Some Intel Skylake CPU models (SKL-H/S/Xeon E3 v5, family 6, model 94,
stepping 3) had reports of possible system hangs when revision 0xdc
of microcode, that is included in microcode-20200609 update to address
CVE-2020-0543, CVE-2020-0548, and CVE-2020-0549, was applied[1].  In order
to address this, microcode updates to the newer revision had been disabled
by default on these systems, and the previously published microcode revision
0xd6 was used by default for the OS-driven microcode update.  The revision
0xea seems[2] to have fixed the aforementioned issue, hence it is enabled
by default (but can be disabled explicitly; see below).

[1] https://github.com/intel/Intel-Linux-Processor-Microcode-Data-Files/issues/31#issuecomment-644885826
[2] https://github.com/intel/Intel-Linux-Processor-Microcode-Data-Files/issues/31#issuecomment-857806014

For the reference, SHA1 checksums of 06-5e-03 microcode files containing
microcode revisions in question are listed below:
 * 06-5e-03, revision 0xd6: 86c60ee7d5d0d7115a4962c1c61ceecb0fd3a95a
 * 06-5e-03, revision 0xdc: 5e1020a10678cfc60980131c3d3a2cfd462b4dd7
 * 06-5e-03, revision 0xe2: 031e6e148b590d1c9cfdb6677539eeb4899e831c
 * 06-5e-03, revision 0xea: e6c37056a849fd281f2fdb975361a914e07b86c8
 * 06-5e-03, revision 0xec: 6458bf25da4906479a01ffdcaa6d466e22722e01

Please contact your system vendor for a BIOS/firmware update that contains
the latest microcode version.  For the information regarding microcode versions
required for mitigating specific side-channel cache attacks, please refer
to the following knowledge base articles:
 * CVE-2017-5715 ("Spectre"):
   https://access.redhat.com/articles/3436091
 * CVE-2018-3639 ("Speculative Store Bypass"):
   https://access.redhat.com/articles/3540901
 * CVE-2018-3620, CVE-2018-3646 ("L1 Terminal Fault Attack"):
   https://access.redhat.com/articles/3562741
 * CVE-2018-12130, CVE-2018-12126, CVE-2018-12127, and CVE-2019-11091
   ("Microarchitectural Data Sampling"):
   https://access.redhat.com/articles/4138151
 * CVE-2019-0117 (Intel SGX Information Leak),
   CVE-2019-0123 (Intel SGX Privilege Escalation),
   CVE-2019-11135 (TSX Asynchronous Abort),
   CVE-2019-11139 (Voltage Setting Modulation):
   https://access.redhat.com/solutions/2019-microcode-nov
 * CVE-2020-0543 (Special Register Buffer Data Sampling),
   CVE-2020-0548 (Vector Register Data Sampling),
   CVE-2020-0549 (L1D Cache Eviction Sampling):
   https://access.redhat.com/solutions/5142751
 * CVE-2020-8695 (Information disclosure issue in Intel SGX via RAPL interface),
   CVE-2020-8696 (Vector Register Leakage-Active),
   CVE-2020-8698 (Fast Forward Store Predictor):
   https://access.redhat.com/articles/5569051
 * CVE-2020-24489 (VT-d-related Privilege Escalation),
   CVE-2020-24511 (Improper Isolation of Shared Resources),
   CVE-2020-24512 (Observable Timing Discrepancy),
   CVE-2020-24513 (Information Disclosure on Some Intel Atom Processors):
   https://access.redhat.com/articles/6101171

The information regarding disabling microcode update is provided below.

To prevent usage of the latest 06-5e-03 microcode revision for a specific kernel
version, please create a file "disallow-intel-06-5e-03" inside
/lib/firmware/<kernel_version> directory, run
"/usr/libexec/microcode_ctl/update_ucode" to remove it to firmware directory
where microcode is available for late microcode update, and run
"dracut -f --kver <kernel_version>", so initramfs for this kernel version
is regenerated, for example:

    touch /lib/firmware/3.10.0-862.9.1/disallow-intel-06-5e-03
    /usr/libexec/microcode_ctl/update_ucode
    dracut -f --kver 3.10.0-862.9.1

To avoid  addition of the latest microcode for all kernels, please create file
"/etc/microcode_ctl/ucode_with_caveats/disallow-intel-06-5e-03", run
"/usr/libexec/microcode_ctl/update_ucode" for late microcode updates,
and "dracut -f --regenerate-all" for early microcode updates:

    mkdir -p /etc/microcode_ctl/ucode_with_caveats
    touch /etc/microcode_ctl/ucode_with_caveats/disallow-intel-06-5e-03
    /usr/libexec/microcode_ctl/update_ucode
    dracut -f --regenerate-all

Please refer to /usr/share/doc/microcode_ctl/README.caveats for additional
information.