Signing a Linux Kernel for Secure Boot

Referenced Surface Linux Key Signing

Instructions are for ubuntu, but should work similar for other distros, if they are using shim and grub as bootloader. If your distro is not using shim (e.g. Linux Foundation Preloader), there should be similar steps to complete the signing (e.g. HashTool instead of MokUtil for LF Preloader) or you can install shim to use instead. The ubuntu package for shim is called shim-signed, but please inform yourself on how to install it correctly, so you do not mess up your bootloader.

Since the most recent GRUB2 update (2.02+dfsg1-5ubuntu1) in Ubuntu, GRUB2 does not load unsigned kernels anymore, as long as Secure Boot is enabled. Users of Ubuntu 18.04 will be notified during upgrade of the grub-efi package, that this kernel is not signed and the upgrade will abort.

Instructions adapted from the Ubuntu Blog. Before following, please backup your /boot/EFI directory, so you can restore everything. Follow these steps on your own risk.

Create Signing Keys

Create the config to create the signing key, save as mokconfig.cnf:

 # This definition stops the following lines failing if HOME isn't
 # defined.
 HOME                    = .
 RANDFILE                = $ENV::HOME/.rnd 
 [ req ]
 distinguished_name      = req_distinguished_name
 x509_extensions         = v3
 string_mask             = utf8only
 prompt                  = no

 [ req_distinguished_name ]
 countryName             = <YOURcountrycode>
 stateOrProvinceName     = <YOURstate>
 localityName            = <YOURcity>
 0.organizationName      = <YOURorganization>
 commonName              = Secure Boot Signing Key
 emailAddress            = <YOURemail>

 [ v3 ]
 subjectKeyIdentifier    = hash
 authorityKeyIdentifier  = keyid:always,issuer
 basicConstraints        = critical,CA:FALSE
 extendedKeyUsage        = codeSigning,1.3.6.1.4.1.311.10.3.6
 nsComment               = "OpenSSL Generated Certificate"

Adjust all parts with <YOUR*> to your details.

Create the public and private key for signing the kernel:

 openssl req -config ./mokconfig.cnf \
         -new -x509 -newkey rsa:2048 \
         -nodes -days 36500 -outform DER \
         -keyout "MOK.priv" \
         -out "MOK.der"

Convert the key also to PEM format (mokutil needs DER, sbsign needs PEM):

 openssl x509 -in MOK.der -inform DER -outform PEM -out MOK.pem

Enroll MOK Key

Enroll the key to your shim installation:
```
 sudo mokutil --import MOK.der
```
You will be asked for a password, you will just use it to confirm your key selection in the next step, so choose any.
Restart your system. You will encounter a blue screen of a tool called MOKManager.

Select “Enroll MOK” and then “View key”. Make sure it is your key you created in step 2. Afterwards continue the process and you must enter the password which you provided in step 4. Continue with booting your system.
Verify your key is enrolled via:
```
 sudo mokutil --list-enrolled
```

Sign your Kernel

Sign your installed kernel (it should be at /boot/vmlinuz-[KERNEL-VERSION]-surface-linux-surface):

 sudo sbsign --key MOK.priv --cert MOK.pem /boot/vmlinuz-[KERNEL-VERSION]-generic --output /boot/vmlinuz-[KERNEL-VERSION]-generic.signed

Copy the initram of the unsigned kernel, so we also have an initram for the signed one.
```
 sudo cp /boot/initrd.img-[KERNEL-VERSION]-generic{,.signed}
```

Update GRUB

Update your grub-config
```
 sudo update-grub
```

Reboot and Test

Reboot your system and select the signed kernel. If booting works, you can remove the unsigned kernel:

 sudo mv /boot/vmlinuz-[KERNEL-VERSION]-generic{.signed,}
 sudo mv /boot/initrd.img-[KERNEL-VERSION]-generic{.signed,}
 sudo update-grub

Now your system should run under a signed kernel and upgrading GRUB2 works again. If you want to upgrade the custom kernel, you can sign the new version easily by following above steps again from step seven on. Thus BACKUP the MOK-keys (MOK.der, MOK.pem, MOK.priv).