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CLI

On Debian general config is in /etc/default/grub and scripts are in /etc/grub.d/

grub-update                      # Create boot image
grub-install /dev/sdX            # Install boot loader

Configure CPU mitigations

Examples:

# Newer kernels turns all off
GRUB_CMDLINE_LINUX_DEFAULT="<params> mitigations=off"

# Get most performance back
GRUB_CMDLINE_LINUX_DEFAULT="<params> nopti nospectre_v2 nospec"

# Disable everything for older kernels
GRUB_CMDLINE_LINUX_DEFAULT="<params> noibrs noibpb nopti nospectre_v2 nospectre_v1 l1tf=off nospec_store_bypass_disable no_stf_barrier mds=off mitigations=off"

Setting different default kernel

Find kernel id

grep "menuentry .*id_option.*gnulinux" /boot/grub/grub.cfg

Ids look like gnulinux-5.11.0-18-generic-advanced-c0da4c73-db83-44f2-9a69-f6ad6a4dc7b4 Once you've identified your new default id change the following line in /etc/default/grub:

GRUB_DEFAULT=0

to something like

GRUB_DEFAULT="gnulinux-5.11.0-18-generic-advanced-c0da4c73-db83-44f2-9a69-f6ad6a4dc7b4"

and run sudo grub-update

Rescue System

When booting into rescue system

  mount /dev/... /mnt               # Mount / filesystem
  mount /dev/... /mnt/boot          # Mount /boot
  mount --bind /dev /mnt/dev        # Bind mount /dev
  mount --bind /sys /mnt/sys        # Bind mount /sys
  mount --bind /proc /mnt/proc      # Bind mount /proc
  chroot /mnt

With these preparations you should be able to run fdisk, apt/yum/..., grub-update