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 )

Installation

Arch Linux (AUR)

• Import Bandie's GPG key through running gpg --recv-keys E2D7876915312785DC086BFCC1E133BC65A822DD.
• Use your favourite AUR helper to install grub2-signing-extension.

From github

• Import Bandie's GPG key through running gpg --recv-keys E2D7876915312785DC086BFCC1E133BC65A822DD.
• Download the grub2-signing-extension and it's signature.
• Run gpg --verify grub2-signing-extension*.tar.gz.asc to make sure that everything is alright.
• Unpack the tar archive. tar xvf grub2-signing-extension*.tar.gz
• Change into the grub2-signing-extension directory.
• Run make install as root.

You will now have grub-sign, grub-unsign, grub-verify and grub-update-kernel-signature as runable scripts.

Enabling GRUB2 check_signatures feature

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

• Run gpg --gen-key as root. Please use a secure passphrase.
• Activate the gpg-agent for root so that you are able to sign and verify files in a su environment. To do that:
  • Edit the file /root/.gnupg/gpg.conf and add the line use-agent. Save the file.
  • Create /root/.gnupg/gpg-agent.conf with the following content

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

• Export your public key through running gpg --export -o ~/pubkey.
• mount /boot (assuming your /boot partition is in your /etc/fstab)
• (Re)install GRUB2. The following command will install root's public key into the core and instruct to load the modules gcry_sha256 gcry_dsa and gcry_rsa at start so that GRUB2 will be able to do verifications.
  • grub-install /dev/sda -k /root/pubkey --modules="gcry_sha256 gcry_dsa gcry_rsa"
• 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
    

• Rungrub-mkconfig -o /boot/grub/grub.cfg to make the new configuration valid.
• Sign your bootloader running grub-sign and enter your GPG passphrase.

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

How to update the signatures on changes

On every change at the GRUB2 core files you need to run grub-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 grub-update-kernel-signature instead.

Files

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

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

Exit codes

You might be interested in the exit codes of grub-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 grub-verify to see, which signature is bad. Remove the signature and run grub-unsign, after this grub-sign. Alternatively, if you just updated your kernel/initramfs/grub.cfg, run grub-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 grub-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 grub-verify. If you think this happened through an update shortly done by you, you may want to run gpg-agent --daemon ; grub-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

• Generate a GRUB2 password string through running grub-mkpasswd-pbkdf2. Please take care because in the GRUB2 standard installation the keyboard layout is set to en_US.

• Copy the generated grub.pbkdf2.[...] string 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 [...this grub.pbkdf2.* string from the clipboard...]
  EOF
  

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

      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 grub-unsign to unsign the bootloader.

• Run grub-mkconfig -o /boot/grub/grub.cfg to write the new config.

• Run grub-sign to sign the new changings.