when the match function did not identify a device, which leads to
random access in the following rtc_open() call where the pointer is
dereferenced

Instead of relying on the char pointer, check the return value of
class_find_device. If a device is found then the name pointer is valid
as well.
Reported-by: NIngo Molnar <mingo@elte.hu>
Cc: John Stultz <john.stultz@linaro.org>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

kernel/time/alarmtimer.c

@@ -669,11 +669,13 @@ static int __init has_wakealarm(struct device *dev, void *name_ptr)
  */
 static int __init alarmtimer_init_late(void)
 {
+	struct device *dev;
 	char *str;
 
 	/* Find an rtc device and init the rtc_timer */
-	class_find_device(rtc_class, NULL, &str, has_wakealarm);
-	if (str)
+	dev = class_find_device(rtc_class, NULL, &str, has_wakealarm);
+	/* If we have a device then str is valid. See has_wakealarm() */
+	if (dev)
 		rtcdev = rtc_class_open(str);
 	if (!rtcdev) {
 		printk(KERN_WARNING "No RTC device found, ALARM timers will"