This is a quick recovery and fix for the machines rendered unbootable after the grub2/shim yum update.

It is written for CentOS 8.2.2004 but similar should work for any CentOS
8 or 7 as long as you get the correct shim file, that is, the one from the latest installation media.

I am running on an x86_64 architecture (see uname -i). Please use the correct shim file for your architecture (shim--15-11.el8..rpm)

I have tested this by breaking a machine and then recovering it. It works for me.

I hope someone finds it useful. Let me know.

Regards Alan

HOW TO BOOT AN UNBOOTABLE MACHINE
=================================

1) Download a copy of rEFind. This is a UEFI boot manager. Burn it to a USB key.

# wget -O refind.zip http://sourceforge.net/projects/refind/files/0.12.0/refind-flashdrive-0.12.0.zip/download
# unzip refind.zip
# cd refind-flashdrive-0.12.0
# dd if=refind-flashdrive-0.12.0.img bs=4096 of=/dev/sdX (sdX is the device for your USB key, this will be erased, use the whole device use sdX not sdX1)
1800+0 records in
1800+0 records out
7372800 bytes (7.4 MB, 7.0 MiB) copied, 0.980893 s, 7.5 MB/s

2) Turn off secureboot in your UEFI hardware.

3) Boot the USB key. You should get a colourful screen with icons and a filename below.

Use the left/right arrow keys to select the correct grubx64.efi. Hit space to boot.

Your usual grub menu should appear and the system should boot normally.

HOW TO FIX THE PROBLEM
=====================

1) We need to downgrade the shim package. Now your system is running get an older copy of the correct shim package for your architecture from the CentOS installation media (e.g. CentOS-8.2.2004-x86_64-dvd1.iso) and install it.

# mount CentOS-8.2.2004-x86_64-dvd1.iso /mnt
# cd /mnt/BaseOS/Packages
# cp shim-x64-15-11.el8.x86_64.rpm /root
# cd /root
# umount /mnt

OR

Get the package from a CentOS mirror:

# cd /root
# wget http://ucmirror.canterbury.ac.nz/linux/CentOS/8.2.2004/BaseOS/x86_64/os/Packages/shim-x64-15-11.el8.x86_64.rpm

2) We can now reinstall the older shim package using yum. This will downgrade the package to the working version.

# yum install shim-x64-15-11.el8.x86_64.rpm

Last metadata expiration check: 2:11:11 ago on Sun 02 Aug 2020 11:31:06
NZST. Dependencies resolved.

====================================================================================================================================================================================
 Package Architecture Version Repository                                     Size
====================================================================================================================================================================================
Downgrading:
 shim-x64 x86_64 15-11.el8
@commandline                                  647 k

Transaction Summary
====================================================================================================================================================================================
Downgrade  1 Package

Total size: 647 k Is this ok [y/N]: y Downloading Packages:
Running transaction check Transaction check succeeded. Running transaction test Transaction test succeeded. Running transaction
  Preparing : 1/1
  Downgrading      : shim-x64-15-11.el8.x86_64 1/2
  Cleanup          : shim-x64-15-13.el8.x86_64 2/2
  Verifying        : shim-x64-15-11.el8.x86_64 1/2
  Verifying        : shim-x64-15-13.el8.x86_64 2/2
Installed products updated.

Downgraded:
shim-x64-15-11.el8.x86_64

Complete!

3) Your system should now boot normally.

4) add “exclude=shim*” to /etc/yum.conf to prevent the broken one being reinstalled.  You should now be able to run ‘yum update’. Remove the exclude= when a proper fix becomes available.