The Network Lock Manager (NLM) protocol implementation in the NFS client functionality in the Linux kernel before 3.0 allows local users to cause a denial of service (system hang) via a LOCK_UN flock system call.
https://github.com/torvalds/linux/commit/0b760113a3a155269a3fba93a409c640031dd68f
https://bugzilla.redhat.com/show_bug.cgi?id=709393
http://www.openwall.com/lists/oss-security/2011/06/23/6