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<title>GRUB hardening</title>
</head>
<body>
<div class="section">
<h1>GRUB hardening</h1>
<p>
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.
</p>
<p>
<a href="./">Back to previous index</a>
</p>
</div>
<div class="section">
<h1>
GRUB secure boot with GPG
</h1>
<p>
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
<a href="https://www.gnu.org/software/gnupg/">GPG project website</a>.
GRUB has some GPG support built in, for checking signatures.
</p>
<p>
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.
</p>
<p>
First extract the old grubtest.cfg and remove it from the libreboot image:
<pre>
cbfstool my.rom extract -n grubtest.cfg -f my.grubtest.cfg
cbfstool my.rom remove -n grubtest.cfg
</pre>
</p>
<p>
Helpful links:
<ul>
<li><a href="https://www.gnu.org/software/grub/manual/html_node/Security.html#Security">
GRUB manual #security</a></li>
<li><a href="http://git.savannah.gnu.org/cgit/grub.git/tree/docs/grub.texi">
GRUB info pages</a></li>
<li><a href="https://libreboot.org/faq/#firmware-hddssd">
SATA connected storage considered dangerous until proven otherwise.</a></li>
<li><a href="https://www.coreboot.org/GRUB2#Security">
Coreboot GRUB security howto</a></li>
</ul>
</p>
</div>
<div class="section">
<h1>
GRUB Password
</h1>
<p>
The security of this setup depends on a good GRUB password as GPG signature
checking can be disabled through the interactive console:
</p>
<pre>set check_signatures=no</pre>
<p>
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 </path/to/filename>.sig when verifying
</path/to/filename> and as such it is not possible to supply signatures
in an alternate location.
</p>
<p>
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.
<b>This should be different than your LUKS passphrase and user password.</b>
</p>
<p>
Use of the <i>diceware method</i> 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 pass<em>words</em>).
</p>
-->
<p>
The GRUB password can be entered in two ways:
<ul>
<li>plaintext</li>
<li>protected with <a href="https://en.wikipedia.org/wiki/Pbkdf2">PBKDF2</a></li>
</ul>
We will (obviously) use the later. Generating the PBKDF2 derived key is done
using the <b>grub-mkpasswd-pbkdf2</b> utility. You can get it by installing
GRUB version 2. Generate a key by giving it a password:
</p>
<pre>grub-mkpasswd-pbkdf2</pre>
<p>
Its output will be a string of the following form:
grub.pbkdf2.sha512.10000.HEXDIGITS.MOREHEXDIGITS
</p>
<p>
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:
</p>
<pre>
set superusers="root"
password_pbkdf2 root grub.pbkdf2.sha512.10000.711F186347156BC105CD83A2ED7AF1EB971AA2B1EB2640172F34B0DEFFC97E654AF48E5F0C3B7622502B76458DA494270CC0EA6504411D676E6752FD1651E749.8DD11178EB8D1F633308FD8FCC64D0B243F949B9B99CCEADE2ECA11657A757D22025986B0FA116F1D5191E0A22677674C994EDBFADE62240E9D161688266A711</pre>
<p>
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!
</p>
<p>
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
<b>--unrestricted</b> to a menuentry definition:
</p>
<pre>menuentry 'Load Operating System (incl. fully encrypted disks) [o]' --hotkey='o' --unrestricted {
...</pre>
<p>
Another good thing to do, if we chose to load signed on-disk GRUB configurations,
is to remove (or comment out) <b>unset superusers</b> in function try_user_config:
</p>
<pre>
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
}</pre>
<p>
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.
</p>
<p>
As far as basic password setup is concerned we are done and we can now move on to signing.
</p>
</div>
<div class="section">
<h1>
GPG keys
</h1>
<p>
First generate a GPG keypair to use for signing. Option RSA (sign only) is ok.
</p>
<p>
<b>Warning:</b> GRUB does not read ASCII armored keys.
When attempting to trust ... a key filename it will print error: bad signature
</p>
<pre>
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</pre>
<p>
Now that we have a key, we can sign some files with it. We have to sign:
<ul>
<li>a kernel</li>
<li>(if we have one) an initramfs</li>
<li>(if we wish to transfer control to it) an on-disk grub.cfg</li>
<li>grubtest.cfg (this is so one can go back to grubtest.cfg after signature
checking is enforced. You can always get back to grub.cfg by pressing ESC,
but afterwards grubtest.cfg is not signed and it will not load.</li>
</ul>
Suppose that we have a pair of <b>my.kernel</b> and <b>my.initramfs</b>
and an on-disk <b>libreboot_grub.cfg</b>. We sign them by issuing the
following commands:
</p>
<pre>
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
</pre>
<p>
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):
</p>
<pre>
trust (cbfsdisk)/boot.key
set check_signatures=enforce
</pre>
<p>
What remains now is to include the modifications into the image (rom):
</p>
<pre>
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
</pre>
<p>
... and flashing it.
</p>
</div>
<div class="section">
<p>
Copyright © 2017 Fedja Beader <fedja@protonmail.ch><br/>
Permission is granted to copy, distribute and/or modify this document
under the terms of the Creative Commons Attribution-ShareAlike 4.0 International license
or any later version published by Creative Commons;
A copy of the license can be found at <a href="../cc-by-sa-4.0.txt">../cc-by-sa-4.0.txt</a>
</p>
<p>
Updated versions of the license (when available) can be found at
<a href="https://creativecommons.org/licenses/by-sa/4.0/legalcode">https://creativecommons.org/licenses/by-sa/4.0/legalcode</a>
</p>
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