Securing-GRUB-2

Tutorial about securing the access of GRUB 2 at Linux startup.

Table of Content

• Securing-GRUB-2
  • GRUB
  • Boot process of Linux
    • 1. BIOS
    • 2. MBR
    • 3. GRUB
    • 4. Kernel
    • 5. Init
  • The "flaw" of GRUB
    • Access to a shell with root privileges
  • GRUB's configuration
  • Secure GRUB with a hashed password
  • Allow automatic boot

GRUB

First of all GNU GRUB (GRand Unified Bootloader) is a boot loader and boot manager from the GNU Project.
Executed after the booting of the BIOS or UEFI it allows user to select in which operating system he wants to boot and it serves in the same time as a Linux boot loader.

Boot process of Linux

They are usually 5 different stages of Linux boot process involved during the startup of a Linux distro :

1. BIOS

The BIOS (Basic Input Output System) is stored in a memory location of the motherboard called EEPROM (Electrically-Erasable Programmable Read-only Memory). It's a set of functions (written in Assembly Language) which allow the first interaction with the physical material (Hard drives, Keyboard, Mouse, ...).
It scans all the internal and external devices and interfaces connected to the motherboard and performs some system integrity checks.
Then, depending of the boot order configured in the BIOS, it loads and executes the 512 bytes of the disk, known as the MBR.

Nowadays the BIOS is replaced by UEFI (Unified Extensible Firmware Interface) that can act as a boot loader, boot manager and a replacement of GRUB. Moreover, UEFI has more adavantages such as : secure boot, user-friendly interface, support GPT parition, support a large of architecture, ...

2. MBR

The first 512 bytes of the disk are called the MBR (Master Boot Record). It conatains :

• Boot code or Bootstrap code which contains informations about the boot loader (446 bytes)
• Partitions table to index all the partitions of the disk (64 bytes)
• Boot signature to check if the disk is bootable or not (2 bytes)

In a nutshell, the MBR is charged of loading and executing the boot loader in our case : GRUB.

3. GRUB

Here comes our famous GRUB ! 😄
It loads all the available operating system (Linux kernels precisely) or other boot loaders like Windows Boot Manager. If we don't do anything at the GRUB screen, it loads and executes automatically the default Linux kernel (vmlinuz) and initrd (inital ramdisk) images.

Loaded into the RAM, initrd is a file system which is used for the Linux startup process. It contains all the additional modules and drivers for the kernel.

4. Kernel

The Linux kernel first mounts the root file system set in grub.conf in the line root=.
Then it executes the /sbin/init program as the fisrt program with root privileges which executes some others scripts.

/sbin/init is actually a symbolic link to the init system of the OS. It is either SysV or systemd, there are in charged of launching all the necessary services for the OS. Nowadays systemd is the most used and it is compatible with SysV init scripts.
If it is SysV all the scripts and programs are located in /etc/systemd/system/ and /lib/systemd/system/
So /sbin/init is a symbolic link to /lib/systemd/systemd

Since init is the first program to be executed by Linux kernel, it has the PID (Process IDentifier) of 1.
We can do a ps command to check the first PID :
ps -ef | grep init

The kernel then establishes a temporary root file system with initrd until the real file system is mounted. It also contains necessary drivers compiled inside as we saw earlier.

5. Init

First the init program reads its initialization files which are in /etc/init.d/ (/etc/inittab before with SysV). It sets everthing the system needs for its initialization.
Then it set the default run level. A run level is a configuration of processes.

There are 7 run level from 0 to 1 :

Level	Description
0	Halt
1	Single user mode
2	Multi-user
3	Full multi-user mode
4	Unused
5	X11 (Full multi-user graphical mode)
6	Reboot

To check the current run level you can do :
who -r
sudo runlevel
systemctl get-default for systemd
It should normally output the run level 5 or 3

Modern Linux systems use systemd which refers with this :

Level	Target
0	poweroff.target
1	rescue.target
2,3,4	multi-user.target
5	graphical.target
6	reboot.target

To set a run level with systemd you can do this command :
sudo systemctl isolate name.target

The "flaw" of GRUB

As we mentionned earlier, the kernel executes normally the /sbin/init program. However we can shortcut this procedure by executing another program instead. For instance we can excute a shell that simply gives us an acess to the machine with root privileges on top of that. 😕

Access to a shell with root privileges

Here we are going to see how to have access to the machine without typing any login or password.

We are working with a simple default Debian 10 with a user account in it.

First of all we boot the machine and arrive at the GRUB boot menu :

At this stage press "e" to edit the boot option of the kernel :

Now we go to the line (near the bottom) that contains "/boot/vmlinuz..."

We just replace "ro" (read only) by "rw" (read and write) in order to write in the disk and add the init option with the location of shell in argument "init=/bin/bash".

Here we are telling to the kernel to execute the bash shell (the most commun shell, usually by default) instead of the init program (/sbin/init)

Here is the line once modified :

Here are some options that we can give :

• root=device : specifies the disk where we want to mount the root file system (Exemple : /dev/sdaX, LABEL, UUID
• init=binary : specifies the initial program executed by the kernel (Normally /sbin/init)
• single : allow to start in single user (root) with minimum services
• ro : the file system will be mounted in read-only and do a file system consistency check (fsck)
• rw : the file stystem will be mounted in read and write and doesn't do a fsck.
• quiet : dont' use verbose mode.

Now we simply press "F10" to boot with the option we've added.

Since the init process is executed with root privileges ... Tada 💥 we have access to the machine with the root user ! You could do anything you want at this stage :

That's why it is very important to secure our GRUB.

GRUB's configuration

The file of GRUB's configuration is located in /boot/grub/grub.cfg which is generated by /etc/default/grub and scripts in /etc/grub.d/.
However to modify GRUB's configuration we are going to modify only the scripts in /etc/default/grub and /etc/grub.d/ then do update-grub2 to apply the modification.

The scripts in /etc/grub.d are numbered to be executed in a certain order, here are some common ones :

Script	Description
00_header	generate the header of grub.cfg
05_debian_theme	generate the interface's theme
10_linux	generate the localisation of Linux's kernel
30_os-prober	generate the detected OS
40_custom	for adding our own input

Secure GRUB with a hashed password

What we are actually doing is to add a login/password to GRUB configuration's files whenever the user wants to edit the boot option. To do this, we have to add a login and passord in GRUB's configuration. We can add a plain text password but every user could see it by checking GRUB's configuration. To prevent this we are going to add a hashed password with GRUB's tool : grub-mkpasswd-pbkdf2
This command creates a hashed password readable by GRUB.

Here are the steps to add the login and hashed password :

1. Generate the password to /etc/grub.d/40_custom

   grub-mkpasswd-pbkdf2 | sudo tee -a /etc/grub.d/40_custom

   Enter two times the password for GRUB and once the password for sudo in order to have the right to write in 40_custom.

   "| tee -a" will show the output of grub-mkpasswd-pbkdf2 command and append to /etc/grub.d/40_custom

2. Add the login to /etc/grub.d/40_custom

   sudo vim /etc/grub.d/40_custom

   Or nano if you want a simplier text editor

   And add :

   set superusers="username"  
   password_pbkdf2 username hashed_password

   It should look like this :

   

   Finally don't forget to update GRUB'configuration :
   sudo update-grub2

   update-grub2 is actually a symbolic link that executes grub-mkconfig -o /path/to/grub.cfg

   Now every time you want to boot using GRUB or edit the boot process, it will ask you a login/password.

⚠️
If we can boot a live USB/disk on our machine and the partition /boot is not encrypted we can easily modify the GRUB configuration to bypass what we did before.
So to really secure our GRUB we have to make sure that the /boot partition is encrypted.

Allow automatic boot

The request of login/password every boot could be very annoying. To overcome this we can make GRUB to only ask it when we are editing the boot option. For this we just add the unrestricted mode that allow user to boot without asking the password.
To do that, we just add "--unrestricted" to the variable CLASS in /etc/grub.d/10_custom :

sudo sed -i 's/class os/class os --unrestricted/' /etc/grub.d/10_linux

sed : stream editor, to filter and transorm text
-i : save the changes in the file
's/word1/word2/' : replace the word1 by word2
/filepath/... : the file we want to modify

Or you can simply manually add "--unrestricted" by editing the configuration file

It should look like this :

Don't forget to do a sudo update-grub2 to apply all the changes.

Now the GRUB will ask a login/password only if we are editing the boot configuration.

---

Created : 11/28/2020
Updated : 03/29/2021
Author : AnthonyF