#1. Install the mkutil package to be able to do signed.
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install mokutil
#2. generate the signature file:
openssl req -new -x509 -newkey rsa:2048 -keyout MOK.priv -outform DER -out MOK.der -nodes -days 36500 -subj "/CN=VirtualBox/"
#3. Then add it to the kernel:
sudo /usr/src/linux-headers-$(uname -r)/scripts/sign-file sha256 ./MOK.priv ./MOK.der $(modinfo -n vboxdrv)
#4. Register it for the Secure Boot.
IMPORTANT! That will ask you for a password, put the one you want, you will only have to use it once in the next reboot.
sudo mokutil --import MOK.der
#5 Restart - At boost menu Choose Enroll MDK > Continue > Yes > type password at step 4