Matthew Schlachter

(Debian Linux) Script for Signing all DKMS Modules with a Machine Owner Key after Updates

In order for some packages to work properly with secure boot enabled on my machine running Debian Linux, I've needed to sign their modules using a Machine Owner Key (MOK). This works well, but isn't especially robust because the signed modules are replaced with unsigned modules during updates. In order to simplify the process after running apt upgrade or similar, I created a script that re-signs all modules with my MOK as needed.


In order to use this script, you will need to have first generated and enrolled a MOK, by following either the official documentation or the steps in my post about connecting a DSLR as a webcam. You will also need to have modules that need to be signed, which I assume you do if you're reading this post.

The script


# Variables
VERSION="$(uname -r)"
SHORT_VERSION="$(uname -r | cut -d . -f 1-2)"

# Small helper functions for printing success/error messages
echosuccess() { printf "\e[32m%s\e[0m\n" "$*"; }
echoerr() { printf "\e[01;31m%s\e[0m\n" "$*" >&2; }

# Input the passphrase for the MOK
cd "$MODULES_DIR/updates/dkms"
echo -n "Passphrase for the private key: "  # Give us a friendly password prompt
read -s KBUILD_SIGN_PIN  # Store the passphrase for the private key
export KBUILD_SIGN_PIN  # Export the variable containing the password

# Sign all unsigned modules in the DKMS directory
for module in "$MODULES_DIR/updates/dkms"/*; do
  module_name="$(basename $module .ko)"
  if $(sudo modinfo "$module_name" | grep signature > /dev/null)
    echosuccess "Module '$module_name' already signed"
    echo "Signing module '$module_name'"
    sudo --preserve-env=KBUILD_SIGN_PIN "$KBUILD_DIR"/scripts/sign-file sha256 /var/lib/shim-signed/mok/MOK.priv /var/lib/shim-signed/mok/MOK.der "$module"
    sudo modinfo "$module_name" | grep signature > /dev/null && echosuccess "Signing module '$module_name' succeeded" || echoerr "Signing module '$module_name' failed"

I have the script saved as ~/bin/sign-modules; make sure that the script has execution permissions set (e.g. chmod a+x ~/bin/sign-modules) and that the script's directory is included in your $PATH variable (e.g. by having PATH="$PATH:~/bin" in your ~/.bashrc file).

With all this in place, making sure your modules are signed should be as simple as running sign-modules after each time that you run apt upgrade.