fanotify: flush outstanding perm requests on group destroy

When an fanotify listener is closing it may cause a deadlock between the
listener and the original task doing an fs operation.  If the original task
is waiting for a permissions response it will be holding the srcu lock.  The
listener cannot clean up and exit until after that srcu lock is syncronized.
Thus deadlock.  The fix introduced here is to stop accepting new permissions
events when a listener is shutting down and to grant permission for all
outstanding events.  Thus the original task will eventually release the srcu
lock and the listener can complete shutdown.

Reported-by: Andreas Gruenbacher <agruen@suse.de>
Cc: Andreas Gruenbacher <agruen@suse.de>
Signed-off-by: Eric Paris <eparis@redhat.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 032b837fcd11..b966b7230f47 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -195,6 +195,14 @@ static int prepare_for_access_response(struct fsnotify_group *group,
         re->fd = fd;
 
         mutex_lock(&group->fanotify_data.access_mutex);
+
+        if (group->fanotify_data.bypass_perm) {
+                mutex_unlock(&group->fanotify_data.access_mutex);
+                kmem_cache_free(fanotify_response_event_cache, re);
+                event->response = FAN_ALLOW;
+                return 0;
+        }
+                
         list_add_tail(&re->list, &group->fanotify_data.access_list);
         mutex_unlock(&group->fanotify_data.access_mutex);
 
@@ -364,9 +372,28 @@ static ssize_t fanotify_write(struct file *file, const char __user *buf, size_t
 static int fanotify_release(struct inode *ignored, struct file *file)
 {
         struct fsnotify_group *group = file->private_data;
+        struct fanotify_response_event *re, *lre;
 
         pr_debug("%s: file=%p group=%p\n", __func__, file, group);
 
+#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
+        mutex_lock(&group->fanotify_data.access_mutex);
+
+        group->fanotify_data.bypass_perm = true;
+
+        list_for_each_entry_safe(re, lre, &group->fanotify_data.access_list, list) {
+                pr_debug("%s: found group=%p re=%p event=%p\n", __func__, group,
+                         re, re->event);
+
+                list_del_init(&re->list);
+                re->event->response = FAN_ALLOW;
+
+                kmem_cache_free(fanotify_response_event_cache, re);
+        }
+        mutex_unlock(&group->fanotify_data.access_mutex);
+
+        wake_up(&group->fanotify_data.access_waitq);
+#endif
         /* matches the fanotify_init->fsnotify_alloc_group */
         fsnotify_put_group(group);