fanotify: Fix use after free in mask checking

We cannot use the event structure returned from
fsnotify_add_notify_event() because that event can be freed by the time
that function returns. Use the mask argument passed into the event
handler directly instead. This also fixes a possible problem when we
could unnecessarily wait for permission response for a normal fanotify
event which got merged with a permission event.

We also disallow merging of permission event with any other event so
that we know the permission event which we just created is the one on
which we should wait for permission response.

Reported-and-tested-by: Jiri Kosina <[e-mail]>
Reported-and-tested-by: Dave Jones <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

fs/notify/fanotify/fanotify.c

···
 {
 	struct fanotify_event_info *old, *new;
 
-#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
-	/* dont merge two permission events */
-	if ((old_fsn->mask & FAN_ALL_PERM_EVENTS) &&
-	    (new_fsn->mask & FAN_ALL_PERM_EVENTS))
-		return false;
-#endif
 	pr_debug("%s: old=%p new=%p\n", __func__, old_fsn, new_fsn);
 	old = FANOTIFY_E(old_fsn);
 	new = FANOTIFY_E(new_fsn);
···
 	bool do_merge = false;
 
 	pr_debug("%s: list=%p event=%p\n", __func__, list, event);
+
+#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
+	/*
+	 * Don't merge a permission event with any other event so that we know
+	 * the event structure we have created in fanotify_handle_event() is the
+	 * one we should check for permission response.
+	 */
+	if (event->mask & FAN_ALL_PERM_EVENTS)
+		return NULL;
+#endif
 
 	list_for_each_entry_reverse(test_event, list, list) {
 		if (should_merge(test_event, event)) {
···
 		fsnotify_destroy_event(group, fsn_event);
 		if (IS_ERR(notify_fsn_event))
 			return PTR_ERR(notify_fsn_event);
-		/* We need to ask about a different events after a merge... */
-		event = FANOTIFY_E(notify_fsn_event);
-		fsn_event = notify_fsn_event;
 	}
 
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
-	if (fsn_event->mask & FAN_ALL_PERM_EVENTS)
+	if (mask & FAN_ALL_PERM_EVENTS)
 		ret = fanotify_get_response_from_access(group, event);
 #endif
 	return ret;