Full disk encryption, including /boot: Unlocking LUKS devices from GRUB

1 Introduction

So called “full disk encryption” is often a misnomer, because there is typically a separate plaintext partition holding /boot. For instance the Debian Installer does this in its “encrypted LVM” partitioning method. Since not all bootloaders are able to unlock LUKS devices, a plaintext /boot is the only solution that works for all of them.

However, GRUB2 is (since Jessie) able to unlock LUKS devices with its cryptomount command, which therefore enables encryption of the /boot partition as well: using that feature reduces the amount of plaintext data written to disk. It is especially interesting when GRUB is installed to a read-only media, for instance as coreboot payload flashed to a write-protected chip. On the other hand, it is incompatible with some other features that only enabled later at initramfs stage, such as splash screens or remote unlocking.

Since enabling unlocking LUKS devices from GRUB isn’t exposed to the d-i interface (as of Buster), people have come up with various custom workarounds. But as of Buster cryptsetup(8) defaults to a new LUKS header format version, which isn’t supported by GRUB as of 2.04. Hence the pre-Buster workarounds won’t work anymore. Until LUKS version 2 support is added to GRUB2, the device(s) holding /boot needs to be in LUKS format version 1 to be unlocked from the boot loader.

This document describes a generic way to unlock LUKS devices from GRUB for Debian Buster.

2 Encrypting the device holding /boot

There are two alternatives here:

These two alternatives are described in the two following sub-sections.

We assume the system resides on a single drive /dev/sda, partitioned with d-i’s “encrypted LVM” scheme:

root@debian:~# lsblk -o NAME,FSTYPE,MOUNTPOINT /dev/sda
NAME                    FSTYPE      MOUNTPOINT
├─sda1                  ext2        /boot
└─sda5                  crypto_LUKS
  └─sda5_crypt          LVM2_member
    ├─debian--vg-root   ext4        /
    └─debian--vg-swap_1 swap        [SWAP]

Note: The partition layout of your system may differ.

2.1 Formatting the existing /boot partition to LUKS1

Since the installer creates a separate (plaintext) /boot partition by default in its “encrypted LVM” partitioning method, the simplest solution is arguably to re-format it as LUKS1, especially if the root device is in LUKS2 format.

That way other partitions, including the one holding the root file system, can remain in LUKS2 format and benefit from the stronger security guaranties and convenience features of the newer version: more secure (memory-hard) Key Derivation Function, backup header, ability to offload the volume key to the kernel keyring (thus preventing access from userspace), custom sector size, persistent flags, unattended unlocking via kernel keyring tokens, etc.

Furthermore every command in this sub-section can be run from the main system: no need to reboot into a live CD or an initramfs shell.

  1. Before copying content of the /boot directory, remount it read-only to make sure data is not modified while it’s being copied.

    root@debian:~# mount -oremount,ro /boot
  2. Archive the directory elsewhere (on another device), and unmount it afterwards.

    root@debian:~# install -m0600 /dev/null /tmp/boot.tar
    root@debian:~# tar -C /boot --acls --xattrs --one-file-system -cf /tmp/boot.tar .
    root@debian:~# umount /boot

    (If /boot has sub-mountpoints, like /boot/efi, you’ll need to unmount them as well.)

  3. Optionally, wipe out the underlying block device (assumed to be /dev/sda1 in the rest of this sub-section).

    root@debian:~# dd if=/dev/urandom of=/dev/sda1 bs=1M status=none
    dd: error writing '/dev/sda1': No space left on device
  4. Format the underlying block device to LUKS1. (Note the --type luks1 in the command below, as Buster’s cryptsetup(8) defaults to LUKS version 2 for luksFormat.)

    root@debian:~# cryptsetup luksFormat --type luks1 /dev/sda1
    This will overwrite data on /dev/sda1 irrevocably.
    Are you sure? (Type uppercase yes): YES
    Enter passphrase for /dev/sda1:
    Verify passphrase:
  5. Add a corresponding entry to crypttab(5) with mapped device name boot_crypt, and open it afterwards.

    root@debian:~# uuid="$(blkid -o value -s UUID /dev/sda1)"
    root@debian:~# echo "boot_crypt UUID=$uuid none luks" | tee -a /etc/crypttab
    root@debian:~# cryptdisks_start boot_crypt
    Starting crypto disk...boot_crypt (starting)...
    Please unlock disk boot_crypt:  ********
    boot_crypt (started)...done.
  6. Create a file system on the mapped device. Assuming source device for /boot is specified by its UUID in the fstab(5) – which the Debian Installer does by default – reusing the old UUID avoids editing the file.

    root@debian:~# grep /boot /etc/fstab
    # /boot was on /dev/sda1 during installation
    UUID=c104749f-a0fa-406c-9e9a-3fc01f8e2f78 /boot           ext2    defaults        0       2
    root@debian:~# mkfs.ext2 -m0 -U c104749f-a0fa-406c-9e9a-3fc01f8e2f78 /dev/mapper/boot_crypt
    mke2fs 1.44.5 (15-Dec-2018)
    Creating filesystem with 246784 1k blocks and 61752 inodes
    Filesystem UUID: c104749f-a0fa-406c-9e9a-3fc01f8e2f78
  7. Finally, mount /boot again from fstab(5), and copy the saved tarball to the new (and now encrypted) file system.

    root@debian:~# mount -v /boot
    mount: /dev/mapper/boot_crypt mounted on /boot.
    root@debian:~# tar -C /boot --acls --xattrs -xf /tmp/boot.tar

    (If /boot had sub-mountpoints, like /boot/efi, you’ll need to mount them back as well.)

You can skip the next sub-section and go directly to Enabling cryptomount in GRUB2. Note that init(1) needs to unlock the /boot partition again during the boot process. See Avoiding the extra password prompt for details and a proposed workaround. (You’ll need to substitute / resp. sda5 with /boot resp. sda1 in that section, however only steps 1-3 are relevant here: no need to copy the key file to the initramfs image since /boot can be unlocked and mounted later during the boot process.)

2.2 Moving /boot to the root file system

The previous sub-section described how to to re-format the /boot partition as LUKS1. Alternatively, it can be moved to the root file system, assuming the latter is not held by any LUKS2 device. (As shown below, LUKS2 devices created with default parameters can be “downgraded” to LUKS1.)

The advantage of this method is that the original /boot partition can be preserved and used in case of disaster recovery (if for some reason the GRUB image is lacking the cryptodisk module and the original plaintext /boot partition is lost, you’d need to reboot into a live CD to recover). Moreover increasing the number of partitions increases usage pattern visibility: a separate /boot partition, even encrypted, will likely leak the fact that a kernel update took place to an attacker with access to both pre- and post-update snapshots.

On the other hand, the downside of that method is that the root file system can’t benefit from the nice LUKS2 improvements over LUKS1, some of which were listed above. Another (minor) downside is that space occupied by the former /boot partition (typically 256MiB) becomes unused and can’t easily be reclaimed by the root file system.

2.2.1 Downgrading LUKS2 to LUKS1

Check the LUKS format version on the root device (assumed to be /dev/sda5 in the rest of this sub-section):

root@debian:~# cryptsetup luksDump /dev/sda5 | grep -A1 "^LUKS"
LUKS header information
Version:        2

Here the LUKS format version is 2, so the device needs to be converted to LUKS version 1 to be able to unlock from GRUB. Unlike the rest of this document, conversion can’t be done on an open device, so you’ll need reboot into a live CD or an initramfs shell. (The (initramfs) prompt strings in this sub-section indicates commands that are executed from an initramfs shell.) Also, if you have valuable data in the root partition, then make sure you have a backup (at least of the LUKS header)!

Run cryptsetup convert --type luks1 DEVICE to downgrade. However if the device was created with the default parameters then in-place conversion will fail:

(initramfs) cryptsetup convert --type luks1 /dev/sda5

This operation will convert /dev/sda5 to LUKS1 format.

Are you sure? (Type uppercase yes): YES
Cannot convert to LUKS1 format - keyslot 0 is not LUKS1 compatible.

This is because its first key slot uses Argon2 as Password-Based Key Derivation Function (PBKDF) algorithm:

(initramfs) cryptsetup luksDump /dev/sda5 | grep "PBKDF:"
        PBKDF:      argon2i

Argon2 is a memory-hard function that was selected as the winner of the Password-Hashing Competition; LUKS2 devices use it by default for key slots, but LUKS1’s only supported PBKDF algorithm is PBKDF2. Hence the key slot has to be converted to PBKDF2 prior to LUKS format version downgrade.

(initramfs) cryptsetup luksConvertKey --pbkdf pbkdf2 /dev/sda5
Enter passphrase for keyslot to be converted:

Now that all key slots use the PBKDF2 algorithm, the device shouldn’t have any LUKS2-only features left, and can be converted to LUKS1.

(initramfs) cryptsetup luksDump /dev/sda5 | grep "PBKDF:"
        PBKDF:      pbkdf2
(initramfs) cryptsetup convert --type luks1 /dev/sda5

This operation will convert /dev/sda5 to LUKS1 format.

Are you sure? (Type uppercase yes): YES
(initramfs) cryptsetup luksDump /dev/sda5 | grep -A1 "^LUKS"
LUKS header information

2.2.2 Moving /boot to the root file system

(The moving operation can be done from the normal system. No need to reboot into a live CD or an initramfs shell if the root file system resides in a LUKS1 device.)

  1. To ensure data is not modified while it’s being copied, remount /boot read-only.

    root@debian:~# mount -oremount,ro /boot
  2. Recursively copy the directory to the root file system, and replace the old /boot mountpoint with the new directory.

    root@debian:~# cp -axT /boot /boot.tmp
    root@debian:~# umount /boot
    root@debian:~# rmdir /boot
    root@debian:~# mv -T /boot.tmp /boot

    (If /boot has sub-mountpoints, like /boot/efi, you’ll need to unmount them first, and then remount them once /boot has been moved to the root file system.)

  3. Comment out the fstab(5) entry for the /boot mountpoint. Otherwise at reboot init(1) will mount it and therefore shadow data in the new /boot directory with data from the old plaintext partition.

    root@debian:~# grep /boot /etc/fstab
    ## /boot was on /dev/sda1 during installation
    #UUID=c104749f-a0fa-406c-9e9a-3fc01f8e2f78 /boot           ext2    defaults        0       2

3 Enabling cryptomount in GRUB2

Enable the feature and update the GRUB image:

root@debian:~# echo "GRUB_ENABLE_CRYPTODISK=y" >>/etc/default/grub
root@debian:~# update-grub
root@debian:~# grub-install /dev/sda

If everything went well, /boot/grub/grub.cfg should contain insmod cryptodisk (and also insmod lvm if /boot is on a Logical Volume).

Note: The PBKDF parameters are determined via benchmark upon key slot creation (or update). Thus they only makes sense if the environment in which the LUKS device is open matches (same CPU, same RAM size, etc.) the one in which it’s been formatted. Unlocking from GRUB does count as an environment mismatch, because GRUB operates under tighter memory constraints and doesn’t take advantage of all crypto-related CPU instructions. Concretely, that means unlocking a LUKS device from GRUB might take a lot longer than doing it from the normal system. Since GRUB’s LUKS implementation isn’t able to benchmark, you’ll need to do it manually. It’s easier for PBKDF2 as there is a single parameter to play with (iteration count) — while Argon2 has two (iteration count and memory) — and changing it affects the unlocking time linearly: for instance halving the iteration count would speed up unlocking by a factor of two. (And of course, making low entropy passphrases twice as easy to brute-force. There is a trade-off to be made here. Balancing convenience and security is the whole point of running PBKDF benchmarks.)

root@debian:~# cryptsetup luksDump /dev/sda1 | grep -B1 "Iterations:"
Key Slot 0: ENABLED
    Iterations:             1000000
root@debian:~# cryptsetup luksChangeKey --pbkdf-force-iterations 500000 /dev/sda1
Enter passphrase to be changed:
Enter new passphrase:
Verify passphrase:

(You can reuse the existing passphrase in the above prompts. Replace /dev/sda1 with the LUKS1 volume holding /boot; in this document that’s /dev/sda1 if /boot resides on a separated encrypted partition, or /dev/sda5 if /boot was moved to the root file system.)

Note: cryptomount lacks an option to specify the key slot index to open. All active key slots are tried sequentially until a match is found. Running the PBKDF algorithm is a slow operation, so to speed up things you’ll want the key slot to unlock at GRUB stage to be the first active one. Run the following command to discover its index.

root@debian:~# cryptsetup luksOpen --test-passphrase --verbose /dev/sda5
Enter passphrase for /dev/sda5:
Key slot 0 unlocked.
Command successful.

4 Avoiding the extra password prompt

The device holding the kernel (and the initramfs image) is unlocked by GRUB, but the root device needs to be unlocked again at initramfs stage, regardless whether it’s the same device or not. This is because GRUB boots with the given vmlinuz and initramfs images, but there is currently no way to securely pass cryptographic material (or Device Mapper information) to the kernel. Hence the Device Mapper table is initially empty at initramfs stage; in other words, all devices are locked, and the root device needs to be unlocked again.

To avoid extra passphrase prompts at initramfs stage, a workaround is to unlock via key files stored into the initramfs image. Since the initramfs image now resides on an encrypted device, this still provides protection for data at rest. After all for LUK1 the volume key can already be found by userspace in the Device Mapper table, so one could argue that including key files to the initramfs image – created with restrictive permissions – doesn’t change the threat model for LUKS1 devices. Please note however that for LUKS2 the volume key is normally offloaded to the kernel keyring (hence no longer readable by userspace), while key files lying on disk are of course readable by userspace.

  1. Generate the shared secret (here with 512 bits of entropy as it’s also the size of the volume key) inside a new file.

    root@debian:~# mkdir -m0700 /etc/keys
    root@debian:~# ( umask 0077 && dd if=/dev/urandom bs=1 count=64 of=/etc/keys/root.key conv=excl,fsync )
    64+0 records in
    64+0 records out
    64 bytes copied, 0.000698363 s, 91.6 kB/s
  2. Create a new key slot with that key file.

    root@debian:~# cryptsetup luksAddKey /dev/sda5 /etc/keys/root.key
    Enter any existing passphrase:
    root@debian:~# cryptsetup luksDump /dev/sda5 | grep "^Key Slot"
    Key Slot 0: ENABLED
    Key Slot 1: ENABLED
    Key Slot 2: DISABLED
    Key Slot 3: DISABLED
    Key Slot 4: DISABLED
    Key Slot 5: DISABLED
    Key Slot 6: DISABLED
    Key Slot 7: DISABLED
  3. Edit the crypttab(5) and set the third column to the key file path for the root device entry.

    root@debian:~# cat /etc/crypttab
    root_crypt UUID=… /etc/keys/root.key luks,discard,key-slot=1

    The unlock logic normally runs the PBKDF algorithm through each key slot sequentially until a match is found. Since the key file is explicitly targeting the second key slot, its index is specified with key-slot=1 in the crypttab(5) to save useless expensive PBKDF computations and reduce boot time.

  4. In /etc/cryptsetup-initramfs/conf-hook, set KEYFILE_PATTERN to a glob(7) expanding to the key path names to include to the initramfs image.

    root@debian:~# echo "KEYFILE_PATTERN=\"/etc/keys/*.key\"" >>/etc/cryptsetup-initramfs/conf-hook
  5. In /etc/initramfs-tools/initramfs.conf, set UMASK to a restrictive value to avoid leaking key material. See initramfs.conf(5) for details.

    root@debian:~# echo UMASK=0077 >>/etc/initramfs-tools/initramfs.conf
  6. Finally re-generate the initramfs image, and double-check that it 1/ has restrictive permissions; and 2/ includes the key.

    root@debian:~# update-initramfs -u
    update-initramfs: Generating /boot/initrd.img-4.19.0-4-amd64
    root@debian:~# stat -L -c "%A  %n" /initrd.img
    -rw-------  /initrd.img
    root@debian:~# lsinitramfs /initrd.img | grep "^cryptroot/keyfiles/"

    (cryptsetup-initramfs normalises and renames key files inside the initramfs, hence the new file name.)

Should be safe to reboot now :-) If all went well you should see a single passphrase prompt.

5 Using a custom keyboard layout

GRUB uses the US keyboard layout by default. Alternative layouts for the LUKS passphrase prompts can’t be loaded from /boot or the root file system, as the underlying devices haven’t been mapped yet at that stage. If you require another layout to type in your passphrase, then you’ll need to manually generate the core image using grub-mkimage(1). A possible solution is to embed a memdisk containing the keymap inside the core image.

  1. Create a memdisk (in GNU tar format) with the desired keymap, for instance dvorak’s. (The XKB keyboard layout and variant passed to grub-kbdcomp(1) are described in the setxkbmap(1) manual.)

    root@debian:~# memdisk="$(mktemp --tmpdir --directory)"
    root@debian:~# grub-kbdcomp -o "$memdisk/keymap.gkb" us dvorak
    root@debian:~# tar -C "$memdisk" -cf /boot/grub/memdisk.tar .
  2. Generate an early configuration file to embed inside the image.

    root@debian:~# uuid="$(blkid -o value -s UUID /dev/sda1)"
    root@debian:~# cat >/etc/early-grub.cfg <<-EOF
    	terminal_input --append at_keyboard
    	keymap (memdisk)/keymap.gkb
    	cryptomount -u ${uuid//-/}
    	set root=(cryptouuid/${uuid//-/})
    	set prefix=/grub
    	configfile grub.cfg

    Note: This is for the case of a separate /boot partition. If /boot resides on the root file system, then replace /dev/sda1 with /dev/sda5 (the LUKS device holding the root file system) and set prefix=/boot/grub; if it’s in a logical volume you’ll also need to set root=(lvm/DMNAME).

    Note: You might need to remove the first line if you use a USB keyboard, or tweak it if GRUB doesn’t see any PC/AT keyboard among its available terminal input devices. Start by specifing terminal_input in an interactive GRUB shell in order to determine the suitable input device. (Choosing an incorrect device might prevent unlocking if no input can be be entered.)

  3. Finally, manually create and install the GRUB image. Don’t use grub-install(1) here, as we need to pass an early configuration and a ramdisk. Instead, use grub-mkimage(1) with suitable image file name, format, and module list.

    root@debian:~# grub-mkimage \
        -c /etc/early-grub.cfg -m /boot/grub/memdisk.tar \
        -o "$IMAGE" -O "$FORMAT" \
        diskfilter cryptodisk luks gcry_rijndael gcry_sha256 \
        memdisk tar keylayouts configfile \
        at_keyboard usb_keyboard uhci ehci \
        ahci part_msdos part_gpt lvm ext2

    (Replace with ahci with a suitable module if the drive holding /boot isn’t a SATA drive supporting AHCI. Also, replace ext2 with a file system driver suitable for /boot if the file system isn’t ext2, ext3 or ext4.)

    The value of IMAGE and FORMAT depend on whether GRUB is in EFI or BIOS mode.

    1. For EFI mode: IMAGE="/boot/efi/EFI/debian/grubx64.efi" and FORMAT="x86_64-efi".

    2. For BIOS mode: IMAGE="/boot/grub/i386-pc/core.img", FORMAT="i386-pc" and set up the image as follows:

      root@debian:~# grub-bios-setup -d /boot/grub/i386-pc /dev/sda

    You can now delete the memdisk and the early GRUB configuration file, but note that subquent runs of grub-install(1) will override these changes.

– Guilhem Moulin guilhem@debian.org, Sun, 09 Jun 2019 16:35:20 +0200