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.
git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=e9f57ebcba563e0cd532926cab83c92bb4d79360
people.canonical.com/~ubuntu-security/cve/2016/CVE-2016-1575.html
www.halfdog.net/Security/2016/UserNamespaceOverlayfsXattrSetgidPrivilegeEscalation/
www.openwall.com/lists/oss-security/2016/02/24/7
www.openwall.com/lists/oss-security/2021/10/18/1
launchpad.net/bugs/1534961