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:
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 </path/to/filename>.sig when verifying </path/to/filename> 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:
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.
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:
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>
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