The overlayfs implementation in the linux (aka Linux kernel) package before 3.19.0-21.21 in Ubuntu through 15.04 does not properly check permissions for file…

The overlayfs implementation in the linux (aka Linux kernel) package before 3.19.0-21.21 in Ubuntu through 15.04 does not properly check permissions for file creation in the upper filesystem directory, which allows local users to obtain root access by leveraging a configuration in which overlayfs is permitted in an arbitrary mount namespace.