fs/namespace.c in the Linux kernel before 4.0.2 processes MNT_DETACH umount2 system calls without verifying that the MNT_LOCKED flag is unset, which allows local users to bypass intended access restrictions and navigate to filesystem locations beneath a mount by calling umount2 within a user namespace.
http://lists.opensuse.org/opensuse-security-announce/2016-06/msg00054.html
http://lists.opensuse.org/opensuse-security-announce/2016-06/msg00056.html
http://lists.opensuse.org/opensuse-security-announce/2016-08/msg00000.html
https://bugzilla.redhat.com/show_bug.cgi?id=1226751
https://github.com/torvalds/linux/commit/ce07d891a0891d3c0d0c2d73d577490486b809e1
https://groups.google.com/forum/message/raw?msg=linux.kernel/HnegnbXk0Vs/RClojwJzAFEJ