Hardening GNU GRUB


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This article only applies to those people who use the GNU GRUB bootloader as their default payload (options besides GNU GRUB are also available in Libreboot). Whenever this article refers to GNU GRUB, or configuration files used in GNU GRUB, it is referring exclusively to those files hosted in CBFS (coreboot file system) in the Libreboot ROM image. In this configuration, GNU GRUB is running on bare metal as a coreboot payload (instead of relying on BIOS or UEFI services, like it does on most x86 based configurations).

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

GNU GRUB provides many advanced security features, which most people don’t know about but are fully documented on the Libreboot website. Read on!

This article doesn’t cover how to dump your ROM, or flash a new one. Please read other sections in the Libreboot documentation if you don’t know how to do that. As such, this is an expert only guide. There is a great possibility for bricking your system if you follow this guide incorrectly, or otherwise don’t know what you’re doing.

GRUB secure boot with GPG

GNU GRUB contains code, based on GnuPG, that can verify PGP signatures on any type of file, on any storage medium supported by GNU GRUB (it supports basically everything, including CBFS which is short for coreboot file system and it is what we will focus on in this article). We will be using this functionality to verify the signature of a Linux kernel, at boot time. In conjunction with reproducible builds (both Libreboot and your Linux kernel), this can greatly improve system security; Debian is an excellent example of a GNU+Linux distribution that is fully reproducible nowadays (in stable releases).

For your reference: a reproducible build is one where, given a precise (and well documented) development setup, the exact same binary can be produced each time the source code is compiled when that very same development setup is replicated by another person. In other words, the file checksum (e.g. SHA512 hash) will be exactly the same at all times. In practise, this means that metadata such as time stamps are not included in the binary, or if they are, they are constant (in many scenarios, it’s based on the date of a Git commit ID that the build is based on, if the software is built from a Git repository). More information about reproducible builds can be found here:

https://reproducible-builds.org/

Reproducibility is a key goal of the Libreboot project, though it has not yet achieved that goal. However, it is an important part of any secure system. We suggest that, when securing your Libreboot system as instructed by this guide, you should also use a reproducible GNU+Linux distribution (because checking GPG signatures on a non-reproducible binary, such as a Linux kernel, is meaningless if that binary can be compromised as a result of literally not being able to verify that the source code actually corresponds to the provided binary, which is exactly what reproducible builds allow). If someone else compiles an executable for you, and that executable is non-reproducible, you have no way to verify that the source code they provided actually corresponds to the binary they gave you. Based on these facts, we can observe that checking GPG signatures will improve your operational security, but only in specific circumstances under controlled conditions.

This tutorial assumes you have a Libreboot image (ROM) that you wish to modify, which from now on we will refer to simply as my.rom. It should go without saying that this ROM uses the GNU GRUB bootloader as payload. This page shows how to modify grubtest.cfg, which means that 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 new setup, you should transfer the new settings to grub.cfg to make your machine truly 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

You can build cbfstool in the Libreboot build system. Run this command:

./build module cbutils

This assumes that you already downloaded coreboot:

./download coreboot

This, in turn, assumes that you have installed the build dependencies for Libreboot. On Ubuntu 20.04 and other apt-get distros, you can do this:

./build dependencies ubuntu2004

The cbfstool executables will be under each coreboot directory, under each coreboot/boardname/ directory for each board. Just pick one, presumably from the coreboot directory for your board. Libreboot creates multiple coreboot archives for different board revisions, on different boards.

References:

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 useful because it allows you to occasionally boot unsigned live CD/USB media and such. You might consider supplying signatures on a USB stick, but the signature checking code currently looks for /path/to/filename.sig when verifying /path/to/filename and, as such, it will be impossible to supply signatures in any other location (unless the software is modified accordingly).

It’s worth noting that this is not your LUKS password but, rather, a password that you must enter in order to use restricted functionality (such as the GNU GRUB terminal for executing commands). This behaviour protects your system from an attacker simply booting a live USB key (e.g. live GNU+Linux distribution) for the purpose of flashing modified boot firmware, which from your perspective is compromised boot firmware. This should be different than your LUKS passphrase and user password.

GNU GRUB supports storing salted, hashed passwords in the configuration file. This is a far more secure configuration, because an attacker cannot simply read your password as plain text inside said file.

Use of the diceware method is strongly recommended, for generating secure passphrases (as opposed to passwords). The diceware method involves rolling 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). This high level of entropy is precisely what makes such pass phrases secure, even if an attacker knows exactly which dictionary you used!

The GRUB password can be stored in one of two ways:

We will obviously use the latter method. 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:

NOTE: This utility is included under the grub/ directory, when you build GRUB using the Libreboot build system. Run the following commands (assuming you have the correct build dependencies installed) to build GNU GRUB, from the Libreboot Git repository:

./download grub

./build module grub

The following executable will then be available under the grub/ directory:

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 obtained for the password you entered. In other words, do not use the hash that you see above!

With this configuration in place, you must now enter the passphrase every single time you boot your computer. This completely restricts an attacker in such a way that they cannot simply boot an arbitrary operating system on your computer. NOTE: An attacker could still open your system and re-flash new firmware externally. You should implement some detection mechanism, such as epoxy applied in a random pattern on every screw; this slows down the attack and means that you will know someone tampered with it because they cannot easily re-produce the exact same blob of epoxy in the same pattern (when you apply it, swirl it around a bit for a few minutes while it cures. The purpose is not to prevent disassembly, but to slow it down and make it detectable when it has occured).

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
}

The unset superusers command disables password authentication, which will allow the attacker to boot an arbitrary operating system, regardless of signature checking. The default Libreboot configuration is tweaked for easy of use by end users, and it is not done with security in mind (though security is preferred). Thus, Libreboot is less restrictive by default. What you are doing, per this article, is making your system more secure but at the expense of user-friendliness.

That just about covers it, where password setup is concerned!

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 on the screen.

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 must sign:

Suppose that we have a pair of my.kernel and my.initramfs and an on-disk libreboot_grub.cfg. We will sign them by running 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 need 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 Libreboot 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

Now, flash it. If it works, copy it over to grub.cfg in CBFS.

Markdown file for this page: https://libreboot.org/docs/gnulinux/grub_hardening.md

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