tick/broadcast: Prevent NULL pointer dereference

When a disfunctional timer, e.g. dummy timer, is installed, the tick core
tries to setup the broadcast timer.

If no broadcast device is installed, the kernel crashes with a NULL pointer
dereference in tick_broadcast_setup_oneshot() because the function has no
sanity check.

Change-Id: I1fedbe948036f8ded04a89d276f7a7c403696280
Reported-by: Mason <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Anna-Maria Gleixner <[email protected]>
Cc: Richard Cochran <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Daniel Lezcano <[email protected]>
Cc: Peter Zijlstra <[email protected]>,
Cc: Sebastian Frias <[email protected]>
Cc: Thibaud Cornic <[email protected]>
Cc: Robin Murphy <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jiri Slaby <[email protected]>
Signed-off-by: mydongistiny <[email protected]>

kernel/time/tick-broadcast.c

@@ -853,6 +853,9 @@ void tick_broadcast_setup_oneshot(struct clock_event_device *bc)
 {
 	int cpu = smp_processor_id();
 
+	if (!bc)
+		return;
+
 	/* Set it up only once ! */
 	if (bc->event_handler != tick_handle_oneshot_broadcast) {
 		int was_periodic = bc->mode == CLOCK_EVT_MODE_PERIODIC;