On Thu, 30 Jul 2020 Sandro Bonazzola wrote...
I think you can follow here: Bug 1861977 https://bugzilla.redhat.com/show_bug.cgi?id=1861977 - RHSA-2020:3216 grub2 security update renders system unbootable
Thanks very much...I can confirm the following does work...
1) booting to rhel-8 (centos-8) troubleshooting mode from a boot CD / USB 2) chrooted to /mnt/sysimage 3) setup networking 4) yum downgrade shim-x64 grub2* 5) add exclude=grub2* shim* to /etc/yum.conf 6) reboot
N.B. If you are on CentOS-stream then the repos will NOT have the older packages so just point to the CentOS repo ( or download from that repo and rpm -Uvh --force to install them )