14 DAY TRIAL // On July 5, 2018, Canonical published a security advisory where it informs users of the Ubuntu 14.04 LTS (Trusty Tahr) operating system series about a regression causing boot failures on some AMD PCs.
Last month, on June 20, 2018, Canonical released an AMD microcode firmware update for the Ubuntu 18.04 LTS (Bionic Beaver), Ubuntu 17.10 (Artful Aardvark), Ubuntu 16.04 LTS (Xenial Xerus), and Ubuntu 14.04 LTS (Trusty Tahr) operating systems to address the well-known Spectre microprocessor side-channel vulnerability.
Unearthed by Jann Horn of Google Project Zero, the Spectre security flaw revealed the fact that devices powered by modern processors using branch prediction and speculative execution may allow local attackers to expose sensitive information, including kernel memory, by reading the memory via so-called side-channel attacks.
Unfortunately, the processor microcode firmware for AMD CPUs (amd64-microcode) released by Canonical for its supported Ubuntu Linux release caused boot failures on some PCs. Canonical has apologized for the issue and released a patch to fix the unfortunate regression, urging users to update their PCs immediately.
"USN-3690-1 provided updated microcode for AMD processors to address CVE-2017-5715 (aka Spectre). Unfortunately, the update caused some systems to fail to boot. This update reverts the update for Ubuntu 14.04 LTS. We apologize for the inconvenience," said Canonical in the security advisory.
Only Ubuntu 14.04 LTS users appear to be affected at this time
At this time, Canonical fixed the said regression caused by the AMD microcode firmware update only for users of the Ubuntu 14.04 LTS (Trusty Tahr) operating system series and its official derivatives, including Kubuntu, Lubuntu, Xubuntu, etc. Canonical says that he update it released on July 5, 2018, to fix boot failures reverts the previous one from June 20.
Therefore, to fix the problem, Ubuntu 14.04 LTS users are urged to update the AMD microcode firmware update to the amd64-microcode - 3.20180524.1~ubuntu0.14.04.2+really20130710.1 package that's now available in the official software repositories. To update your system, follow the instructions at https://wiki.ubuntu.com/Security/Upgrades, and remember to reboot your PCs after installing the new microcode version.