fix(M4): format-storage requires a board-id-derived confirmation token

[d33mobile]

Finding (ISSUES.md M4 — format-storage is unauthenticated and chains to fail-open admin)

serial/commands.c:68 registers it with requires_admin = false, and commands.c:110 allow-lists it while storage is unmounted. The handler's only guard is "is storage mounted" (commands_system.c:219-224) — not identity.

The mount-failure precondition is the design intent (you cannot login when storage is down)... The fallout is not: after format-storage + CONFIRM, all keys and the WiFi config are destroyed... and the device reboots into a state where login 0 0 succeeds. The attacker then runs add-key 0 me + set-key-admin 0 and owns the lock.

• Require physical proof-of-presence for format-storage, or derive the confirm token from the board's unique ID rather than the fixed CONFIRM.

The fix

cmd_format_storage (serial/commands_system.c) no longer accepts the fixed literal CONFIRM. It now derives the confirmation token from pico_get_unique_board_id(): the operator must type the board's 16-hex uppercase unique id to proceed.

Why this works even when storage is down — the mount-failure entry point is design intent and cannot rely on login (storage is unavailable, so cmd_login refuses). This remediation needs no storage and no clock. Crucially, while storage is unmounted the dispatcher (commands.c:110) allow-lists only format-storage/help/?, so status (the only command that prints the board id) is not reachable, and the handler never echoes the token — it only echoes the operator's own keystrokes. A remote/blind attacker who reaches the console in the unmounted state therefore cannot learn the token. Reading it requires physical access to the device (via a probe, or a prior status while healthy). Input is uppercase-normalised for hex a-f; the read is bounded to the token length and the buffer is sized PICO_UNIQUE_BOARD_ID_SIZE_BYTES * 2 + 1 (16 hex + NUL), preserving the prior safe bounds.

The existing storage_is_mounted() gate is kept unchanged.

Deferred to M1

The second M4 sub-item — "force a provisioning state after format in which the door cannot open until an admin key is enrolled" — overlaps M1 (the fail-open-admin cluster). M1's cmd_login rework already makes the only credential-free path a genuinely unprovisioned device that self-closes once an admin key is added, and the door already fails closed with no admin key / unset clock. This PR is kept focused on the confirm-token hardening to avoid duplicating M1; the provisioning-state item is covered there.

Host-test status

The board-id path is only compiled compile-only in the coverage build; the pico/unique_id.h host stub already declares pico_get_unique_board_id (added for status), and harness_commands links a SPY for cmd_format_storage, so no stub/harness changes were needed. All CI-gated host gates pass (0):

• make -C test asan — 0
• make -C test valgrind — 0
• make -C test coverage — 0
• ./ci --action=check (clang-format lint) — 0

fuzz_*.c harnesses untouched.

verify-on-hw

The board-id read (pico_get_unique_board_id against the real flash chip) and the post-format reboot into the unprovisioned state cannot be exercised on the host — confirm on hardware that the printed/typed token matches the status board id and that the device reboots correctly after a successful format.