** DISPUTED ** kernel/ptrace.c in the Linux kernel through 4.4.1 mishandles
uid and gid mappings, which allows local users to gain privileges by
establishing a user namespace, waiting for a root process to enter that
namespace with an unsafe uid or gid, and then using the ptrace system call.
NOTE: the vendor states "there is no kernel bug here."

Jann Horn discovered a ptrace issue with user namespaces in the Linux
kernel. The namespace owner could potentially exploit this flaw by ptracing
a root owned process entering the user namespace to elevate its privileges
and potentially gain access outside of the namespace.
(http://bugs.launchpad.net/bugs/1527374, CVE-2015-8709)

sbeattie

published fix has been reverted and replaced by upstream commit. Published kernels are not vulnerable to this, the status is tracking the state of the replaced commit coming through the trees.