feat(image): remote LUKS unlock via a Tor onion service in the initramfs

Encrypted image setups can now bake a Tor onion service into the initramfs
so the dropbear unlock shell stays reachable behind NAT or a dynamic IP.
When the user opts in, configure_encryption installs tor + busybox, drops
the mkinitcpio hooks (ordered `netconf tor dropbear encryptssh`), generates
the v3 onion keys offline in the image chroot, and prints the stable
.onion address. Unlock with `torsocks ssh root@<onion-address>`.

The runtime hook syncs the clock via NTP first (RTC-less boards boot at
1970, which Tor's consensus checks reject) and starts the onion service
pointing at dropbear on 127.0.0.1:22.

Hardening baked in from an adversarial review of the shipped path:
- cmdline.txt boot path (RPi4-class firmware boot) now sets the same
  ip=::::<host>:eth0:dhcp net.ifnames=0 params as the boot.txt path, so
  the initramfs actually gets a network and the onion can publish.
- the initramfs bakes in libnss_dns.so.2 so the NTP hostname resolves.
- the hook extracts DHCP DNS with sed instead of sourcing the lease files,
  which would run attacker-controlled DHCP option strings as root pre-boot.
- NTP is attempted unconditionally (bounded), not gated on DHCP-provided DNS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lim/configuration/initcpio/tor_hook

@@ -0,0 +1,47 @@
+#!/usr/bin/ash
+# mkinitcpio runtime hook: start Tor so the dropbear unlock shell is
+# reachable as an onion service while encryptssh waits for the passphrase.
+# Installed to /etc/initcpio/hooks/tor by linux-image-manager.
+
+# netconf's ipconfig drops its DHCP lease data (incl. DNS) into
+# /tmp/net-*.conf; busybox's resolver only reads /etc/resolv.conf.
+_tor_write_resolv_conf() {
+    [ -s /etc/resolv.conf ] && return 0
+    local conf dns
+    for conf in /tmp/net-*.conf; do
+        [ -f "$conf" ] || continue
+        # Extract ONLY the DNS fields with sed; never source these files —
+        # they hold attacker-controllable DHCP option strings (hostname,
+        # domain, rootpath), and sourcing would run them as root pre-boot.
+        for dns in $(sed -n 's/^IPV4DNS[01]=//p' "$conf"); do
+            [ -n "$dns" ] && [ "$dns" != "0.0.0.0" ] \
+                && echo "nameserver $dns" >> /etc/resolv.conf
+        done
+    done
+    [ -s /etc/resolv.conf ]
+}
+
+run_hook() {
+    # Tor rejects consensus documents when the clock is far off; boards
+    # without an RTC boot in 1970, so sync before starting Tor. Bounded:
+    # a failed sync must never block the boot.
+    msg "tor: syncing clock via NTP..."
+    # Best-effort DNS; a tor_ntp IP literal needs none, so never gate on it.
+    _tor_write_resolv_conf || msg "tor: no DNS from DHCP (fine if tor_ntp is an IP)"
+    /usr/local/bin/busybox timeout 30 \
+        /usr/local/bin/busybox ntpd -n -q -p "${tor_ntp:-pool.ntp.org}" \
+        || msg "tor: NTP sync failed, keeping current clock"
+
+    msg "tor: starting onion service for remote unlock..."
+    mkdir -p /var/lib/tor
+    chmod 0700 /var/lib/tor /etc/tor/onion
+    chmod 0600 /etc/tor/onion/hs_ed25519_secret_key
+    tor -f /etc/tor/torrc --RunAsDaemon 1 --Log "notice file /tmp/tor.log" \
+        || msg "tor: failed to start, unlock stays reachable via direct IP"
+}
+
+run_cleanuphook() {
+    # Nothing from the initramfs may keep running after the pivot.
+    /usr/local/bin/busybox killall tor 2>/dev/null
+    return 0
+}

lim/configuration/initcpio/tor_install

@@ -0,0 +1,37 @@
+#!/bin/bash
+# mkinitcpio install hook: Tor onion service for remote LUKS unlock.
+# Installed to /etc/initcpio/install/tor by linux-image-manager.
+
+build() {
+    add_binary /usr/bin/tor
+    # Full busybox for the ntpd applet: boards without an RTC (e.g. most
+    # Raspberry Pis) wake up in 1970, which Tor's consensus checks reject.
+    # A different target path keeps mkinitcpio's own busybox untouched.
+    add_binary /usr/bin/busybox /usr/local/bin/busybox
+    # glibc resolves the NTP server hostname via getaddrinfo(), which dlopen()s
+    # these NSS modules at runtime — add_binary only follows NEEDED libs, so
+    # without them DNS silently fails, ntpd never syncs, and the clock stays
+    # at 1970. glibc's built-in default (no nsswitch.conf) is "dns ... files".
+    add_binary /usr/lib/libnss_dns.so.2
+    add_binary /usr/lib/libnss_files.so.2
+    add_file /etc/tor/initramfs-torrc /etc/tor/torrc
+    add_file /etc/tor/initramfs-onion/hostname /etc/tor/onion/hostname
+    add_file /etc/tor/initramfs-onion/hs_ed25519_public_key /etc/tor/onion/hs_ed25519_public_key
+    add_file /etc/tor/initramfs-onion/hs_ed25519_secret_key /etc/tor/onion/hs_ed25519_secret_key
+    add_runscript
+}
+
+help() {
+    cat <<HELPEOF
+Starts a Tor onion service in early userspace so the dropbear unlock
+shell stays reachable under the .onion address baked into the image,
+even behind NAT or a dynamic IP.
+
+Place it between netconf and dropbear:
+  HOOKS=(... netconf tor dropbear encryptssh ...)
+
+Optional kernel parameter:
+  tor_ntp=<server>  NTP server used to set the clock before Tor starts
+                    (default: pool.ntp.org)
+HELPEOF
+}

lim/configuration/initcpio/torrc

@@ -0,0 +1,7 @@
+# Tor configuration for the initramfs onion unlock service.
+# Baked into the initramfs as /etc/tor/torrc by the "tor" mkinitcpio hook;
+# the onion keys come from /etc/tor/initramfs-onion on the system.
+DataDirectory /var/lib/tor
+HiddenServiceDir /etc/tor/onion
+HiddenServicePort 22 127.0.0.1:22
+SocksPort 0

lim/configuration/packages/server/tor.txt

@@ -0,0 +1,3 @@
+# Packages for the Tor onion unlock service in the initramfs
+tor
+busybox  # full applet set: ntpd for clock sync on RTC-less boards

lim/image/encryption.py

@@ -9,6 +9,7 @@ from pathlib import Path
 from lim import catalog, fsutil, packages, runner, ui
 from lim.image.plan import ImagePlan
 from lim.image.session import ImageSession, chroot_bash, install_packages
+from lim.image.tor import configure_tor_unlock
 
 MKINITCPIO_HOOKS_PREFIX = (
     "base udev autodetect microcode modconf kms keyboard keymap consolefont block"
@@ -32,9 +33,10 @@ def _configure_mkinitcpio(plan: ImagePlan, root: Path) -> None:
     fsutil.replace_in_file(
         "BINARIES=()", "BINARIES=(/usr/lib/libgcc_s.so.1)", mkinitcpio_path
     )
+    tor_hook = "tor " if plan.tor_unlock else ""
     fsutil.replace_in_file(
         f"HOOKS=({MKINITCPIO_HOOKS_PREFIX} {MKINITCPIO_HOOKS_SUFFIX})",
-        f"HOOKS=({MKINITCPIO_HOOKS_PREFIX} sleep netconf dropbear encryptssh "
+        f"HOOKS=({MKINITCPIO_HOOKS_PREFIX} sleep netconf {tor_hook}dropbear encryptssh "
         f"{MKINITCPIO_HOOKS_SUFFIX})",
         mkinitcpio_path,
     )
@@ -90,9 +92,15 @@ def _configure_bootloader(plan: ImagePlan, session: ImageSession, root: Path) ->
     else:
         cmdline_txt_path = session.boot_mount_path / "cmdline.txt"
         ui.info(f"Configuring {cmdline_txt_path}...")
+        # Firmware-boot boards (e.g. RPi4 cmdline.txt) need the same early
+        # networking as the boot.txt branch, or the initramfs netconf hook
+        # brings up no interface and remote unlock (dropbear + tor) is
+        # unreachable — the onion never even publishes.
+        hostname = (root / "etc/hostname").read_text().strip()
         fsutil.replace_in_file(
             "root=/dev/mmcblk0p2",
-            f"{cryptdevice} rootfstype={plan.root_filesystem}",
+            f"{cryptdevice} rootfstype={plan.root_filesystem} "
+            f"ip=::::{hostname}:eth0:dhcp net.ifnames=0 biosdevname=0",
             cmdline_txt_path,
         )
         ui.info(f"Content of {cmdline_txt_path}:{cmdline_txt_path.read_text()}")
@@ -112,6 +120,10 @@ def configure_encryption(
     ui.info(f"Adding {authorized_keys} to dropbear...")
     runner.run(["cp", "-v", str(authorized_keys), str(dropbear_root_key_path)], sudo=True)
 
+    if plan.tor_unlock:
+        # Hook files and onion keys must exist before mkinitcpio bakes the image.
+        configure_tor_unlock(plan, root)
+
     _configure_mkinitcpio(plan, root)
     _register_encrypted_root(plan, session, root)
     _configure_bootloader(plan, session, root)

lim/image/plan.py

@@ -17,6 +17,7 @@ class ImagePlan:
     luks_memory_cost: str | None = None
     raspberry_pi_version: str | None = None
     encrypt_system: bool = False
+    tor_unlock: bool = False
     root_filesystem: str = ""
     image_folder: Path = field(default_factory=Path)
 

lim/image/setup.py

@@ -49,6 +49,10 @@ def _choose_and_verify_image(plan: ImagePlan) -> None:
     if plan.operation_system == "linux":
         choosers.choose_linux_image(plan)
         plan.encrypt_system = ui.confirm("Should the system be encrypted?")
+        if plan.encrypt_system:
+            plan.tor_unlock = ui.confirm(
+                "Should the system be remotely unlockable via a Tor onion service?"
+            )
         ui.info("Generating os-image...")
         transfer.download_image(plan)
     else:

lim/image/tor.py

@@ -0,0 +1,79 @@
+"""Tor onion service in the initramfs for remote LUKS unlock.
+
+The onion keys are generated offline inside the image chroot
+(``tor --DisableNetwork 1`` writes them without touching the network)
+and baked into the initramfs by the "tor" mkinitcpio hook, so the
+dropbear unlock shell stays reachable under a stable .onion address
+even behind NAT or a dynamic IP.
+"""
+
+from pathlib import Path
+
+from lim import config, packages, runner, ui
+from lim.errors import LimError
+from lim.image.plan import ImagePlan
+from lim.image.session import chroot_bash, install_packages
+
+# Paths inside the image (relative to the mounted root).
+ONION_DIR = "etc/tor/initramfs-onion"
+TORRC_PATH = "etc/tor/initramfs-torrc"
+
+# An empty -f torrc keeps the image's /etc/tor/torrc (User tor, ...) out of
+# the keygen run; with DisableNetwork the keys appear within a second, the
+# loop only cushions slow qemu-emulated chroots.
+_KEYGEN_SCRIPT = f"""
+mkdir -p /{ONION_DIR}
+chmod 0700 /{ONION_DIR}
+: > /tmp/tor-keygen-torrc
+tor -f /tmp/tor-keygen-torrc --DisableNetwork 1 \\
+    --DataDirectory /tmp/tor-keygen-data \\
+    --HiddenServiceDir /{ONION_DIR} \\
+    --HiddenServicePort "22 127.0.0.1:22" \\
+    --SocksPort 0 --RunAsDaemon 0 --Log "notice stderr" &
+tor_pid=$!
+for _ in $(seq 1 30); do
+    [ -s /{ONION_DIR}/hostname ] && break
+    sleep 1
+done
+kill "$tor_pid" 2>/dev/null || true
+rm -rf /tmp/tor-keygen-data /tmp/tor-keygen-torrc
+[ -s /{ONION_DIR}/hostname ]
+"""
+
+
+def _install_initcpio_files(root: Path) -> None:
+    source_dir = config.CONFIGURATION_PATH / "initcpio"
+    for source, target in (
+        (source_dir / "tor_install", root / "etc/initcpio/install/tor"),
+        (source_dir / "tor_hook", root / "etc/initcpio/hooks/tor"),
+        (source_dir / "torrc", root / TORRC_PATH),
+    ):
+        ui.info(f"Installing {target}...")
+        runner.run(
+            ["install", "-D", "-m", "0644", str(source), str(target)], sudo=True
+        )
+
+
+def _generate_onion_keys(root: Path) -> str:
+    hostname_path = root / ONION_DIR / "hostname"
+    if hostname_path.is_file():
+        ui.info("Onion keys already exist, keeping the existing address.")
+    else:
+        ui.info("Generating onion service keys (offline, inside the chroot)...")
+        chroot_bash(root, _KEYGEN_SCRIPT, error_msg="Onion key generation failed.")
+        if not hostname_path.is_file():
+            raise LimError(f"Onion key generation produced no {hostname_path}.")
+    return hostname_path.read_text().strip()
+
+
+def configure_tor_unlock(plan: ImagePlan, root: Path) -> str:
+    """Install everything the "tor" mkinitcpio hook bakes in; return the onion address."""
+    ui.info("Setting up remote unlock via Tor onion service...")
+    install_packages(
+        plan.distribution, root, " ".join(packages.get_packages("server/tor"))
+    )
+    _install_initcpio_files(root)
+    onion_address = _generate_onion_keys(root)
+    ui.success(f"Onion unlock address: {onion_address}")
+    ui.info(f"Unlock later with: torsocks ssh root@{onion_address}")
+    return onion_address