Step 2: Booting with ESP

The UEFI utilizes a partition called EFI, which contains a boot code, to boot the system. The EFI System Partition (ESP) is a special partition on the GPT that stores EFI programs such as the bootloaders and utility software. Usually, the ESP mounts at /boot/efi. Therefore, /boot/efi/EFI is likely the beginning point for many of the EFI directory trees. The UEFI system looks for a partition with a unique GUID that distinguishes it as being the ESP. When more than one boot device (loadable kernel) is present, the UEFI boot manager chooses which ESP to utilize depending on the set boot manager order. Instead of using the predetermined order, you may choose an operating system to boot. You can specify your choice using the efibootmgr program.

Step 3: UEFI Secure Boot

Optionally, the UEFI boot manager makes sure to verify whether Secure Boot is turned on or off. In case Secure Boot is not set, the boot manager will cause the ESP to use the GRUB bootloader.

Other than that, the boot manager will ask the bootloader for a certificate. Then, the boot manager checks it against the keys in the UEFI Secure Boot key database before running the bootloader. Practically, to register the binary, most Linux systems utilize a small signed binary (Red Hat uses the shim package, for example). Additionally, the GRUB 2 loader can also be signed with a key that is user-managed via a Machine Owner Key (MOK) list. All boot binaries, including UEFI firmware drivers, EFI programs, and the OS, must have their digital signature validated by the firmware. Only after this step can the computer boot. This protects the OS from possible threats such as pre-boot malware, rootkits, and software upgrades that aren't secure [6].

Step 4: The Bootloader Phase

The GRUB 2 EFI boot loads after a successful Secure Boot verification process. The GRUB 2 bootloader, in turn, validates the kernel. In a nutshell, a bootloader is the very first piece of software that a computer's boot process activates. It is in charge of loading and handing off control to an OS kernel such as Linux.