The fs_pin implementation in the Linux kernel before 4.0.5 does not ensure the internal consistency of a certain list data structure, 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, related to fs/fs_pin.c and include/linux/fs_pin.h.
https://github.com/torvalds/linux/commit/820f9f147dcce2602eefd9b575bbbd9ea14f0953
https://bugzilla.redhat.com/show_bug.cgi?id=1249849
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