xen/events: avoid NULL pointer dereference in dom0 on large machines

Using the pvops kernel a NULL pointer dereference was detected on a
large machine (144 processors) when booting as dom0 in
evtchn_fifo_unmask() during assignment of a pirq.

The event channel in question was the first to need a new entry in
event_array[] in events_fifo.c. Unfortunately xen_irq_info_pirq_setup()
is called with evtchn being 0 for a new pirq and the real event channel
number is assigned to the pirq only during __startup_pirq().

It is mandatory to call xen_evtchn_port_setup() after assigning the
event channel number to the pirq to make sure all memory needed for the
event channel is allocated.
Signed-off-by: Juergen Gross <jgross@suse.com>
Cc: <stable@vger.kernel.org> # 3.14+
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/events/events_base.c

@@ -526,20 +526,26 @@ static unsigned int __startup_pirq(unsigned int irq)
 	pirq_query_unmask(irq);
 
 	rc = set_evtchn_to_irq(evtchn, irq);
-	if (rc != 0) {
-		pr_err("irq%d: Failed to set port to irq mapping (%d)\n",
-		       irq, rc);
-		xen_evtchn_close(evtchn);
-		return 0;
-	}
+	if (rc)
+		goto err;
+
 	bind_evtchn_to_cpu(evtchn, 0);
 	info->evtchn = evtchn;
 
+	rc = xen_evtchn_port_setup(info);
+	if (rc)
+		goto err;
+
 out:
 	unmask_evtchn(evtchn);
 	eoi_pirq(irq_get_irq_data(irq));
 
 	return 0;
+
+err:
+	pr_err("irq%d: Failed to set port to irq mapping (%d)\n", irq, rc);
+	xen_evtchn_close(evtchn);
+	return 0;
 }
 
 static unsigned int startup_pirq(struct irq_data *data)