About two weeks ago, Canonical patched a regression that would lead to boot failures on some AMD machines using the Ubuntu 14.04 LTS (Trusty Tahr) operating system series, which was caused by a microcode firmware update for AMD processors that was supposed to mitigate the well-known Spectre microprocessor side-channel security vulnerability.
Earlier this month, on July 2, Canonical released a Linux kernel security update for Ubuntu 18.04 LTS (Bionic Beaver) users, addressing a total of six security vulnerabilities, one of which introduced a regression also causing boot failures, though it doesn’t appear to be limited to AMD processors only, but also to Intel machines.
As Canonical allows users of previous Ubuntu LTS (Long Term Support) versions to use the Linux kernel of newer Ubuntu LTS versions, the patch for the said regression causing boot failures is also available for Ubuntu 16.04.4 LTS (Xenial Xerus) users that have the HWE (Hardware Enablement) kernel from Ubuntu 18.04 LTS (Bionic Beaver) installed by default.
All Ubuntu 18.04 LTS and Ubuntu 16.04 LTS users are urged to update their installations to the new Linux kernel versions that are available in the main software repositories as soon as possible and reboot their computers after a successful installation.
Submitted by: Arnfried Walbrecht