The overlayfs implementation in the Linux kernel through 4.5.2 does not properly maintain POSIX ACL xattr data, which allows local users to gain privileges by leveraging a group-writable setgid directory.
https://launchpad.net/bugs/1534961
http://www.openwall.com/lists/oss-security/2021/10/18/1
http://www.openwall.com/lists/oss-security/2016/02/24/7
http://people.canonical.com/~ubuntu-security/cve/2016/CVE-2016-1575.html