Ubuntu Security Notice 4916-2 - USN-4916-1 fixed vulnerabilities in the Linux kernel. Unfortunately, the fix for CVE-2021-3493 introduced a memory leak in some situations. This update fixes the problem. It was discovered that the overlayfs implementation in the Linux kernel did not properly validate the application of file system capabilities with respect to user namespaces. A local attacker could use this to gain elevated privileges. Various other issues were also addressed.