Canonical was forced to release a second round of Ubuntu updates that address the recently disclosed CPU vulnerabilities after some users complained that their systems no longer booted after installing the initial patches.
On January 9, Canonical released Ubuntu updates designed to mitigate Spectre and Meltdown, two recently disclosed attack methods that work against processors from Intel, AMD, ARM, Qualcomm and IBM. The Linux kernel updates mitigate the vulnerabilities that allow the Meltdown (CVE-2017-5754) and Spectre (CVE-2017-5753 and CVE-2017-5715) attacks.
Shortly after the kernel was updated to version 4.4.0-108, some Ubuntu users started complaining that their systems had failed to boot. Restoring the system to an earlier version apparently resolved the problem.