fanotify: fix double free of pending permission events

Commit 85816794240b ("fanotify: Fix use after free for permission
events") introduced a double free issue for permission events which are
pending in group's notification queue while group is being destroyed.
These events are freed from fanotify_handle_event() but they are not
removed from groups notification queue and thus they get freed again
from fsnotify_flush_notify().

Fix the problem by removing permission events from notification queue
before freeing them if we skip processing access response.  Also expand
comments in fanotify_release() to explain group shutdown in detail.

Fixes: 85816794240b9659e66e4d9b0df7c6e814e5f603
Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Douglas Leeder <douglas.leeder@sophos.com>
Tested-by: Douglas Leeder <douglas.leeder@sophos.com>
Reported-by: Heinrich Schuchard <xypron.glpk@gmx.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/notify/fanotify/fanotify.c

@@ -70,8 +70,15 @@ static int fanotify_get_response(struct fsnotify_group *group,
 	wait_event(group->fanotify_data.access_waitq, event->response ||
 				atomic_read(&group->fanotify_data.bypass_perm));
 
-	if (!event->response) /* bypass_perm set */
+	if (!event->response) {	/* bypass_perm set */
+		/*
+		 * Event was canceled because group is being destroyed. Remove
+		 * it from group's event list because we are responsible for
+		 * freeing the permission event.
+		 */
+		fsnotify_remove_event(group, &event->fae.fse);
 		return 0;
+	}
 
 	/* userspace responded, convert to something usable */
 	switch (event->response) {

fs/notify/fanotify/fanotify_user.c

@@ -359,6 +359,11 @@ static int fanotify_release(struct inode *ignored, struct file *file)
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
 	struct fanotify_perm_event_info *event, *next;
 
+	/*
+	 * There may be still new events arriving in the notification queue
+	 * but since userspace cannot use fanotify fd anymore, no event can
+	 * enter or leave access_list by now.
+	 */
 	spin_lock(&group->fanotify_data.access_lock);
 
 	atomic_inc(&group->fanotify_data.bypass_perm);
@@ -373,6 +378,13 @@ static int fanotify_release(struct inode *ignored, struct file *file)
 	}
 	spin_unlock(&group->fanotify_data.access_lock);
 
+	/*
+	 * Since bypass_perm is set, newly queued events will not wait for
+	 * access response. Wake up the already sleeping ones now.
+	 * synchronize_srcu() in fsnotify_destroy_group() will wait for all
+	 * processes sleeping in fanotify_handle_event() waiting for access
+	 * response and thus also for all permission events to be freed.
+	 */
 	wake_up(&group->fanotify_data.access_waitq);
 #endif
 

fs/notify/notification.c

@@ -73,7 +73,8 @@ void fsnotify_destroy_event(struct fsnotify_group *group,
 	/* Overflow events are per-group and we don't want to free them */
 	if (!event || event->mask == FS_Q_OVERFLOW)
 		return;
-
+	/* If the event is still queued, we have a problem... */
+	WARN_ON(!list_empty(&event->list));
 	group->ops->free_event(event);
 }
 
@@ -124,6 +125,21 @@ int fsnotify_add_event(struct fsnotify_group *group,
 	return ret;
 }
 
+/*
+ * Remove @event from group's notification queue. It is the responsibility of
+ * the caller to destroy the event.
+ */
+void fsnotify_remove_event(struct fsnotify_group *group,
+			   struct fsnotify_event *event)
+{
+	mutex_lock(&group->notification_mutex);
+	if (!list_empty(&event->list)) {
+		list_del_init(&event->list);
+		group->q_len--;
+	}
+	mutex_unlock(&group->notification_mutex);
+}
+
 /*
  * Remove and return the first event from the notification list.  It is the
  * responsibility of the caller to destroy the obtained event

include/linux/fsnotify_backend.h

@@ -326,6 +326,8 @@ extern int fsnotify_add_event(struct fsnotify_group *group,
 			      struct fsnotify_event *event,
 			      int (*merge)(struct list_head *,
 					   struct fsnotify_event *));
+/* Remove passed event from groups notification queue */
+extern void fsnotify_remove_event(struct fsnotify_group *group, struct fsnotify_event *event);
 /* true if the group notification queue is empty */
 extern bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group);
 /* return, but do not dequeue the first event on the notification queue */