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-<arch>-15-11.el8.<arch>.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.
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