In the Linux kernel 4.15.x through 4.19.x before 4.19.2, map_write() in
kernel/user_namespace.c allows privilege escalation because it mishandles
nested user namespaces with more than 5 UID or GID ranges. A user who has
CAP_SYS_ADMIN in an affected user namespace can bypass access controls on
resources outside the namespace, as demonstrated by reading /etc/shadow.
This occurs because an ID transformation takes place properly for the
namespaced-to-kernel direction but not for the kernel-to-namespaced
direction.

Jann Horn discovered that the Linux kernel mishandles mapping UID or GID
ranges inside nested user namespaces in some situations. A local attacker
could use this to bypass access controls on resources outside the
namespace.

tyhicks