Some Intel Skylake CPU models (SKL-U/Y, family 6, model 78, stepping 3)
have reports of system hangs when revision 0xdc of microcode, that is included
since microcode-20200609 update to address CVE-2020-0543, CVE-2020-0548,
and CVE-2020-0549, is applied[1].  In order to address this, microcode update
to the newer revision has been disabled by default on these systems,
and the previously published microcode revision 0xd6 is used by default
for the OS-driven microcode update.

[1] https://github.com/intel/Intel-Linux-Processor-Microcode-Data-Files/issues/31

For the reference, SHA1 checksums of 06-4e-03 microcode files containing
microcode revisions in question are listed below:
 * 06-4e-03, revision 0xd6: 06432a25053c823b0e2a6b8e84e2e2023ee3d43e
 * 06-4e-03, revision 0xdc: cd1733458d187486999337ff8b51eeaa0cfbca6c
 * 06-4e-03, revision 0xe2: 41f4513cf563605bc85db38056ac430dec948366
 * 06-4e-03, revision 0xea: 5a54cab9f22f69b819d663e5747ed6ea2a326c55
 * 06-4e-03, revision 0xec: d949a8543d2464d955f5dc4b0777cac863f48729

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
 * CVE-2021-0127 (Intel Processor Breakpoint Control Flow):
   https://access.redhat.com/articles/6716541

The information regarding enforcing microcode update is provided below.

To enforce usage of the latest 06-4e-03 microcode revision for a specific kernel
version, please create a file "force-intel-06-4e-03" inside
/lib/firmware/<kernel_version> directory, run
"/usr/libexec/microcode_ctl/update_ucode" to add it to firmware directory
where microcode will be available for late microcode update, and run
"dracut -f --kver <kernel_version>", so initramfs for this kernel version
is regenerated and the microcode can be loaded early, for example:

    touch /lib/firmware/3.10.0-862.9.1/force-intel-06-4e-03
    /usr/libexec/microcode_ctl/update_ucode
    dracut -f --kver 3.10.0-862.9.1

After that, it is possible to perform a late microcode update by executing
"/usr/libexec/microcode_ctl/reload_microcode" or by writing value "1" to
"/sys/devices/system/cpu/microcode/reload" directly.

To enforce addition of this microcode for all kernels, please create file
"/etc/microcode_ctl/ucode_with_caveats/force-intel-06-4e-03", run
"/usr/libexec/microcode_ctl/update_ucode" for enabling late microcode updates,
and "dracut -f --regenerate-all" for enabling early microcode updates:

    mkdir -p /etc/microcode_ctl/ucode_with_caveats
    touch /etc/microcode_ctl/ucode_with_caveats/force-intel-06-4e-03
    /usr/libexec/microcode_ctl/update_ucode
    dracut -f --regenerate-all

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