Setting up secure boot with fully encrypted filesystems on Alpine Linux


Tl;dr

This article explains how to install Alpine Linux on a Supermicro server, with a fully encrypted filesystems (yes, including /boot) and harden the boot sequence against an evil maid attack using secure boot.

Problem statement

Recently I splurged some currency1 on getting a new Supermicro server, with NVMe cards in addition to traditional 2.5” SATA SSDs.

What was a bit of a nasty surprise was that “legacy” boot2 doesn’t really dig the new NVMe hotness. In other words, no boot from NVMe without UEFI.

Also, the Supermicro servers have a wonderful html5-based IPMI implementation3 which just begs to be used for entering the initial full disk encryption password.

But the trouble is – the server is destined to live in a shared colo facility4 – and even if you trust the people running the place, are you really going to entrust your data to a setup that’s trivially hackable via physical attack (modified bootloader)?

Let’s make it harder for a casual attacker to ruin our day.

And all of that using Alpine Linux5 and ZFS. Because ZFS is awesome, and so is Alpine.

And to top it off, the entire install should be completely non-interactive; because next time I want to reinstall from scratch, I’d rather not repeat the same series of steps manually.

A bit of background

With the benefit of hindsight, I have a few observations about the task at hand:

  1. Even with Secure boot, the security of the machine hinges on the integrity of the BIOS and UEFI itself, not to mention the setup password. So this is an “oh, I’m going to ruin a casual attacker’s day” type of deal, rather than something industrial grade.
  2. Another hidden assumption is that the IPMI (remote access) interface that Supermicro provides is reasonably secure. After all, if you enter a password via a compromised IPMI KVM, you’re toast. So maybe have a strong IPMI password, a reasonable SSL certificate installed, and your prayer book of choice ready6.

That said, my initial scope was:

I just want this damn thing to boot from NVMe

which rather quickly scope-creeped into:

I want to have everything (but bootloader) encrypted, and bootloader authorized via secure boot.7

Which is cool, but I’ll still try and keep it simple8.

Arch diagram

Again, with the benefit of hindsight, the final architecture diagram is as follows:

architecture

So if we trust BIOS and Secure boot, then the only unencrypted part is boot.efi. And entering the /boot filesystem password is all we need to boot the system.

Pre-flight

Since I want to have entire install unattended, several things need to be pre-filled:

  1. The “answers file” for setup-alpine (containing keymap, hostname, network interface config, etc)
  2. Root password (encrypted)
  3. Root’s SSH authorized keys
  4. Partition password for /boot (to be changed post-install to something secure)

In addition, the disk names are sort of set in stone (/dev/nvme[01]n1), and I’m expecting to RAID1 (mirror) two NVMe cards together.

Everything else can be derived at runtime:

  1. Partition password (file) for /boot, ZFS, and swap
  2. Secureboot certificate

Solution

In order to hack this solution up, I’ve drawn inspiration from several guides:

Without further ado, here’s what I came up with (modulo a few unimportant bits):


# ========================================================
#
# To start off, boot from alpine extended (so you get ZFS)
#
# ========================================================

# This would be the preflight data one has to input:
cat - >/root/answers <<-__EOF__
KEYMAPOPTS="us us"
# This would be your hostname:
HOSTNAMEOPTS="-n shiny"
# Domain name + DNS server:
DNSOPTS="-d wejn.org 1.1.1.1"
INTERFACESOPTS="auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp
    hostname shiny
"

TIMEZONEOPTS="-z Europe/Zurich"
PROXYOPTS=none
APKREPOSOPTS="-1"
SSHDOPTS="-c openssh"
NTPOPTS="-c chrony"
# Leave the following as is (see below why):
DISKOPTS="-z --please-dont-do-anything"
__EOF__

# --------------------------------------------------------
#
# FIXME: DNSOPTS (domainname) aren't honored when using DHCP.
# Untested workaround is probably running `setup-dns -d wejn.org 1.1.1.1`
# before running `setup-alpine`.
#
# Run basic setup, which will fail at disk setup time due to garbled $DISKOPTS
setup-alpine -e -f /root/answers

# --------------------------------------------------------
# Change root's password (non-interactive alternative to "passwd"
# You can gen the hash via: `mkpasswd -m SHA-512 -P 3 3<<<mypassword`
sed -i 's@^root::@root:$6$somesalthere.$[--thiswouldbetherootpwhash--]:@' /etc/shadow

# Setup ssh access for a given set of ssh keys
mkdir -m 0700 /root/.ssh
cat - >/root/.ssh/authorized_keys <<-__EOF__
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIItGb5Up1jVrePp4v9lffz5xzw4HOXQFS6QumtATZYbA wejn@sigh
__EOF__

# Tweak /etc/issue, clear /etc/motd
cat - >/etc/issue <<-__EOF__

Terminal ready on \l.

$(ssh-keygen -l -v -f /etc/ssh/ssh_host_ecdsa_key.pub)

__EOF__
> /etc/motd

# --------------------------------------------------------
# Enable community repository
sed -i -r 's,#(.*/v.*/community),\1,' /etc/apk/repositories
apk update

# Add required packages
apk add zfs sfdisk e2fsprogs mdadm cryptsetup xkcdpass gnupg e2fsprogs-extra openssl sbsigntool

# --------------------------------------------------------
# Partition disks
#
# The partition layout:
# p1 - ESP, EFI System Partition (on swraid)
# p2 - /boot, LUKS-on-swraid
# p3 - swap, LUKS-on-swraid
# p4 - zfs, ZFS encrypted pool
#
for i in /dev/nvme?n?p?; do
    dd if=/dev/zero of=$i bs=4k count=1k
    mdadm --zero-superblock $i
done
for i in /dev/nvme0n1 /dev/nvme1n1; do
  dd if=/dev/zero of=$i bs=4k count=1k
  cat - <<-__EOF__ | sfdisk --quiet --label gpt $i
${i}p1: start=1M,size=100M,bootable,type=C12A7328-F81F-11D2-BA4B-00A0C93EC93B
${i}p2: size=400M,type=A19D880F-05FC-4D3B-A006-743F0F84911E
${i}p3: size=8G,type=A19D880F-05FC-4D3B-A006-743F0F84911E
${i}p4: type=6A898CC3-1DD2-11B2-99A6-080020736631
__EOF__
done

# --------------------------------------------------------
# Spin up the raid on all partitions
modprobe raid1
echo raid1 >> /etc/modules-load.d/raid1.conf
mdadm --create --level=1 --metadata=1.0 --raid-devices=2 /dev/md0 /dev/nvme0n1p1 /dev/nvme1n1p1
mdadm --create --level=1 --metadata=1.0 --raid-devices=2 /dev/md1 /dev/nvme0n1p2 /dev/nvme1n1p2
mdadm --create --level=1 --metadata=1.0 --raid-devices=2 /dev/md2 /dev/nvme0n1p3 /dev/nvme1n1p3
mdadm --detail --scan > /etc/mdadm.conf
rc-update add mdadm-raid
rc-update add mdadm boot
rc-update add mdadm-raid boot

# --------------------------------------------------------
# Setup the encrypted swap, with keyfile in /etc/fstab.swap_keyfile
echo -n $(xkcdpass -n 25) > /etc/fstab.swap_keyfile
chmod go= /etc/fstab.swap_keyfile
cryptsetup luksFormat -q -d /etc/fstab.swap_keyfile /dev/md2
cryptsetup luksOpen -d /etc/fstab.swap_keyfile /dev/md2 encswap
mkswap /dev/mapper/encswap
swapon /dev/mapper/encswap

# --------------------------------------------------------
# Setup the ZFS pool, with key in /etc/fstab.zfs_keyfile
modprobe zfs
echo -n $(xkcdpass -n 25) > /etc/fstab.zfs_keyfile
chmod go= /etc/fstab.zfs_keyfile
zpool create -f \
  -o ashift=12 -O acltype=posixacl -O canmount=off -O compression=lz4 \
  -O dnodesize=auto -O normalization=formD -O relatime=on -O xattr=sa \
  -O encryption=aes-256-gcm \
  -O keyformat=passphrase -O keylocation=file:///etc/fstab.zfs_keyfile \
  -O mountpoint=/ -R /mnt \
  nvmetank mirror /dev/nvme0n1p4 /dev/nvme1n1p4
# Change keylocation to prompt (for now):
zfs set keylocation=prompt nvmetank
# to change ZFS key:
# zfs change-key -l -o keylocation=prompt -o keyformat=passphrase nvmetank
# emergency load:
# zfs load-key -L prompt nvmetank

# Setup the mountpoints:
zfs create -o mountpoint=none -o canmount=off nvmetank/ROOT
zfs create -o mountpoint=legacy -o canmount=off nvmetank/ROOT/alpine
mount -t zfs nvmetank/ROOT/alpine /mnt/

# Make sure the ZFS boots :)
rc-update add zfs-import sysinit
rc-update add zfs-mount sysinit

# --------------------------------------------------------
# Format + mount the ESP as /efi (now that we have root mounted as /mnt)
mkdir /mnt/efi
mkfs.vfat -F32 -n efi /dev/md0
mount -t vfat /dev/md0 /mnt/efi/

# --------------------------------------------------------
# Setup the /boot partition with /etc/fstab.boot_keyfile and fixed password
mkdir /mnt/boot
echo -n $(xkcdpass -n 25) > /etc/fstab.boot_keyfile
chmod go= /etc/fstab.boot_keyfile
cryptsetup luksFormat -q --type luks1 -d /etc/fstab.boot_keyfile /dev/md1
echo -n i-am-begging-to-get-pwned-here > /root/boot.ephemeral_keyfile
# FIXME: You want to change this^^ static password. :-) That's the one you
# will need to enter upon boot, tho. So choose wisely.
cryptsetup luksAddKey /dev/md1 -d /etc/fstab.boot_keyfile /root/boot.ephemeral_keyfile
cryptsetup luksOpen -d /etc/fstab.boot_keyfile /dev/md1 encboot
mkfs.ext4 /dev/mapper/encboot
mount -t ext4 /dev/mapper/encboot /mnt/boot

# --------------------------------------------------------
# Finish the alpine setup (install the system, sans reasonable bootloader)
setup-disk /mnt/


# --------------------------------------------------------
#
# Up to this point package installations and tweaks to
# /etc were transferred to the new system.
#
# But from this point on, no changes in / are transferred.
# So tread carefully.


# --------------------------------------------------------
# Make passwordless zfs mount (it's safe because /boot is encrypted)
chroot /mnt ln -s /etc/fstab.zfs_keyfile /crypto_keyfile.bin
zfs set keylocation=file:///crypto_keyfile.bin nvmetank

# --------------------------------------------------------
# Tweak mkinitfs' features to enable bunch of stuff we need (and setup-disk didn't detect)
sed -i 's/zfs/nvme raid cryptsetup cryptkey ext4 zfs/' /mnt/etc/mkinitfs/mkinitfs.conf
mkinitfs -c /mnt/etc/mkinitfs/mkinitfs.conf -b /mnt/ $(ls /mnt/lib/modules/)

# --------------------------------------------------------
# Make sure encrypted swap and /boot are mounted
cat - >>/mnt/etc/conf.d/dmcrypt <<-__EOF__

target='encswap'
source='/dev/md2'
key='/etc/fstab.swap_keyfile'

target='encboot'
source='/dev/md1'
key='/etc/fstab.boot_keyfile'

__EOF__
cat - >>/mnt/etc/fstab <<-__EOF__
/dev/mapper/encswap none swap sw,defaults 0 0
__EOF__
sed -i 's,^UUID=.*/boot,/dev/mapper/encboot /boot,' /mnt/etc/fstab

chroot /mnt rc-update add dmcrypt boot
chroot /mnt rc-update add swap boot

# --------------------------------------------------------
# Set up grub bootloader
mount -t proc /proc /mnt/proc
mount --rbind /dev /mnt/dev
mount --make-rslave /mnt/dev
mount --rbind /sys /mnt/sys
chroot /mnt apk add grub grub-efi efibootmgr
chroot /mnt apk del syslinux
chattr -i /mnt/boot/ldlinux*
rm -f /mnt/boot/*.c32 /mnt/boot/ldlinux* /mnt/boot/extlinux.conf /mnt/boot/boot
cat - >/mnt/etc/default/grub <<-__EOF__
GRUB_DISTRIBUTOR="Alpine"
GRUB_TIMEOUT=2
GRUB_DISABLE_SUBMENU=y
GRUB_DISABLE_RECOVERY=true
GRUB_PRELOAD_MODULES="luks cryptodisk part_gpt lvm"
GRUB_ENABLE_CRYPTODISK=y
GRUB_DEVICE=nvmetank/ROOT/alpine
GRUB_DISABLE_LINUX_PARTUUID=true
GRUB_DISABLE_LINUX_UUID=true
__EOF__
sed -i 's/^modules=.*/modules=nvme,zfs/' /mnt/etc/update-extlinux.conf
chroot /mnt grub-mkconfig -o /boot/grub/grub.cfg
chroot /mnt grub-install --target=x86_64-efi --efi-directory=/efi
# The grub install partially fails (the call to efibootmgr does) because of raid.
# Because grub tries to give two devices to efibootmgr (who doesn't like that).
# 
# It's fine, tho, we can use the fallback /EFI/boot/bootx64.efi path within ESP.
mkdir /mnt/efi/EFI/boot
mv /mnt/efi/EFI/alpine/grubx64.efi /mnt/efi/EFI/boot/bootx64.efi
rmdir /mnt/efi/EFI/alpine
# Oh, and grub, f-off with grubenv, will ya? It's on RAID anyway.
rm /mnt/boot/grub/grubenv

# --------------------------------------------------------
# Time to setup the glorious Secure boot
mkdir -m 0700 /mnt/boot/secureboot
# Generate the cert
openssl req -new -x509 -newkey rsa:2048 -keyout /mnt/boot/secureboot/sb.key \
    -out /mnt/boot/secureboot/sb.crt -nodes -days 36500 -subj "/CN=Wejn SB CA/"
openssl x509 -in /mnt/boot/secureboot/sb.crt -out /mnt/boot/secureboot/sb.cer -outform DER
# Sign grub. This is, btw, the command you'll be re-running if you update grub
# binary sometime later (along with the "mv" a few lines above, I guess).
sbsign --key /mnt/boot/secureboot/sb.key --cert /mnt/boot/secureboot/sb.crt \
    /mnt/efi/EFI/boot/bootx64.efi
mv /mnt/efi/EFI/boot/bootx64.efi.signed /mnt/efi/EFI/boot/bootx64.efi
# Copy the "sb.cer" so it can be added to bios. I have it as all of: PK, KEK, db.
cp /mnt/boot/secureboot/sb.cer /mnt/efi/sb.cer

# Carry over root's authorized_keys
cp -a /root/.ssh/ /mnt/root/

# --------------------------------------------------------
#
# This section is completely optional, but recommended:
#
# Make this system recoverable by gpg-encrypting keys
# and certs using GPG to the ESP.
#
# That way if something fails, at least you have a way
# to run recovery.
#
cat - > /mnt/root/wejn.asc <<-__EOF__
-----BEGIN PGP PUBLIC KEY BLOCK-----
[...snip...]
-----END PGP PUBLIC KEY BLOCK-----
__EOF__
chroot /mnt gpg --import /root/wejn.asc
echo 'CC15927C8B00192715EAE5C08911469B691A16D4:6:' | chroot /mnt gpg --import-ownertrust
cat - >/mnt/root/save-keys <<-'__EOF__'
for i in /etc/fs*key* /boot/secureboot/sb.*; do
  cat $i | gpg --encrypt -r box@ --armor > /efi/$(basename $i).asc
done
__EOF__
chroot /mnt ash /root/save-keys
chroot /mnt ash -c 'ls -l /efi/*asc'

# --------------------------------------------------------
# NOT OPTIONAL -- cleanup :-)
umount -l /mnt/dev
umount -l /mnt/proc
umount -l /mnt/sys

I think the script illustrates rather well what I’ve done. More than prose could. My reasoning behind just dumping a script with comments is this – if someone doesn’t understand it based on the above, then maybe they’ll need to do some (light) reading anyway.

Before I wrap this up, I’d like to point out a few things I’m fond of:

  1. The gpg-encrypt of some vital data (encryption keys, certs) to ESP partition (so I can recover from my horrendous memory) is a future lifesaver
  2. The ssh key randomart in /etc/issue is also pretty handy9.
  3. The use of xkcdpass -n 25 should provide enough entropy for all of the fs passwords. I mean, unless I’m mistaken, it’s some 7776^25. :-) And pretty easy to type in on the prompt w/o a mistake (even if loooong).
  1. okay, okay, I spent a rather generous non-cash bonus award I got at work, so it was nearly ~free. Still, splurged. Maybe coulda bought a bigger flatscreen tv? ;)

  2. You know, the good old boot process based on Master boot record); yes, I’m that old.

  3. That – html5 or not – comes in handy when you screw up your networking config.

  4. Because despite being overwhelmingly small (and drawing ~35W), it’s still as noisy as many of its more powerful brethren.

  5. Hey, Debian, you’re getting rather windows-y lately… with your GBs upon GBs of by-default-installed crap. And let’s not even mention the whole systemd bloatware.

  6. Or maybe spend quality time coding some authenticated challenge-response algo as a GRUB module to avoid this headache? Well, if you – gentle reader – implement this, don’t be a stranger!

  7. And maybe a pony and a hot chocolate, while we’re dreaming.

  8. By which I mean: NO shim, NO kernel/initrd signing, absolute minimum of stuff unencrypted.

  9. Since nobody in their right mind would ever connect without verifying sigs, yes?