The collect_mounts function in fs/namespace.c in the Linux kernel before 4.0.5 does not properly consider that it may execute after a path has been unmounted, which allows local users to cause a denial of service (system crash) by leveraging user-namespace root access for an MNT_DETACH umount2 system call.
https://github.com/torvalds/linux/commit/cd4a40174b71acd021877341684d8bb1dc8ea4ae
https://bugzilla.redhat.com/show_bug.cgi?id=1248486
http://www.openwall.com/lists/oss-security/2015/06/04/5
http://www.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.0.5
http://openwall.com/lists/oss-security/2015/05/29/5