Ubuntu Security Notice 2644-2 - The Fix for CVE-2015-1328 introduced a regression into the Linux kernel's overlayfs file system. The removal of a directory that only exists on the lower layer results in a kernel panic. Philip Pettersson discovered a privilege escalation when using overlayfs mounts inside of user namespaces. A local user could exploit this flaw to gain administrative privileges on the system. Various other issues were also addressed.
369222ceb6590020b1886ad884d11700b41c94fa8ea8a7d63f869a68478feba4