The sys_amd64 IRET Handler in the kernel in FreeBSD 9.3 and 10.1 allows local users to gain privileges or cause a denial of service (kernel panic).
https://www.freebsd.org/security/advisories/FreeBSD-SA-15:21.amd64.asc
http://www.securitytracker.com/id/1033376
http://www.securityfocus.com/bid/76485
http://www.securityfocus.com/archive/1/536321/100/0/threaded