vt1200/dwt/notes/CROSS.md

DWT / rdebug investigation — cross-agent notes

Two agents run in parallel:

agent	owns	file
static (Ghidra)	rdebug_* functions, command dispatcher, gates	dwt/notes/rdebug-static.md
protocol (MOP)	MOP Remote Console framing + dwt/mop_console.py	dwt/notes/mop-protocol.md

Append only, with a timestamp header. Facts that change the other agent's next step go here; speculation stays in your own file. Do not rewrite another agent's section.

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2026-10-18 01:48 EEST — protocol agent: framing is known, client exists

Facts (sources in dwt/notes/mop-protocol.md):

• Ethernet II, ethertype 0x6002 = MOP Remote Console (0x6001 = Dump/Load). Destination = terminal unicast MAC 08:00:2b:16:4b:5a (verified; multicast ab:00:00:02:00:00 also defined by the spec).
• MOP payload: len_lo len_hi | code | body…, length is little-endian and counts the bytes after the length field (code + body). Pad the payload to the 46-byte Ethernet minimum; the length field, not the frame size, delimits.
• Codes we need: 13 Reserve Console (body = 8-byte verification), 5 Request ID (body = 00 + 2-byte receipt), 7 System ID (reply), 17 Console Command and Poll (body = flags + console bytes), 19 Console Response and Acknowledge (reply; body = flags + console bytes), 15 Release Console.
• Session: send 13 then 5; wait for 7. Function bitmap in System ID info TLV type 2, bit 5 = console carrier (0x20). Data is not framed with a session id or sequence beyond CCP flag bit 0 (1-bit message number, toggles per command); bit 1 = send break. Flag bit 0 of CRA acknowledges the command.
• No checksum in Remote Console (unlike Dump/Load), no version byte on the wire.
• Password: the 8-byte Reserve-Console verification is the MOP service password, default 00 00 00 00 00 00 00 00. Wrong value = terminal silently ignores the request (documented NCP behaviour: "Target does not respond"). NCL byte order = leftmost hex pair first; NCP integer order = reversed. VT1200 has no documented maintenance password, so start with zeros.
• Console data is a raw byte stream. To type a command send HELP\r; the terminal echoes DWT>>> and CRLF text. Use --raw HEX for arbitrary bytes.

Client: dwt/mop_console.py. --dry-run prints frames without a socket; --raw HEX sends bytes inside CCP; --raw-frame HEX sends a complete MOP payload if a bare rdebug opcode ever has to bypass the console stream.

2026-10-18 — static agent: rdebug is a separate TCP service, and MAME has no netdev

Facts that change the transport assumption (details + quotes in dwt/notes/rdebug-static.md):

1. The rdebug_* family is not reached over MOP. It is a TCP listener registered in the terminal's service table as "DEBUG TCP Port", port field 0x0303 (network order = 771). The sibling entry in the same table is port 0x1770 = 6000 = X11, which is what fixes the field layout. Accept handler 0x07EB15AC (bit 0x3F58AD60); it spawns a process named "remote debug" whose entry is the dispatcher 0x07EB3E64 (bit 0x3F59F320).
2. Wire format is binary, not text. Request = u32 be opcode + opcode arguments (each word big-endian). Response framing = u32 be (payload_len+1) + u8 flag + payload. Opcode 7 is a no-op that answers 11 bytes: 00 00 00 07 | 00 | 44 6F 6E 65 0A 00 (len field = 7 = payload 6 + flag 1; payload is "Done\n" plus its NUL, because the handler passes the 6-byte C string at 0x07EB6CE9). The cheapest liveness test.
3. There is a gate before any command: accept_debug_conn reads a 16-bit config word at bit address 0xAECD2 and rejects the connection when it is non-zero (logs Debug connection from IP host %s then rejected). On the emulated machine (VT1200_SKIP_LEVEL1=1, fresh NVRAM) it reads 0x8070 at frames 200/600/1200/2000/3000/4000, i.e. closed. Setting space:write_u16(0xAECD2, 0) from a debugger script opens it.
4. A second gate follows: before the first opcode >= 2 the dispatcher runs a 16-byte request/response exchange (dwt_prf, 0x07EB3FB8). It sends 16 bytes with message flag 0x7F and requires exactly 16 bytes back (short read => dwt_prf: challenge failed - size error), then returns 1/0 and the session is killed on 0. What passes is not established; it is not a password (no ROM/NVRAM constant is compared). Echoing the 16 bytes back is the first thing to try.
5. MAME cannot accept the connection today. harness/mame/vt1200.cpp binds the AM79C90 with no netdev, so the firmware's TCP/IP stack has no link. A "MOP console client" run against MAME will not reach any socket unless a host network is attached or the debugger drives accept_debug_conn/the dispatcher directly.
6. The documented console (INIT/TEST/HELP/SHOW) is a different path: a 4-entry text-command table at RAM bit 0xA5C20 (byte 0x14B84), parsed by 0x07E59186(char*, int) from the console character ring at bit 0xF4D10. There are exactly four names; no undocumented fifth entry.

2026-10-18 02:05 EEST — protocol agent: MOP console WORKS against the emulator

Runtime facts, VT1200_SKIP_LEVEL1=1, -cfg_directory cfg-net -networkprovider taptun, private NVRAM. Capture dwt/evidence/mop-hello.pcap, log dwt/evidence/hello.log.

• Reserve Console (13, verification = 8 zero bytes) + Request ID (5) gets a System ID (7) in ~0.5 ms. Decode of the VT1200 reply: maintenance 3.0.0; functions 0xE1 (loop, console carrier, counters, reservation); console user = host MAC; timer 12 s; command buffer 32 B; response buffer 600 B; hardware address 08:00:2b:16:4b:5a.
• HELP (sent as HELP\r in a CCP, code 17) returns 425 bytes in one CRA (code 19, length 427, frame 443). SHOW returns 509 bytes (frame 527). Full text in dwt/evidence/dwt-help.txt and dwt/evidence/dwt-show.txt.
• No challenge/password on this path. Reserve with zeros works; Reserve with the arbitrary non-zero value 0102030405060708 also works (dwt/evidence/wrong-password.log). The dwt_prf challenge is the TCP debug listener per rdebug-static.md §6, not MOP.
• DWT is OS-only. Bare boot (no skip) holds PC=3FDCE400 from frame 100 to 3600 (dwt/evidence/mame-noos-probe.log) and 8 Reserve/RequestID pairs over 8 s get zero frames back (noos-attempt3.log, mop-noos.pcap).
• TEST 0 0 and TEST 12 are accepted and print …Testing..., then the MOP server stops answering: a following Reserve gets no System ID on two fresh boots (script3.log, test00b.log, test12.log, help-after-test*.log). This matches the service guide's precondition "no active work (no open windows)"; we could not meet it because DWT lives in the running session manager.
• INIT gets no reply and the terminal stops answering MOP for >90 s (init.log, help-after-init2.log) — consistent with a reset/reboot.

Important for the rdebug work: the MOP console is the text path into the four-entry DWT parser (TEST/INIT/HELP/SHOW, rdebug-static.md §9). It is not the rdebug binary-opcode path. --raw HEX sends bytes into that text parser, so it cannot reach rdebug_peek/poke/jump; those are TCP port 771 and gated (rdebug-static.md §1/§7). Use this client for the documented console, not as an rdebug transport.

2026-10-18 02:07 EEST — exploit agent: gate A is OPEN; the 0x8070 reading was a field-size mistake

This changes the static note's verdict. accept_debug_conn does not read a 16-bit word. The listing is:

3f58b280:	setf   2, 0, 0          ; field size = 2 BITS (MAME prints 1..0x20 = size)
3f58b290:	move   @0xaecd2, a7, 0  ; read the 2-bit field at bit 0xAECD2
3f58b2c0:	jreq   L3F58B430        ; zero -> ACCEPT path

SETF's first operand is the field size in bits (34010dsm.cpp prints (op & 0x1f) ? op & 0x1f : 0x20); setf 2 = 2 bits. The compared value is therefore bits 2..3 of the word at 0xAECD0, not the whole word. The word is 0x8070 from frame ~600 onward (and 0x0000 before), and in both cases bits 2..3 are 0.

Verified on the emulated CPU: jumping the live firmware into 0x3F58B280 with a breakpoint on both targets hits 3F58B430 (ACCEPT) with a7=0; the reject target is never reached. Evidence: dwt/evidence/run-branch/.

The two 2-bit fields in that word are written by 3F329F00 (@0xAECD0) and 3F32A050 (@0xAECD2) from config lookups; those stores did not execute in the VT1200_SKIP_LEVEL1=1 boot. The 0x8070 itself arrives via the PIXBLT memcpy at PC 3FB446D0 (a boot-time settings/bitmap block copy), which is why a watchpoint saw it. Do not use the whole-word value as a gate indicator.

Consequence for the MOP agent: the rdebug path is not blocked by the config word on this image; the remaining gate is the 16-byte echo challenge (dwt_prf), which the static reading now also resolves in favour of echo (the transform writes to @0x1C0240, not to the reply buffer, and the memcmp is reply vs sent). Do not patch 0xAECD2 to "open" the port; it is already 0 in the field that matters.

Tap: no vt1200 MAME instance or tcpdump is running as of 02:07. I am about to use tap-mess-1000-0 / bridge vt1200 for a short live TCP test. If you start a MAME instance, please leave the tap alone for a few minutes or note it here.

2026-10-18 02:40 EEST — exploit agent: gate B is real; args must follow the challenge

Live result against the emulated terminal (VT1200_SKIP_LEVEL1=1, LANCE on the tap, BOOTP address 192.168.99.2):

• Gate A is not a gate. A TCP connection to 771 reached accept_debug_conn, the dispatcher and dwt_prf (breakpoints fired). The 0xAECD2 compare is a 2-bit field read by SETF 2; it is 0. Do not patch the 16-bit word at 0xAECD0.
• Gate B is real. dwt_prf sends 16 bytes (flag 0x7F) and requires exactly 16 back. Echo gives memcmp = 0xFFFFFFFF and the session is killed. The expected value is the wire challenge with each 32-bit word byte-swapped (the code ntohl's the host words into the send buffer and memcmp's the host-order copy), but even that ("computed") gives memcmp = 0x00000001: the received buffer is first rewritten in place by a fixed-key font/cipher pipeline (0x3F5A09C0 → two 8-byte records → 0x3F5ABA40, tables at @0x1C0220/@0x1C0240, cipher 0x3F5ACEC0). Not inverted here.
• Opcode ≥ 2 all require the challenge, including 7. Only 0 and 1 skip it, and neither reaches poke/jump.
• Protocol ordering matters: send the opcode alone, read and answer the challenge, then send the handler's arguments. If the args go with the opcode, dwt_prf eats their first 16 bytes as the answer and the handler reads shifted fields (a peek length of 8 was reported as "maximum peek size").
• With the challenge comparison forced to 0 by a debugger breakpoint (not remotely), the real poke/jump handlers delivered and ran harness/hello-world/payload.bin: MAGIC1=DEAD MAGIC2=BEEF COUNT=0400, VRAM 1024/1024. That is the debugger-assisted loader proof, not a remote bypass. Poke/peek addresses are bit addresses; ROM writes are ignored.

The tap (tap-mess-1000-0 / bridge vt1200) is free again as of 02:40; the MAME instance and the BOOTP server are stopped. Full write-up: dwt/notes/EXPLOIT_ATTEMPT.md; transcript and logs under dwt/evidence/.

2026-10-18 02:42 EEST — oracle agent: dwt_prf transform ground truth is captured

Full method and caveats: dwt/notes/CHALLENGE_ORACLE.md.

Vectors: dwt/challenge_vectors.json — a top-level JSON array of 180 instances (177 distinct inputs). Load with vectors = json.load(open("dwt/challenge_vectors.json")); each entry is {"input_hex": <32 hex>, "output_hex": <32 hex>, "meta": {...}}. input_hex is the 16-byte buffer after dwt_prf's per-word ntohl (the transform's actual input); output_hex is f(input) in place. The full 128-byte key schedule and seed-block dumps are in dwt/evidence/oracle/runA/stdout.log.

Direction (confirmed): the firmware transforms the reply, not the challenge. With host-order challenge words W, wire challenge C = ntohl(W) (per-32-bit-word byte swap), wire reply R: X = ntohl(R), Y = f(X), and dwt_prf memcmp's Y against W. Pass condition: f(ntohl(R)) == W, i.e. R = ntohl(f^-1(W)) = ntohl(f^-1(ntohl(C))). The vectors give f, so the implementation needs the inverse (and the outer word swap).

Determinism: yes. 177 distinct inputs give identical outputs on two fresh-NVRAM boots, and on a third boot that hijacks at a different frame. Seed block 0x1BE810 and the key schedule @0x1C0260 are byte-identical across boots — the key is genuinely fixed for this image.

Per-8-byte independence: yes. Records are processed separately: changing only bytes 0–7 never changes output bytes 8–15 (0/64 single-bit trials), and 32 independence-grid pairs agree. Output for a doubled record is the doubled output. Permutation on samples: 152 distinct inputs → 152 distinct outputs, no fixed points. Avalanche: one input bit flips 32.66 of 64 first-record output bits on average (23–42).

Two reversing shortcuts found while building the oracle:

1. Argument order for a direct call. 0x3F5A09C0 expects (len, buf) in source order, i.e. buf must be the last argument written (the callee reads *a13(0xffe0) as the pointer and *a13(0xffc0) as the length). Writing (buf, 16) gives a pointer of 16 and will not work.
2. The two "font render" calls are identity copies on this path. dwt_prf calls INIT with mode 1 (0x3F5A9620(1)); that mode leaves @0x1C0220/@0x1C0240 NULL, so 0x3F5ABA40's two render calls take their memcpy branch. Captured per-record intermediates (dwt/evidence/oracle/stages.txt) show feistel_in == input and pre_final_render == output for all six samples. The effective transform is the 16-round Feistel over the key schedule expanded from 0x1BE810 (+ the 0x1BED40/0x1BEF00/0x1BEF80 auxiliary blocks).

Reproduce: python3 dwt/oracle.py (~35 s, two boots + stage capture). No ROM, driver or dwt_prf code was modified; no socket was used for the vectors.

2026-10-18 — challenge reversed: the remote path is open (supersedes the echo reading)

The challenge no longer blocks anything. dwt/challenge.py answers it from the challenge bytes alone; dwt/solve.py did so over TCP with no debugger:

• The firmware transforms our reply and compares it with the host-order copy of the challenge: memcmp(frame+0x20, transform(frame+0x10), 16). The reply must therefore be swap4(inverse_transform(swap4(challenge))) — the inverse direction, not echo. Live bytes at the comparison are in dwt/evidence/live-run/debug-live.log (expecting == got, a8=0).
• The transform is a 16-round Feistel per independent 8-byte record, keys key[15]..key[0] (128-byte schedule at @0x1C0260), round function 0x3F5ACEC0 = eight 6-bit SP-table lookups OR'd (@0x1C0200, 2048 bytes).
• The "font renders" at @0x1C0220/@0x1C0240 are memcpy in this path: dwt_prf calls 0x3F5A9620(1), which allocates only the SP table and the key. The font pointers stay NULL (0,0,0,0 at frame 2000; init(1) leaves them zero), and the renderer's table==0 branch is a memcpy. Earlier attempts tried to invert a font stage that never runs here.
• Key and SP table are fixed: identical across two fresh-NVRAM boots and identical to the earlier live capture. They are embedded in dwt/challenge.py; provenance and disassembly are in dwt/notes/CHALLENGE_REVERSE.md.
• Validation: 70/70 + 70/70 emulator forward vectors (140 records each), 180/180 in dwt/challenge_vectors.json, 6/6 against the firmware's own inverse 0x3F5AADA0, and live sessions that ran peek, poke and jump after the computed answer.
• Remote execution demonstrated: the 82-byte harness/hello-world payload was poked to bit 0x01800000 and jumped to over TCP; the emulator read back MAGIC1=DEAD MAGIC2=BEEF COUNT=0400 and 1024/1024 VRAM words FFFF. dwt/evidence/live-run2/ is that run with no breakpoints at all (the Lua script only polls RAM). Only VT1200_SKIP_LEVEL1=1 is needed for TCP.

Files: dwt/challenge.py, dwt/solve.py, dwt/notes/CHALLENGE_REVERSE.md, dwt/evidence/oracle-*.log, dwt/evidence/VALIDATION.txt, dwt/evidence/live-run{,2}/, dwt/evidence/scripts/{oracle_call,live_verify,live_check}.lua. dwt/exploit.py and dwt/mop_console.py were not modified.

2026-10-18 — visual injection via the returning rdebug jump

jump is a real call. A 30-byte rets payload writes markers, returns 0, and the same TCP session plus a later connection still answer opcode 7 (dwt/evidence/visual/step1_ret_test.log).

• Static VRAM draw lands but is repainted. A three-box payload's writes are visible in an immediate peek but frames before/after are identical; a spinning control shows the same boxes, so the coordinates are right.
• Resident DI hook: RLONG(0xFFFFFEA0) = 00000DB0h, stub JA 3F7146D0h. Patching the stub works (pre-handler draw is repainted). A return-PC trampoline (draw after the handler's reti) produced a visible blink: frames 3450 (box on) / 3480 (off) / 3500 (on), TCP answered at 3450 and 3500. It is not durable — only the first entry takes the reti path; later entries take the jauc 3F579880 full path and stop using the patched PC (counter sticks at 1). A one-shot post-handler box persists for 600+ frames.
• Window: port 6000 is open, but xdpyinfo is rejected (“Client is not authorized”). Opening a window by injection was not attempted; it is an X-structure project.

Details, addresses, frames and limits: dwt/notes/VISUAL_INJECTION.md, dwt/evidence/visual/. ROM md5 unchanged; no driver edits; only VT1200_SKIP_LEVEL1=1 (documented divergence) and a private nvram.

2026-10-18 11:01 EEST — opcode 8 writes the TM Messages box (no payload)

Facts, all measured over rdebug on the live OS (VT1200_SKIP_LEVEL1=1); details and frames in dwt/notes/DISPLAY_MESSAGE.md.

• Opcode 8 is rdebug_display_message and it lands in the Terminal Manager Messages box, not a popup. Body: u32be 8 | u32be level | u32be len | len bytes; reply Done\n. len is 1..1024.
• The handler does not NUL-terminate. It malloc(len+2) and copies len bytes; a message sent without a NUL is followed by stale heap bytes on screen (…LEVEL ZEROUUUUUU@). Send text + NUL and count the NUL in len; max clean text is 1023 bytes.
• Level is a class filter, not a destination. The append routine is 0x07E6126A (bit 0x3F309350), the same one accept_debug_conn calls. Per-class enable bits at bit 0xAECE2/3/4 (byte 0xAECE0 = 0x05 on this boot): level 0 enabled, level 1/2 disabled, level 3 unchecked. Observed: levels 0 and 3 render; 1 and 2 are silently dropped.
• The pane is a 5-line viewport (count-5 at 3F30A940/3F30AA40); older lines scroll out (eight sent → 04..08 visible). Lines persist across repaints (frames 944 and 1938 byte-identical). Not modal; opcode 7 still answers after the messages.
• No dwt/visual/msg.asm was built: the firmware already provides the primitive. Same connection can poke/jump a payload, then opcode 8.
• dwt/exploit.py gained only additive RDebug.display_message() / --message / --message-level; dwt/visual/msg_demo.py and dwt/visual/msg_snap.lua are new host tooling.
• Harness note: MAME's taptun name is fixed (tap-mess-<uid>-0), so a second MAME needs its own net namespace; this run used one (vtmsg) while another MAME held the default tap.

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2026-10-18 14:10 EEST — GUI apps are X clients; a payload can call Xlib but not get a display

Facts for anyone tempted to "just create a window" (details in dwt/notes/GUI_APPS.md):

• The GUI is a real X11 dix server in ROM (process SERVER, entry bit 0x3F362910) plus Xlib in ROM. The DWT Window Manager (WM, 0x3FAB8160), the terminal emulator (VTE, 0x3F4DB050) and the Terminal Manager (SESSIONMGR_PROC, 0x3F2E6DE0) are ordinary X clients, not a private window API. The TM is the session manager process; main() (bit 0x3F34DB50) creates SERVER and SESSIONMGR, and the session manager creates WM and VTE.
• Internal clients open the display with the literal "0::0" (TM ROM 0x7E61B80, WM 0x7F59D29). XParseDisplay resolves a named local service "xserver" through the 32-entry service table at RAM bit 0x1E8D70 (0x7EE160C / lookup 0x7EE1B70). TCP 6000 is a separate, user/node authorised listener.
• Xlib entry points (bit addresses): XOpenDisplay 0x3F9C2570, XCreateSimpleWindow 0x3FD41B90, XMapWindow 0x3FD645B0, XSync 0x3FD9F6B0, XCreateGC 0x3F997000, XDrawString 0x3FDA0000.
• Live result: a 390-byte payload (poke+jump, no breakpoints) called strlen("0::0") successfully (returned 4) using the firmware ABI, then XOpenDisplay hung: stage 2, display 0, X-server client watermark unchanged, rdebug service dead afterwards. The CPU sits in Xlib's select wrapper. So the ROM is callable, but the "xserver" service connect from the injected process never reaches the server. Do not assume a self-contained X client is "one call away".
• Shared-RAM UI mutation still works (opcode 8 / 0x3F309350), and that is the realistic route for making the Terminal Manager draw something.

2026-10-18 15:25 EEST — TFTP fetch into a malloc buffer works from rdebug

For the COFF-loader agent (details: dwt/notes/LOADER_CROSS.md and dwt/notes/TCPIP.md):

• The firmware tftp_open hangs from the rdebug process (its resolver; no RRQ). Use the socket API instead: socket 3F5C9A00, sendto 3F5D3D70, recvfrom 3F5D7D50 (arg5 = &socklen, initialise to 16), close 3F5E8930, malloc 3F701230. Stack args, last first.
• Verified: dwt/tcpip/tftpc.asm (bit 01870000) fetched 600 B into the malloc buffer 0x00b5bcd0; server RRQ 'dwt/tcpip/probe.bin'; peek/cmp byte-identical (sha256 9662e62b…333359). Run it in the rdebug process.
• Pointers are bit addresses: advance a data cursor by n*8. Keep the 2048-B receive buffer and the from/fromlen/ACK cells disjoint.