1 files changed, 30 insertions, 1 deletions

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 39871326afa2..a7308559a26a 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -193,8 +193,10 @@ static void gntdev_put_map(struct grant_map *map)
 
         atomic_sub(map->count, &pages_mapped);
 
-        if (map->notify.flags & UNMAP_NOTIFY_SEND_EVENT)
+        if (map->notify.flags & UNMAP_NOTIFY_SEND_EVENT) {
                 notify_remote_via_evtchn(map->notify.event);
+                evtchn_put(map->notify.event);
+        }
 
         if (map->pages) {
                 if (!use_ptemod)
@@ -599,6 +601,8 @@ static long gntdev_ioctl_notify(struct gntdev_priv *priv, void __user *u)
         struct ioctl_gntdev_unmap_notify op;
         struct grant_map *map;
         int rc;
+        int out_flags;
+        unsigned int out_event;
 
         if (copy_from_user(&op, u, sizeof(op)))
                 return -EFAULT;
@@ -606,6 +610,21 @@ static long gntdev_ioctl_notify(struct gntdev_priv *priv, void __user *u)
         if (op.action & ~(UNMAP_NOTIFY_CLEAR_BYTE|UNMAP_NOTIFY_SEND_EVENT))
                 return -EINVAL;
 
+        /* We need to grab a reference to the event channel we are going to use
+         * to send the notify before releasing the reference we may already have
+         * (if someone has called this ioctl twice). This is required so that
+         * it is possible to change the clear_byte part of the notification
+         * without disturbing the event channel part, which may now be the last
+         * reference to that event channel.
+         */
+        if (op.action & UNMAP_NOTIFY_SEND_EVENT) {
+                if (evtchn_get(op.event_channel_port))
+                        return -EINVAL;
+        }
+
+        out_flags = op.action;
+        out_event = op.event_channel_port;
+
         spin_lock(&priv->lock);
 
         list_for_each_entry(map, &priv->maps, next) {
@@ -624,12 +643,22 @@ static long gntdev_ioctl_notify(struct gntdev_priv *priv, void __user *u)
                 goto unlock_out;
         }
 
+        out_flags = map->notify.flags;
+        out_event = map->notify.event;
+
         map->notify.flags = op.action;
         map->notify.addr = op.index - (map->index << PAGE_SHIFT);
         map->notify.event = op.event_channel_port;
+
         rc = 0;
+
  unlock_out:
         spin_unlock(&priv->lock);
+
+        /* Drop the reference to the event channel we did not save in the map */
+        if (out_flags & UNMAP_NOTIFY_SEND_EVENT)
+                evtchn_put(out_event);
+
         return rc;
 }