Bit of a necro but the newest microcode updates from Intel don't have the microcode.dat file included. If you use a tool to parse the update binaries back down in to a microcode.dat file and attempt to update it via microcode_ctl in CentOS/RHEL 6 it will tell you that the version supercedes what is provided via the microcode_ctl package and doesn't appear to apply (in my case anyways, the microcode version does not change even after a dracut -f -v and reboot). I suspect we may need to wait for this to be pushed out through the repositories. I can link to the tool to make the microcode.dat file if you would like. I hope this info helps.
Code:
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: This updated microcode supersedes microcode provided by Red Hat with
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: the CVE-2017-5715 (ā<80><9c>Spectreā<80><9d>) CPU branch injection vulnerability
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: mitigation.
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: Historically, Red Hat has provided updated microcode, developed by our
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: microprocessor partners, as a customer convenience. Red Hat had
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: temporarily suspended this practice while microcode stabilized. Red
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: Hat is once again providing an updated Intel microcode package
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: (microcode_ctl) and AMD microcode package (linux-firmware) to customers
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: in order to simplify deployment processes and minimize downtime. Weā<80><99>ll
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: continue to update these microcode packages as necessary. Please
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: contact your hardware vendor to determine whether more recent
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: BIOS/firmware updates are recommended because additional improvements
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: may be available.
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: This kbase https://access.redhat.com/articles/3436091 includes a table
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: that maps Intel and AMD CPU processor code family names to updated
2018-06-12T14:50:36.000-07:00 REDACTED DISCLAIMER: Intel and AMD microcode package versions.
Edit: So my original post had a mistake, it should take in the newest microcode update. Mine didn't have any newer version so it appeared that it didn't work. YMMV.