xen/events: fix race in evtchn_fifo_unmask()

Unmasking a fifo event channel can result in unmasking it twice, once
directly in the kernel and once via a hypercall in case the event was
pending.

Fix that by doing the local unmask only if the event is not pending.

This is part of XSA-332.

Cc: stable@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

drivers/xen/events/events_fifo.c

@@ -227,19 +227,25 @@ static bool evtchn_fifo_is_masked(evtchn_port_t port)
 	return sync_test_bit(EVTCHN_FIFO_BIT(MASKED, word), BM(word));
 }
 /*
- * Clear MASKED, spinning if BUSY is set.
+ * Clear MASKED if not PENDING, spinning if BUSY is set.
+ * Return true if mask was cleared.
  */
-static void clear_masked(volatile event_word_t *word)
+static bool clear_masked_cond(volatile event_word_t *word)
 {
 	event_word_t new, old, w;
 
 	w = *word;
 
 	do {
+		if (w & (1 << EVTCHN_FIFO_PENDING))
+			return false;
+
 		old = w & ~(1 << EVTCHN_FIFO_BUSY);
 		new = old & ~(1 << EVTCHN_FIFO_MASKED);
 		w = sync_cmpxchg(word, old, new);
 	} while (w != old);
+
+	return true;
 }
 
 static void evtchn_fifo_unmask(evtchn_port_t port)
@@ -248,8 +254,7 @@ static void evtchn_fifo_unmask(evtchn_port_t port)
 
 	BUG_ON(!irqs_disabled());
 
-	clear_masked(word);
-	if (evtchn_fifo_is_pending(port)) {
+	if (!clear_masked_cond(word)) {
 		struct evtchn_unmask unmask = { .port = port };
 		(void)HYPERVISOR_event_channel_op(EVTCHNOP_unmask, &unmask);
 	}