Secure Boot Setup

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Revision as of 08:55, 22 November 2023 by Fabian Gruenbichler (talk | contribs) (Update now that PVE supports Secure Boot)
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Using Proxmox VE with Secure Boot

Up to kernel version 6.2.16-7, the Proxmox VE kernel was not out of the box Secure Boot friendly because it did not sign kernel modules at build time, and to get it to boot one had to manually sign all the modules with a DB key after every kernel upgrade.

Since version 6.2.16-8, CONFIG_MODULE_SIG is enabled at build time, which means all the modules shipped together with the kernel in the same package will be trusted by it.

Since Proxmox VE 8.1 and kernel 6.5.11-4, signed packages that support Secure Boot out of the box are available, replacing those shipped by Debian.

Introduction

Secure Boot is a mechanism by which the boot sequence verifies digital signatures of the system which is to be booted. For this the UEFI system has a set of keys and certificates.

  • The platform key (PK). This is the “root” of trust. Once a PK is enrolled and secure boot can be enabled. This key can be used to sign boot loaders. As long as there’s no platform key in the system it is considered to be in “Setup Mode” and no signatures will be validated. Once a “platform key” is enrolled and, if Secure Boot is activated in UEFI, no untrusted bootloaders or kernels can be booted anymore.
  • The key exchange key (KEK). This key is signed by the PK and is used to sign database (db) entries. It can also be used to sign UEFI binaries (such as bootloaders).
  • The database (db) can contain keys, but also signatures or hashes to authorize UEFI binaries. Finally, db keys are trusted by the Linux kernel and can be used to sign modules for DKMS.

Secure Boot is usually setup one of the following 2 ways:

With a custom db key

In this case the system administrator has control over the db key (possibly also the PK and KEK).

This is a simpler case, since the administrator can choose which bootloader to trust. The most straightforward way here is to use Unified Kernel Images with, for example, systemd-boot, since this only requires systemd-boot and the UKI to be sigend with the db key. DKMS modules can also be signed with the db key. Note that this does not directly support MOKs (Machine Owner Keys), these are made available to the kernel only when using the “shim” loader, see the next section. Also note that in this case, grub will refuse to load modules and is therefore much more involved to setup.

Using a shim with Machine Owner Key

In this case the Secure Boot keys are managed by someone else - Microsoft in the case of most UEFI vendors.. UEFI boots a signed shim which has its own set of keys to verify the next stage (either another bootloader or another EFI binary or UKI). For Proxmox products, a custom shim signed by Microsoft which is embedding the public keys used for signing related Proxmox packages is available since November 2023 (Proxmox VE 8.1), and installed by default on new installations.

Machine Owner Keys

In addition to verifying the next stage, the shim also manages a set of keys called “Machine Owner Keys” or MOKs. By default, the kernel trusts MOKs in addition the db key and its own key when loading modules. In this case, a MOK is required to sign the kernel and also for DKMS modules.

MOKs are managed by 3 sets of EFI variables:

  • Non-volatile “bootservice” variables. These are only accessible by boot services (the shim and its MokManager EFI utility). This is where the actual MOKs and configuration is stored by the shim.
  • Volatile runtime mirrors of the state variables. These are how the OS knows which MOK keys actually exist. These are read-only to the OS and only exist if the bootloader (the shim) sets them. They do not themselves persist across restarts and cannot directly be changed. NOTE: When switching away from a shim setup to directly boot with custom keys, the OS will not see the MOKs anymore, since different bootloaders won’t provide these variables.
  • “Request” variables. The OS (usually via mokutil) can set a set of EFI variables to request the shim to make changes to the MOKs. Changes made via the mokutil command line tool do not take effect immediately. Instead, it just tells the shim to run the MokManager utility at the next reboot, so after using mokutil, a reboot is required, the MokManager will appear, and the changes can be reviewed and applied.

Setup instructions for db key variant

Creating custom Secure Boot keys

For this example, we’ll create simple certificates with a “common name” of Example <keytype>. Additionally, a GUID will be associated with our keys, which we generate randomly.

First install the required packages: 

apt install sbsigntool efibootmgr efitools uuid-runtime

Create a directory storing all the files we will generate, and enter it: 

mkdir secureboot
cd secureboot

Then generate the GUID: 

uuidgen --random >GUID.txt

For each of PK, KEK and db we will

  • first create a certificate (*.crt) and key (*.key),
  • then provide a DER representation of the key (*.cer)
  • Create an EFI signature list
  • Sign the certificate with the previous key (or in case of the PK, with itself).

Create the self-signed PK: 

openssl req -x509 -nodes -new -sha256 -days 3650 -newkey rsa:4096 -subj '/CN=Example PK/' -keyout PK.key -out PK.crt
openssl x509 -outform DER -in PK.crt -out PK.cer
cert-to-efi-sig-list -g "$(<GUID.txt)" PK.crt PK.esl
sign-efi-sig-list -g "$(<GUID.txt)" -k PK.key -c PK.crt PK PK.esl PK.auth

Create the KEK, signed by the PK: 

openssl req -x509 -nodes -new -sha256 -days 3650 -newkey rsa:4096 -subj '/CN=Example KEK/' -keyout KEK.key -out KEK.crt
openssl x509 -outform DER -in KEK.crt -out KEK.cer
cert-to-efi-sig-list -g "$(<GUID.txt)" KEK.crt KEK.esl
sign-efi-sig-list -g "$(<GUID.txt)" -k PK.key -c PK.crt KEK KEK.esl KEK.auth

Create the db key, signed by the KEK: 

# openssl req -x509 -nodes -new -sha256 -days 3650 -newkey rsa:4096 -subj '/CN=Example DB/' -keyout db.key -out db.crt
openssl x509 -outform DER -in db.crt -out db.cer
cert-to-efi-sig-list -g "$(<GUID.txt)" db.crt db.esl
sign-efi-sig-list -g "$(<GUID.txt)" -k KEK.key -c KEK.crt db db.esl db.auth

Note that the keys (*.key files) should be stored in a safe place. If no shim (and no MOKs) will be used, the db key needs to be available to the system in order to sign the installed kernels!

Enrolling the custom keys.

There are multiple ways of enrolling the keys. Depending on the UEFI implementation, this may need to be done via the UEFI Setup tool.

While in “Setup Mode” (when no PK is enrolled), many UEFI implementations allow modifying the secure boot keys from within the OS. One easy method in this case is to use sbkeysync. For this, the keys need to be setup in /etc/secureboot/keys as follows: 

mkdir /etc/secureboot
mkdir /etc/secureboot/keys
mkdir /etc/secureboot/keys/PK
cp PK.auth /etc/secureboot/keys/PK/PK.auth
mkdir /etc/secureboot/keys/KEK
cp KEK.auth /etc/secureboot/keys/KEK/KEK.auth
mkdir /etc/secureboot/keys/db
cp db.auth /etc/secureboot/keys/db/db.auth

After this, run sbkeysync -v. This will enroll the KEK and db key, but not the PK.

In case enrolling from the OS fails, putting the certificates (NOT the keys!) onto the ESP, and then manually importing them using your UEFI firmware setup interface should work.

Bootloader setup

Before enrolling the PK we need to make sure we have a signed working bootloader.

Using grub-efi-amd64-signed.

Since grub-efi-amd64-signed is signed by a key trusted by shim-signed, the most straightforward thing to do now is to install grub-efi-amd64-signed. This will ensure that grub is working. 

apt install shim-signed grub-efi-amd64-signed

When using custom keys, we need to ensure that the shim is signed by our db key. The easiest way to do this is to make a full copy of the /boot/efi/EFI/proxmox/ and, if grub or the shim get updated, manually perform this step again (or use systemd.path files or other means to automate this.) 

mkdir -p /boot/efi/EFI/custom
cp -t /boot/efi/EFI/custom/ -a /boot/efi/EFI/proxmox/*
cd /boot/efi/EFI/custom/
sbsign --key /root/secureboot/db.key --cert /root/secureboot/db.crt --output shimx64.efi shimx64.efi
sbsign --key /root/secureboot/db.key --cert /root/secureboot/db.crt --output mmx64.efi mmx64.efi
sbsign --key /root/secureboot/db.key --cert /root/secureboot/db.crt --output fbx64.efi fbx64.efi

When doing this the first time, we also need to create an EFI boot entry for the shim. For this, we need to know which partition the ESP is on. For instance, in a default single disk Proxmox VE installation, this will be on /dev/sda2. This can be seen via 

# findmnt /boot/efi
TARGET    SOURCE    FSTYPE OPTIONS
/boot/efi /dev/sda2 vfat   rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro

/dev/sda is the disk, and 2 the partition number, so our bootloader entry is created as follows: 

efibootmgr --unicode --disk /dev/sda --part 2 --create --label CustomShim --loader /EFI/custom/shimx64.efi

We should verify that this entry is now the default boot entry. 

efibootmgr -v
...
BootOrder: 000N,000X,...
Boot0000* ...
...
Boot000N* CustomShim    HD(2,GPT,d078e8e3-ad59-4e44-af9d-a89429959d81,0x800,0x100000)/File(\EFI\custom\shimx64.efi)
...

The important bit is that the first number under BootOrder matches the number containing \EFI\custom\shimx64.efi in its path output.

Signing the kernel

In order to make sure we can reboot, we need to sign the current kernels using the db key: 

cd /boot
for i in vmlinuz-*; do sbsign --key /root/secureboot/db.key --cert /root/secureboot/db.crt --output "$i" "$i"; done

Make sure future kernel upgrades are signed.

We only signed the currently installed kernels, but kernel upgrades also need to be sigend. For this we can create a post-install hook for the kernel in /etc/kernel/postinst.d.

Create an executable file named /etc/kernel/postinst.d/zz-sign-kernel with the following contents. 

#!/bin/sh

set -e

kver="$1"

sbsign --key /root/secureboot/db.key --cert /root/secureboot/db.crt --output "/boot/vmlinuz-$kver" "/boot/vmlinuz-$kver"

Enrolling the PK

When using sbkeysync it may be enough to just run: 

sbkeysync --pk

If this fails with 

Error writing key update: Invalid argument
Error syncing keystore file /etc/secureboot/keys/PK/PK.auth

we can try another way: 

chattr -i /sys/firmware/efi/efivars/PK-*
efi-updatevar -f /etc/secureboot/keys/PK/PK.auth PK

If this also does not work, your UEFI may only allow doing this via its UEFI Setup utility. Copy the platform key files to the EFI system partition or a USB stick supported by your UEFI Setup, and try to enroll it from there. Otherwise, contact your system manufacturer for help.

DKMS

In order for the kernel to accept DKMS modules they need to be signed.

DKMS signs modules at build time. By default, a key will be found in /var/lib/dkms/mok.pub. When using a shim setup, this key can be enrolled as a MOK directly. To do this, run mokutil --import /var/lib/dkms/mok.pub and reboot. The MokManager tool will show up and the key can be enrolled via its menu.

DKMS can be configured via /etc/dkms/framework.conf via the following variables: 

mok_signing_key=/root/secureboot/db.key
mok_certificate=/root/secureboot/db.cer

Setup instructions for shim + MOK variant

This variant assumes you are currently booting using the GRUB bootloader from a single disk, with the ESP mounted at /boot/efi, or with multiple ESPs managed by proxmox-boot-tool which have been initialized in grub mode.

Required packages

Install the signed packages from Proxmox, which should be trusted by default by your vendor’s UEFI implementation: 

apt install shim-signed grub-efi-amd64-signed mokutil

This should add a boot entry for booting using shim: 

# efibootmgr -v
BootCurrent: 0008
Timeout: 3 seconds
BootOrder: 0008,0002,0001,0003,0004,0005,0006,0000,0007
Boot0000* UiApp FvVol(7cb8bdc9-f8eb-4f34-aaea-3ee4af6516a1)/FvFile(462caa21-7614-4503-836e-8ab6f4662331)
Boot0001* UEFI QEMU DVD-ROM QM00003     PciRoot(0x0)/Pci(0x1,0x1)/Ata(1,0,0)N.....YM....R,Y.
Boot0002* UEFI QEMU QEMU HARDDISK       PciRoot(0x0)/Pci(0x5,0x0)/Pci(0x1,0x0)/SCSI(0,0)N.....YM....R,Y.
Boot0003* UEFI PXEv4 (MAC:4ABDC33F1387) PciRoot(0x0)/Pci(0x12,0x0)/MAC(4abdc33f1387,1)/IPv4(0.0.0.00.0.0.0,0,0)N.....YM....R,Y.
Boot0004* UEFI PXEv6 (MAC:4ABDC33F1387) PciRoot(0x0)/Pci(0x12,0x0)/MAC(4abdc33f1387,1)/IPv6([::]:<->[::]:,0,0)N.....YM....R,Y.
Boot0005* UEFI HTTPv4 (MAC:4ABDC33F1387)        PciRoot(0x0)/Pci(0x12,0x0)/MAC(4abdc33f1387,1)/IPv4(0.0.0.00.0.0.0,0,0)/Uri()N.....YM....R,Y.
Boot0006* UEFI HTTPv6 (MAC:4ABDC33F1387)        PciRoot(0x0)/Pci(0x12,0x0)/MAC(4abdc33f1387,1)/IPv6([::]:<->[::]:,0,0)/Uri()N.....YM....R,Y.
Boot0007* EFI Internal Shell    FvVol(7cb8bdc9-f8eb-4f34-aaea-3ee4af6516a1)/FvFile(7c04a583-9e3e-4f1c-ad65-e05268d0b4d1)
Boot0008* proxmox       HD(2,GPT,e7b6a3b2-77b6-429e-a966-eb65c268ae5e,0x800,0x100000)/File(\EFI\proxmox\shimx64.efi)

The entry number 0008 points at the shim EFI binary, and is set as first boot option in the BootOrder variable.

We also need the following packages that provide tools for key and signature management: 

# apt install sbsigntool mokutil

Generate Machine Owner Key

Next, we create the MOK (only needed if you want to manually sign custom boot components like kernel images): 

# mkdir secureboot
# cd secureboot
# openssl req -x509 -nodes -new -sha256 -days 3650 -newkey rsa:4096 -subj '/CN=Machine Owner Key/' -keyout mok.key -out mok.crt
# openssl x509 -outform DER -in mok.crt -out mok.cer

Enrolling the MOK

Enroll the key using mokutil: 

# mokutil --import /root/secureboot/mok.cert

This command will prompt you for a password that you need to enter to confirm the enrollment after the next reboot!

Rebooting the system now will present you with a shim dialogue (with a timeout of 10 seconds) that allows you to confirm the MOK enrollment. This only needs to be done once for each MOK you generate.

Signing a custom kernel

Proxmox kernels starting with version 6.5.11-4-pve are signed by Proxmox (available in the proxmox-kernel-X.Y.Z-N-pve-signed package). These kernel images do not need to be signed manually.

Only if you want to boot older or custom kernels with Secure Boot enabled, you need to manually sign them using the MOK before rebooting: 

cd /boot
for i in vmlinuz-*; do sbsign --key /root/secureboot/mok.key --cert /root/secureboot/mok.crt --output "$i" "$i"; done

Make sure future kernel upgrades are signed.

We only signed the currently installed kernels, but kernel upgrades also need to be signed if they are not already installed via the -signed package, for example, if you regularly build and deploy custom kernel packages.

For this we can create a post-install hook for the kernel in /etc/kernel/postinst.d.

Create an executable file named /etc/kernel/postinst.d/zz-sign-kernel with the following contents. 

#!/bin/sh

set -e

kver="$1"

sbsign --key /root/secureboot/mok.key --cert /root/secureboot/mok.crt --output "/boot/vmlinuz-$kver" "/boot/vmlinuz-$kver"

DKMS

In order for the kernel to accept DKMS modules they need to be signed.

DKMS signs modules at build time. By default, a key will be found in /var/lib/dkms/mok.pub. This key can be enrolled as a MOK directly. To do this, run mokutil --import /var/lib/dkms/mok.pub and reboot. The MokManager tool will show up and the key can be enrolled via its menu, just like for the ‘main’ MOK we created and enrolled above.

DKMS can also be configured via /etc/dkms/framework.conf via the following variables: 

mok_signing_key=/root/secureboot/mok.key
mok_certificate=/root/secureboot/mok.cer

to use a single MOK for both kernel and DKMS signing.

Enabling and verifying Secure Boot

Enabling Secure Boot should be possible either via shim, or using your UEFI firmware setup utility.

When Secure Boot is enabled, you should see the following output when querying via mokutil: 

# mokutil --sb-state
SecureBoot enabled

Additionally, the kernel will print messages like these on bootup: 

# journalctl -b --grep secure
Jul 17 14:24:33 pve kernel: secureboot: Secure boot enabled
Jul 17 14:24:33 pve kernel: Kernel is locked down from EFI Secure Boot mode; see man kernel_lockdown.7

Disabling Secure Boot

In case any of the validation steps fail, your system might refuse to boot at all. In this case, disabling Secure Boot using your UEFI firmware setup utility should allow you to temporarily boot your system without validation and being locked down in order to fix the issue.

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