xen/events: prevent calling evtchn_get on invalid channels

The event channel number provided to evtchn_get can be provided by
userspace, so needs to be checked against the maximum number of event
channels prior to using it to index into evtchn_to_irq.
Signed-off-by: NDaniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/events.c

@@ -1104,6 +1104,9 @@ int evtchn_get(unsigned int evtchn)
 	struct irq_info *info;
 	int err = -ENOENT;
 
+	if (evtchn >= NR_EVENT_CHANNELS)
+		return -EINVAL;
+
 	mutex_lock(&irq_mapping_update_lock);
 
 	irq = evtchn_to_irq[evtchn];