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.
We apologize for the inconvenience.
Original advisory details:
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.