notify: unused event private race

inotify decides if private data it passed to get added to an event was
used by checking list_empty().  But it's possible that the event may
have been dequeued and the private event removed so it would look empty.

The fix is to use the return code from fsnotify_add_notify_event rather
than looking at the list.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/notify/inotify/inotify_fsnotify.c

@@ -62,13 +62,14 @@ static int inotify_handle_event(struct fsnotify_group *group, struct fsnotify_ev
 	event_priv->wd = wd;
 
 	ret = fsnotify_add_notify_event(group, event, fsn_event_priv);
-	/* EEXIST is not an error */
-	if (ret == -EEXIST)
-		ret = 0;
-
-	/* did event_priv get attached? */
-	if (list_empty(&fsn_event_priv->event_list))
+	if (ret) {
 		inotify_free_event_priv(fsn_event_priv);
+		/* EEXIST says we tail matched, EOVERFLOW isn't something
+		 * to report up the stack. */
+		if ((ret == -EEXIST) ||
+		    (ret == -EOVERFLOW))
+			ret = 0;
+	}
 
 	/*
 	 * If we hold the entry until after the event is on the queue

fs/notify/inotify/inotify_user.c

@@ -386,6 +386,7 @@ void inotify_ignored_and_remove_idr(struct fsnotify_mark_entry *entry,
 	struct fsnotify_event *ignored_event;
 	struct inotify_event_private_data *event_priv;
 	struct fsnotify_event_private_data *fsn_event_priv;
+	int ret;
 
 	ignored_event = fsnotify_create_event(NULL, FS_IN_IGNORED, NULL,
 					      FSNOTIFY_EVENT_NONE, NULL, 0,
@@ -404,10 +405,8 @@ void inotify_ignored_and_remove_idr(struct fsnotify_mark_entry *entry,
 	fsn_event_priv->group = group;
 	event_priv->wd = ientry->wd;
 
-	fsnotify_add_notify_event(group, ignored_event, fsn_event_priv);
-
-	/* did the private data get added? */
-	if (list_empty(&fsn_event_priv->event_list))
+	ret = fsnotify_add_notify_event(group, ignored_event, fsn_event_priv);
+	if (ret)
 		inotify_free_event_priv(fsn_event_priv);
 
 skip_send_ignore:

fs/notify/notification.c

@@ -171,9 +171,7 @@ int fsnotify_add_notify_event(struct fsnotify_group *group, struct fsnotify_even
 	struct list_head *list = &group->notification_list;
 	struct fsnotify_event_holder *last_holder;
 	struct fsnotify_event *last_event;
-
-	/* easy to tell if priv was attached to the event */
-	INIT_LIST_HEAD(&priv->event_list);
+	int ret = 0;
 
 	/*
 	 * There is one fsnotify_event_holder embedded inside each fsnotify_event.
@@ -194,6 +192,7 @@ int fsnotify_add_notify_event(struct fsnotify_group *group, struct fsnotify_even
 
 	if (group->q_len >= group->max_events) {
 		event = &q_overflow_event;
+		ret = -EOVERFLOW;
 		/* sorry, no private data on the overflow event */
 		priv = NULL;
 	}
@@ -235,7 +234,7 @@ int fsnotify_add_notify_event(struct fsnotify_group *group, struct fsnotify_even
 	mutex_unlock(&group->notification_mutex);
 
 	wake_up(&group->notification_waitq);
-	return 0;
+	return ret;
 }
 
 /*