Remotely unlocking an encrypted hard disk (jyn.dev)

/usr/lib/dracut/modules.d/46cryptssh:

    #!/bin/bash
    
    check() {
        require_binaries sshd || return 1
        return 0
    }
    
    depends() {
        return 0
    }
    
    install() {
        if [ ! -e /etc/ssh/dracut ]; then
            mkdir /etc/ssh/dracut &&
            tmp=$(mktemp -d) &&
            head -c128 /dev/random > $tmp/passphrase &&
            ssh-keygen -t ed25519 -f /etc/ssh/dracut/ssh_host_ed25519_key -N"$(base64 < $tmp/passphrase)" &&
            tpm2_createprimary -C o -c $tmp/primary.ctx &&
            tpm2_pcrread -o $tmp/pcr.bin sha256:0,7
            tpm2_createpolicy --policy-pcr -l sha256:0,7 -f $tmp/pcr.bin -L $tmp/pcr.policy
            tpm2_create -C $tmp/primary.ctx -L $tmp/pcr.policy -i $tmp/passphrase -c $tmp/seal.ctx &&
            tpm2_evictcontrol -C o -c $tmp/seal.ctx -o /etc/ssh/dracut/seal || {
                rm -r $tmp /etc/ssh/dracut
                exit 1
            }
            rm -r $tmp
        fi
        for file in /etc/ssh/dracut/*; do
            inst_simple "$file" "/etc/ssh/${file#/etc/ssh/dracut/}"
        done
    
        mkdir -p -m 0700 "$initdir"/root/.ssh
        /usr/bin/install -m 600 /etc/ssh/dracut_authorized_keys "$initdir"/root/.ssh/authorized_keys
    
        inst_binary /usr/sbin/sshd
        inst_binary /usr/sbin/ssh-keygen
        inst_binary /usr/bin/tpm2_unseal
        inst_binary /usr/bin/base64
        inst_simple /usr/lib/libtss2-tcti-device.so
        
        inst_simple "$moddir"/cryptsshd.service "$systemdsystemunitdir"/cryptsshd.service
        inst_simple "$moddir"/sshd_config /etc/ssh/sshd_config
    
        inst_binary /usr/lib/ssh/sshd-session
        inst_binary /usr/lib/ssh/sshd-auth
    
        mkdir -p -m 0755 "$initdir"/var/empty/sshd
        mkdir -p -m 0755 "$initdir"/usr/share/empty.sshd
        mkdir -p  -m 0755 "$initdir"/var/log
        touch "$initdir"/var/log/lastlog
    
        systemctl -q --root "$initdir" enable cryptsshd
    }

    [Unit]
    Description=OpenSSH Daemon for Disk Encryption Passphrase
    DefaultDependencies=no
    Before=cryptsetup.target
    After=network-online.target
    
    [Service]
    Type=notify-reload
    ExecStartPre=/bin/sh -c '/usr/bin/ssh-keygen -p -f /etc/ssh/ssh_host_ed25519_key \
        -N "" -P "$(/usr/bin/tpm2_unseal -c /etc/ssh/seal -p pcr:sha256:0,7 | base64)"'
    ExecStart=/usr/bin/sshd -D
    KillMode=process
    Restart=always
    
    [Install]
    WantedBy=sysinit.target

It's still probably vulnerable to a cold boot attack, since the key is decrypted CPU-side. It would be interesting to perform the actual key operations on the TPM ...

The better solution is to use tpm, unified kernel image and secure boot skipping the network unlock.

The whole process is like this -

1. enable secure boot;

2. generate and install your own secure boot keys (using sbctl);

3. use clevis to enable automatic unlocking of the root fs only when secure boot check passes;

4. generate the unified kernel image (in EFI partition) that is signed by your secure boot key;

4. use efibootmgr to enable booting of said kernel image.

(5.) If your CPU supports it, enable memory encryption in BIOS (to mitigate cold boot attacks).

The unified kernel image doesn't accept additional kernel parameters, so only parameters that are set during generation of the initram are used. The secure boot makes sure no one else has tampered with the boot chain. And TPM stores the disk key securely.

You can still add some additional network level check to make sure that your computer is in your expected location before unlocking.

And you can also include some recovery tools + dropbear in your initram (within the unified kernel image), if you expect that you will have to do some recovery from the other side of the world.

I want something like "kexec" but taking over the entire hardware at the lowest possible level.

Edit, found this:

https://github.com/marcan/takeover.sh

But it's not as low level as I hoped, though it keeps the network running which is nice :)

If you set this up once ("this" meaning adding networking, SSH and tailscale inside initramfs), you can just do the same thing for the next server you set up, and you don't have to worry about the failure of one node affecting the other(s).