grub2-signing-extension for GRUB2

GRUB2 has got a function called "check_signatures" which automatically checks if your GRUB2 files are signed and have a good signature. If the files aren't signed or have a bad signature GRUB2 won't run them to prevent running malicious software. The GRUB2 signing extension are some scripts which helps you to verify, sign and unsign your GRUB2 bootloader files using gpg.

Requirements

You need

• GRUB2 ( sys-boot/grub:2 )
• GNUpg >= 2.1 ( >= app-crypt/gnupg-2.1 )

Preparation

Before you can use the signing and verification feature you need to generate a keypair as root. Please use a secure passphrase.

# gpg --gen-key

To make gpg able to sign and verify files in a su environment we need to activate the gpg-agent for root.

Edit the file /root/.gnupg/gpg.conf and add the line use-agent.

Save the file and create /root/.gnupg/gpg-agent.conf with the following content

pinentry-program /usr/bin/pinentry-curses
no-grab
default-cache-ttl 1800

How to install the GRUB2 check_signatures feature and using the grub2-signing-extension

First, export your public key.

# gpg --export -o ~/pubkey

Next step, mount /boot and (re)install GRUB2. You need to install the public key into the core and instruct to load the modules gcry_sha256 gcry_dsa and gcry_rsa at start. So you need the following arguments to install it this way

grub-install /dev/sda -k /root/pubkey --modules="gcry_sha256 gcry_dsa gcry_rsa"

Now download the grub2-signing-extension and run make install as root. You will now have grub2-sign, grub2-unsign, grub2-verify and grub2-update-kernel-signature as runable scripts.

To enable GRUB2's check_signatures feature insert the following content at the end of the file of /etc/grub.d/00_header

cat << EOF
set check_signatures=enforce
EOF

Run grub-mkconfig -o /boot/grub/grub.cfg to make the new configuration valid.

Now the time is come to sign your GRUB2 bootloader. Just run grub2-sign, enter your passphrase and that's it.

It's also recommended to install a password in GRUB2! [See ADDENDUM]

How to update the signatures on change

On every change at the GRUB2 core files you need to run grub2-unsign first before you make your changes. Please notice, if you reinstall GRUB2, you should do it as it is said above. Otherwise the signature check won't work.

If you do some changes or updates for the kernel or initramfs, you may want to use grub2-update-kernel-signature instead.

Files

If you didn't read the instruction above here is what the scripts does:

• grub2-sign is signing the bootloader files with root's keypair.
• grub2-unsign is removing the signatures of the bootloader files.
• grub2-verify is checking if your signatures are good. If not, you will see which signature is bad.
• grub2-update-kernel-signature is renewing the signatures in /boot/. (without subdirs) regardless if grub2-verify fails.

Exit codes

You might be interested in the exit codes of grub2-verify to use it in your monitoring tools:

0 - Everything is okay
1 - Found bad signatures
2 - No signatures found at all [GRUB2 is completely unsigned]
3 - Missing signatures [There are correct signatures but some files are unsigned]

Troubleshooting

I receive an error

gpg: signing failed: Permission denied

Make sure that the tty you are in belongs to you (root). Do:

chown root:root $(tty)

I forgot to run grub2-unsign before I made changes. What now?

Run grub2-verify to see, which signature is bad. Remove the signature and run grub2-unsign, after this grub2-sign. Alternatively, if you just updated your kernel/initramfs, run grub2-update-kernel-signatures.

How can I switch off GRUB2's check_signature feature?

Open /etc/grub.d/00_header and remove the part

cat << EOF
set check_signatures=enforce
EOF

Run grub2-unsign and grub-mkconfig -o /boot/grub/grub.cfg.

Also you should reinstall grub2, using something like grub-install /dev/sda.

Suddenly I can't boot! This is YOUR FAULT!

No. An important signature is bad. So GRUB2 didn't run this part of code/configuration/kernel/whatever. You could do a chroot using an USB dongle with a GNU/Linux distribution on it. If you're chrooted to your system run grub2-verify. If you think this happened through an update shortly done by you, you may want to run gpg-agent --daemon ; grub2-update-kernel-signatures.

Okay, I really got some bad signatures not caused by me. What do I do now?

Check your system thoroughly. Check it about malicious software. Check it about malicious connections. CHECK EVERYTHING.

ADDENDUM

How to install a GRUB2 password

Run grub-mkpasswd-pbkdf2 and type a password. Please take care because in the GRUB2 standard installation the keyboard layout is set to en_US. Copy the content of grub.pbkdf2.[...] to your clipboard. Open the file /etc/grub.d/00_header and insert this at the end of the file

cat << EOF
set superusers="yourUsername"
export superusers
password_pbkdf2 yourUsername grub.pbkdf2.[...this string from the clipboard...]
EOF

To boot GNU/Linux automatically and without authentication open /etc/grub.d/10_linux and change the following lines like this

 echo "menuentry '$(echo "$title" | grub_quote)' ${CLASS} \$menuentry_id_option 'gnulinux-$version-$type-$boot_device_id' {" | sed "s/^/$submenu_indentation/"
else
 echo "menuentry '$(echo "$os" | grub_quote)' ${CLASS} \$menuentry_id_option 'gnulinux-simple-$boot_device_id' {" | sed "s/^/$submenu_indentation/"
fi

to

 echo "menuentry '$(echo "$title" | grub_quote)' --unrestricted ${CLASS} \$menuentry_id_option 'gnulinux-$version-$type-$boot_device_id' {" | sed "s/^/$submenu_indentation/"
else
 echo "menuentry '$(echo "$os" | grub_quote)' --unrestricted ${CLASS} \$menuentry_id_option 'gnulinux-simple-$boot_device_id' {" | sed "s/^/$submenu_indentation/"
fi

The important changing is the flag --unrestricted.

Run grub2-unsign to unsign the bootloader.

Then run grub-mkconfig -o /boot/grub/grub.cfg to write the new config.

After this run grub2-sign again to sign the new changings.