The eCryptfs subsystem in the Linux kernel before 3.18 allows local users to gain privileges via a large filesystem stack that includes an overlayfs layer, related to fs/ecryptfs/main.c and fs/overlayfs/super.c.
https://github.com/torvalds/linux/commit/69c433ed2ecd2d3264efd7afec4439524b319121
http://www.securitytracker.com/id/1038201
http://www.securityfocus.com/bid/97354