USN-4182-2 introduced a regression in the Intel Microcode for some Skylake processors.

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Ubuntu Security Notice USN-4182-4
December 04, 2019

intel-microcode regression
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A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 14.04 ESM

Summary:

USN-4182-2 introduced a regression in the Intel Microcode for some
Skylake processors.

Software Description:
- intel-microcode: Processor microcode for Intel CPUs

Details:

USN-4182-2 provided updated Intel Processor Microcode. A regression
was discovered that caused some Skylake processors to hang after
a warm reboot. This update reverts the microcode for that specific
processor family.

We apologize for the inconvenience.

Original advisory details:

 Stephan van Schaik, Alyssa Milburn, Sebastian Österlund, Pietro Frigo,
 Kaveh Razavi, Herbert Bos, Cristiano Giuffrida, Giorgi Maisuradze, Moritz
 Lipp, Michael Schwarz, Daniel Gruss, and Jo Van Bulck discovered that Intel
 processors using Transactional Synchronization Extensions (TSX) could
 expose memory contents previously stored in microarchitectural buffers to a
 malicious process that is executing on the same CPU core. A local attacker
 could use this to expose sensitive information. (CVE-2019-11135)

 It was discovered that certain Intel Xeon processors did not properly
 restrict access to a voltage modulation interface. A local privileged
 attacker could use this to cause a denial of service (system crash).
 (CVE-2019-11139)

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 14.04 ESM:
  intel-microcode                 3.20191115.1ubuntu0.14.04.2

After a standard system update you need to reboot your computer.

References:
  https://usn.ubuntu.com/4182-4
  https://usn.ubuntu.com/4182-1
  https://launchpad.net/bugs/1854764