From: Ben Hutchings <ben@decadent.org.uk>
Subject: fanotify: Taint on use of FANOTIFY_ACCESS_PERMISSIONS
Date: Wed, 13 Jul 2016 01:37:22 +0100
Forwarded: not-needed

Various free and proprietary AV products use this feature and users
apparently want it.  But punting access checks to userland seems like
an easy way to deadlock the system, and there will be nothing we can
do about that.  So warn and taint the kernel if this feature is
actually used.

---
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -1661,6 +1661,14 @@ static int do_fanotify_mark(int fanotify
 		umask = FANOTIFY_EVENT_FLAGS;
 	}
 
+#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
+	if (mask & FANOTIFY_PERM_EVENTS) {
+		pr_warn_once("%s (%d): Using fanotify permission checks may lead to deadlock; tainting kernel\n",
+			     current->comm, current->pid);
+		add_taint(TAINT_AUX, LOCKDEP_STILL_OK);
+	}
+#endif
+
 	f = fdget(fanotify_fd);
 	if (unlikely(!f.file))
 		return -EBADF;