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docs(wiki): firmware/dbx-vs-TPM hardening + create disk-os-hardening page

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Real-world park-buzi episode: a UEFI dbx update (delivered via fwupd/LVFS,
NOT apt) revoked a stale GRUB -> panic, and moved PCR 7 -> broke TPM-sealed
LUKS auto-unlock -> passphrase prompt. Recovered by re-sealing PCR 7.

- appliance-provisioning.md: extend the §4 re-seal runbook to name dbx; new
  §4a (fwupd-not-apt, GRUB-panic ordering, PCR-7 re-seal, operator lockdown:
  mask fwupd + remove firmware-updater snap + BIOS-password + passphrase
  escrow) incl. the --test-passphrase-silently-passes-via-TPM trap
  (--disable-external-tokens); gotchas #12/#13.
- disk-os-hardening.md: NEW — resolves a long-dangling wikilink referenced
  from ~18 pages. The *why* of host hardening (5 controls + firmware lockdown);
  commands stay in appliance-provisioning; reconciliation remains the primary
  anti-fraud control.
- index.md: expand the disk-os-hardening catalog line.
- log.md: two note entries.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V
This commit is contained in:
Julian Çuni
2026-06-30 15:21:32 +02:00
parent 9c6741a485
commit 1b86750b0d
4 changed files with 200 additions and 5 deletions
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wiki/decisions/appliance-provisioning.md
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type: reference
tags: [parking, deployment, appliance, hardening, runbook, offline-first]
sources: []
updated: 2026-06-27
updated: 2026-06-30
status: settled
---
@@ -103,9 +103,67 @@ sudo reboot
- Still prompts = PCR mismatch; type the passphrase (NOT locked out), then retry with
`--tpm2-pcrs=0`. The `password` slot + `crypttab.bak` make this fully reversible.
> **Re-seal runbook:** a BIOS update / Secure Boot change alters PCR 7 → the TPM refuses → boot
> falls back to the passphrase prompt (not a brick). After such a change, re-run step 4's
> `systemd-cryptenroll --wipe-slot=tpm2 --tpm2-device=auto --tpm2-pcrs=7 /dev/sda3` to re-bind.
> **Re-seal runbook:** a BIOS update / Secure Boot change / **UEFI dbx (revocation list) update**
> alters PCR 7 → the TPM refuses → boot falls back to the passphrase prompt (not a brick). After
> such a change, re-run step 4's
> `systemd-cryptenroll --wipe-slot=tpm2 --tpm2-device=auto --tpm2-pcrs=7 /dev/sda3` to re-bind, then
> reboot to confirm unattended unlock returned.
### 4a. Firmware / UEFI dbx updates break PCR 7 — and are an OPERATOR threat (VERIFIED 2026-06-30)
The PCR-7 re-seal hazard above is **not** a rare event — the most common trigger is a **UEFI `dbx`
(Secure Boot revocation database) update**, and it bit the real `park-buzi` booth on 2026-06-28:
- **What `dbx` is:** the Secure Boot blocklist of known-vulnerable bootloader/shim hashes
(vendor = Microsoft). It is delivered by **`fwupd`/LVFS — a channel SEPARATE from APT** (the GNOME
"Firmware Updater", which on Ubuntu is the **`firmware-updater` snap**, surfaces it). `apt list
--upgradable` being clean does NOT mean a firmware/dbx update isn't pending.
- **The GRUB panic (root cause):** applying a *new* dbx against a *stale* GRUB/shim revokes the
installed bootloader → Secure Boot refuses to load it → **unbootable / GRUB "panic"**. The fix is
ordering: `apt full-upgrade` (current `grub-efi`/`shim-signed`) FIRST, *then* dbx. A fresh reinstall
ships a current GRUB, so reinstalling recovers it.
- **It moves PCR 7:** even with a current GRUB, applying dbx changes the Secure-Boot-policy
measurement → the TPM (slot 1) refuses to release the key → next boot **drops to the slot-0
passphrase prompt**. Recover with the re-seal runbook above. VERIFIED: on `park-buzi` the dbx
update went through, the box rebooted to a passphrase prompt, the slot-0 passphrase unlocked it,
and `systemd-cryptenroll --wipe-slot=tpm2 … --tpm2-pcrs=7` restored silent auto-unlock.
**Threat-model consequence ([[threat-model]]: the operator is the adversary).** A firmware/dbx update
on a TPM-sealed booth → the booth won't boot unattended and needs the slot-0 passphrase. So the
operator must be unable to *trigger* a firmware update, and must never hold the passphrase. Lock it
down (DONE on `park-buzi` 2026-06-30):
```bash
# 1. Kill the firmware-update DAEMON (the GUI "Update" button then fails with no daemon):
sudo systemctl mask fwupd.service fwupd-refresh.timer
systemctl is-enabled fwupd.service fwupd-refresh.timer # → masked / masked (persists across reboot)
# 2. Remove the operator-facing GUI so the screen is never even presented (Ubuntu = a snap):
sudo snap remove firmware-updater
snap list | grep -i firmware # → no output (re-check: seeded snaps can re-install)
```
Plus: the **BIOS admin password** (§1) must gate *entering setup / changing settings* (a
supervisor/admin password, not just a boot password) so the operator can't disable Secure Boot or
change boot order — either of which also breaks the seal. And the **slot-0 passphrase stays
off-machine / escrowed** (same custody as `EVENT_SIGNING_KEY` / `BACKUP_KEY`); it is an admin-only
recovery secret, used on-site during a maintenance window, never known to operators.
> **Net:** firmware/dbx updates become an **admin-only, on-site, deliberate** action. The booth is
> unattended-bootable only while the firmware/Secure-Boot state is frozen — that is the security
> property, not a bug. Legitimate firmware maintenance now costs: physical presence + the slot-0
> passphrase + a PCR-7 re-enroll.
> **⚠ Gotcha — `cryptsetup … --test-passphrase` SILENTLY passes via the TPM.** Before any
> firmware/dbx change, you must *prove a typed passphrase still unlocks the disk* (the TPM-independent
> safety net). But `sudo cryptsetup open --test-passphrase /dev/sda3` with a TPM2 token enrolled will
> succeed **without prompting** — the TPM auto-answers (it unlocks the tpm2 *slot*, e.g. slot 1), a
> FALSE positive that proves nothing about a human-typeable key. Force a real test with
> `--disable-external-tokens` (→ `No usable token is available.` then it prompts; success on slot 0 =
> the passphrase genuinely works):
> ```bash
> sudo cryptsetup open --test-passphrase /dev/sda3 --disable-external-tokens --verbose
> ```
## 5. GRUB password — EDIT-ONLY (VERIFIED 2026-06-23)
@@ -317,6 +375,16 @@ works; the desktop app is a separate workstream.
6. GRUB password MUST be **edit-only** (`--unrestricted` on entries) or it prompts on EVERY boot →
breaks unattended reboot. Verify `grep -c unrestricted /boot/grub/grub.cfg` ≥1 before rebooting.
### Firmware / dbx gotchas (2026-06-30, §4a)
12. **UEFI dbx ships via `fwupd`/LVFS, NOT APT.** `apt list --upgradable` clean ≠ no firmware update
pending. A new dbx vs a stale GRUB → revoked bootloader → **unbootable / GRUB panic** (`apt
full-upgrade` first, then dbx). And dbx **moves PCR 7** → breaks TPM auto-unlock → passphrase
prompt → re-seal (§4 runbook). Mask `fwupd` + remove the `firmware-updater` snap so the operator
can't trigger it.
13. `cryptsetup … --test-passphrase` **silently passes via the TPM token** (false safety signal). Use
`--disable-external-tokens` to actually force a typed-passphrase test before any firmware change.
### Komodo deploy gotchas (2026-06-27)
7. Periphery `core_address` is **Core's reverse-proxy URL** (`https://komodo.infra.msai.al`), NOT
wiki/decisions/disk-os-hardening.md
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---
type: decision
tags: [parking, hardening, threat-model, luks, tpm, secure-boot, grub, firmware, offline-first]
sources: [parking-system-architecture]
updated: 2026-06-30
status: settled
---
# Disk & OS hardening (booth appliance)
The host-level defences that raise the cost of **offline, physical tamper** of a booth PC: full-disk
encryption, TPM-sealed auto-unlock, Secure Boot, a GRUB edit-lock, an unprivileged operator account,
and locking firmware updates away from the operator. This page is the **rationale (the *why*)**; the
step-by-step verified commands live in the [[appliance-provisioning]] runbook (§3–5c, §4a). Settled
across the first real provisioning (2026-06-23) and the firmware-update episode (2026-06-30).
> ⚠ **This is the secondary control, not the main event.** The load-bearing anti-fraud mechanism is
> [[reconciliation]] over the [[append-only-event-chain|signed event chain]]. Disk/OS hardening
> defends the [[threat-model|outsider-with-the-box]] and raises the cost of offline tamper — it does
> **not** replace reconciliation, and it cannot stop a *legitimate, logged-in* operator from
> committing fraud through the app (that's what the signed ledger + reconciliation are for).
## What it defends against
The appliance sits on-site, physically reachable by the [[threat-model|booth operator (the primary
adversary)]] and by an outsider who can open the case. Without host hardening, either can:
- **Pull the SSD** and read/alter the SQLite ledger offline → FDE (LUKS) defeats this.
- **Boot a live USB** to mount and edit the disk → Secure Boot + TPM-sealing (PCR 7) defeats booting
a tampered/unsigned kernel; FDE keeps the data unreadable.
- **Edit the GRUB cmdline** (`init=/bin/bash`) for a no-login root shell on the *decrypted* disk →
the GRUB edit-lock defeats this (the TPM seal does NOT — see below).
- **Escalate from the operator login** (sudo, `docker`/`lxd` groups) → the unprivileged-operator
model defeats this.
## The five controls and why each is shaped the way it is
| Control | Choice | Why this shape (the load-bearing nuance) |
| --- | --- | --- |
| **FDE** | LUKS, **passphrase** at install (not the installer's "hardware-backed" option) | The 26.04 installer's automated FDE profiler fails on this firmware (`PCR_UNUSABLE`/`dbt`). Passphrase LUKS + a *manual* TPM seal sidesteps it and lets us pick PCRs. The passphrase slot is the **permanent recovery key**. |
| **TPM auto-unlock** | `systemd-cryptenroll`, **PCR 7 only** | Unattended reboot is a hard requirement (no operator types a passphrase). PCR 7 = Secure-Boot policy: catches the attack that matters (disabling Secure Boot) **without** churning on kernel/GRUB updates (PCRs 4/8/9 → would drop to passphrase every boot). **Keep BOTH slots** — slot 0 password (recovery), slot 1 tpm2 (auto-unlock); the TPM is never the only key. |
| **Secure Boot** | Enabled, **Deployed Mode**, stock MS keys | Ubuntu's signed shim needs stock `db`. Reaching the installer with Secure Boot ON is itself proof the MS third-party UEFI CA is trusted. |
| **GRUB edit-lock** | password, **edit-only** (`--unrestricted`) | Closes the `init=/bin/bash` root-shell hole. **The PCR-7 TPM seal does NOT cover this** — editing the cmdline doesn't change PCR 7, so the TPM still releases the key and the attacker lands on the decrypted disk. Edit-only so the box still boots **unattended** (password required only to *edit* entries). |
| **Operator account** | unprivileged, auto-login; separate **admin**+sudo | The operator is the adversary; their OS identity must not be able to escalate. Strip `sudo`, and the latent-escalation groups `lxd`/`docker` (both root-equivalent) + `lpadmin`. Admin is a distinct, no-auto-login identity. |
See [[tpm]] for the TPM-2.0 analysis (why PCR-only sealing, bus-sniff limits, TPM-vs-[[atecc608|ATECC608]]).
## Firmware / UEFI dbx updates — a hardening surface AND an operator threat
Settled 2026-06-30 after a real incident on `park-buzi`. This is the non-obvious one, because it
turns a routine "security update" into a booth-availability risk:
- **UEFI `dbx`** (the Secure Boot revocation database) and BIOS firmware are delivered by
**`fwupd`/LVFS — a channel SEPARATE from APT** (Ubuntu's GNOME "Firmware Updater" = the
`firmware-updater` snap). A clean `apt list --upgradable` does NOT mean no firmware update is pending.
- **It can brick boot:** a new dbx against a *stale* GRUB/shim **revokes the installed bootloader** →
Secure Boot refuses it → unbootable / GRUB panic. Correct order: `apt full-upgrade` (current
`grub-efi`/`shim-signed`) **first**, then dbx.
- **It breaks auto-unlock:** even with a current GRUB, applying dbx **moves PCR 7** → the TPM refuses
the LUKS key → next boot falls back to the slot-0 passphrase prompt (not a brick). Recover with the
PCR-7 re-seal runbook ([[appliance-provisioning]] §4/§4a).
- **Threat-model consequence:** a firmware/dbx update makes the booth need a passphrase to boot
unattended — so the **operator must be unable to trigger one, and must never hold the passphrase.**
Lock it down: `systemctl mask fwupd.service fwupd-refresh.timer`, `snap remove firmware-updater`,
a **BIOS admin password** that gates *entering setup* (so the operator can't disable Secure Boot /
change boot order), and the **slot-0 passphrase escrowed off-machine** (same custody as
`EVENT_SIGNING_KEY` / `BACKUP_KEY`). Firmware maintenance becomes **admin-only, on-site, deliberate**.
> The booth is unattended-bootable **only while the firmware / Secure-Boot state is frozen** — that is
> the security property, not a bug. The cost is that legitimate firmware maintenance now needs physical
> presence + the slot-0 passphrase + a PCR-7 re-enroll.
> **⚠ Verification trap:** `cryptsetup … --test-passphrase` **silently passes via the TPM token** (a
> false safety signal). Before any firmware change, prove a *typed* passphrase still unlocks the disk
> with `--disable-external-tokens` — see [[appliance-provisioning]] §4a.
## Where the commands live
This page is the rationale. The **verified, run-on-real-hardware commands** are in
[[appliance-provisioning]]: §1 BIOS, §2 Secure-Boot live-USB check, §3 encrypted install (the `dbt`
workaround), §4 TPM seal (PCR 7) + re-seal runbook, **§4a firmware/dbx lockdown**, §5 GRUB edit-lock,
§5c admin-vs-operator accounts. Komodo Periphery is folded into the same hardened surface as a
root-capable remote agent — see [[fleet-deployment-komodo]] (bind to the NetBird interface only).
## Relates
- [[appliance-provisioning]] — the runbook (commands); this page is its *why*.
- [[tpm]] — TPM 2.0 analysis (sealing, PCR choice, limits, vs ATECC608).
- [[threat-model]] — the operator-adversary framing this hardening serves.
- [[reconciliation]] / [[append-only-event-chain]] — the **primary** anti-fraud control this
complements, never replaces.
- [[fleet-deployment-komodo]] — Periphery as part of the trusted computing base.