GRUB hardening

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This guide deals with various ways in which you can harden your GRUB configuration, for security purposes. These steps are optional, but highly recommended by the Libreboot project.

GRUB secure boot with GPG

This uses the free implementation of the GPG standard for encryption and signing/verifying data. We will be using this for checking the signature of a Linux kernel at boot time. More information about GPG can be found on the GPG project website. GRUB has some GPG support built in, for checking signatures.

This tutorial assumes you have a libreboot image (rom) that you wish to modify, to which we shall henceforth refer to as “my.rom”. This tutorial modifies grubtest.cfg, this means signing and password protection will work after switching to it in the main boot menu and bricking due to incorrect configuration will be impossible. After you are satisfied with the setup, you should transfer the new settings to grub.cfg to make your machine actually secure.

First extract the old grubtest.cfg and remove it from the libreboot image:

cbfstool my.rom extract -n grubtest.cfg -f my.grubtest.cfg
cbfstool my.rom remove -n grubtest.cfg

Helpful links:

GRUB Password

The security of this setup depends on a good GRUB password as GPG signature checking can be disabled through the interactive console:

set check_signatures=no

This is good in that it allows you to occasionally boot unsigned liveCDs and such. You may think of supplying signatures on an usb key, but the signature checking code currently looks for .sig when verifying and as such it is not possible to supply signatures in an alternate location.

Note that this is not your LUKS password, but it’s a password that you have to enter in order to use “restricted” functionality (such as console). This protects your system from an attacker simply booting a live USB and re-flashing your firmware. This should be different than your LUKS passphrase and user password.

Use of the diceware method is recommended, for generating secure passphrases (as opposed to passwords). Diceware method involves using dice to generate random numbers, which are then used as an index to pick a random word from a large dictionary of words. You can use any language (e.g. English, German). Look it up on a search engine. Diceware method is a way to generate secure passphrases that are very hard (almost impossible, with enough words) to crack, while being easy enough to remember. On the other hand, most kinds of secure passwords are hard to remember and easier to crack. Diceware passphrases are harder to crack because of far higher entropy (there are many words available to use, but only about 50 commonly used symbols in passwords).

–> The GRUB password can be entered in two ways:

We will (obviously) use the later. Generating the PBKDF2 derived key is done using the grub-mkpasswd-pbkdf2 utility. You can get it by installing GRUB version 2. Generate a key by giving it a password: grub-mkpasswd-pbkdf2

Its output will be a string of the following form: grub.pbkdf2.sha512.10000.HEXDIGITS.MOREHEXDIGITS

Now open my.grubtest.cfg and put the following before the menu entries (prefered above the functions and after other directives). Of course use the pbdkf string that you had generated yourself:

set superusers="root"
password_pbkdf2 root grub.pbkdf2.sha512.10000.711F186347156BC105CD83A2ED7AF1EB971AA2B1EB2640172F34B0DEFFC97E654AF48E5F0C3B7622502B76458DA494270CC0EA6504411D676E6752FD1651E749.8DD11178EB8D1F633308FD8FCC64D0B243F949B9B99CCEADE2ECA11657A757D22025986B0FA116F1D5191E0A22677674C994EDBFADE62240E9D161688266A711

Obviously, replace it with the correct hash that you actually got for the password that you entered. Meaning, not the hash that you see above!

As enabling password protection as above means that you have to input it on every single boot, we will make one menu entry work without it. Remember that we will have GPG signing active, thus a potential attacker will not be able to boot an arbitrary operating system. We do this by adding option --unrestricted to a menuentry definition:

menuentry 'Load Operating System (incl. fully encrypted disks)  [o]' --hotkey='o' --unrestricted {
...

Another good thing to do, if we chose to load signed on-disk GRUB configurations, is to remove (or comment out) unset superusers in function try_user_config:

function try_user_config {
   set root="${1}"
   for dir in boot grub grub2 boot/grub boot/grub2; do
      for name in '' autoboot_ libreboot_ coreboot_; do
         if [ -f /"${dir}"/"${name}"grub.cfg ]; then
            #unset superusers
            configfile /"${dir}"/"${name}"grub.cfg
         fi
      done
   done
}

Why? We allowed booting normally without entering a password above. When we unset superusers and then load a signed GRUB configuration file, we can easily use the command line as password protection will be completely disabled. Disabling signature checking and booting whatever an attacker wants is then just a few GRUB commands away.

As far as basic password setup is concerned we are done and we can now move on to signing.

GPG keys

First generate a GPG keypair to use for signing. Option RSA (sign only) is ok.

Warning: GRUB does not read ASCII armored keys. When attempting to trust … a key filename it will print error: bad signature

mkdir --mode 0700 keys
gpg --homedir keys --gen-key
gpg --homedir keys --export-secret-keys --armor > boot.secret.key # backup
gpg --homedir keys --export > boot.key

Now that we have a key, we can sign some files with it. We have to sign:

Suppose that we have a pair of my.kernel and my.initramfs and an on-disk libreboot_grub.cfg. We sign them by issuing the following commands:

gpg --homedir keys --detach-sign my.initramfs
gpg --homedir keys --detach-sign my.kernel
gpg --homedir keys --detach-sign libreboot_grub.cfg
gpg --homedir keys --detach-sign my.grubtest.cfg

Of course some further modifications to my.grubtest.cfg will be required. We have to trust the key and enable signature enforcement (put this before menu entries):

trust (cbfsdisk)/boot.key
set check_signatures=enforce

What remains now is to include the modifications into the image (rom):

cbfstool my.rom add -n boot.key -f boot.key -t raw
cbfstool my.rom add -n grubtest.cfg -f my.grubtest.cfg -t raw
cbfstool my.rom add -n grubtest.cfg.sig -f my.grubtest.cfg.sig -t raw

… and flashing it.

Copyright © 2017 Fedja Beader fedja@protonmail.ch
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License Version 1.3 or any later version published by the Free Software Foundation with no Invariant Sections, no Front Cover Texts, and no Back Cover Texts. A copy of this license is found in ../fdl-1.3.html

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