The create_user_ns function in kernel/user_namespace.c in the Linux kernel before 3.8.6 does not check whether a chroot directory exists that differs from the namespace root directory, which allows local users to bypass intended filesystem restrictions via a crafted clone system call.
https://github.com/torvalds/linux/commit/3151527ee007b73a0ebd296010f1c0454a919c7d
http://www.openwall.com/lists/oss-security/2013/04/16/11