The mountpoint_last function in fs/namei.c in the Linux kernel before 3.15.8 does not properly maintain a certain reference count during attempts to use the umount system call in conjunction with a symlink, which allows local users to cause a denial of service (memory consumption or use-after-free) or possibly have unspecified other impact via the umount program.
http://rhn.redhat.com/errata/RHSA-2015-0062.html
https://bugzilla.redhat.com/show_bug.cgi?id=1122472
http://secunia.com/advisories/60353
https://github.com/torvalds/linux/commit/295dc39d941dc2ae53d5c170365af4c9d5c16212
http://www.kernel.org/pub/linux/kernel/v3.x/ChangeLog-3.15.8