Ubuntu: USN-4712-1: Linux kernel regression

USN-4576-1 fixed a vulnerability in the overlay file system implementation in the Linux kernel. Unfortunately, that fix introduced a regression that could incorrectly deny access to overlay files in some situations. This update fixes the problem.

We apologize for the inconvenience.

Original vulnerability details:

Giuseppe Scrivano discovered that the overlay file system in the Linux kernel did not properly perform permission checks in some situations. A local attacker could possibly use this to bypass intended restrictions and gain read access to restricted files.