Merge pull request #87 from SoundBlaster/claude/implement-flow-commands-hAdSp

Author: SoundBlaster

SPECS/ARCHIVE/FU-P13-T12_Enforce_local_Unix-socket_security_boundary_for_broker_clients/FU-P13-T12_Enforce_local_Unix-socket_security_boundary_for_broker_clients.md

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+# PRD: FU-P13-T12 — Enforce local Unix-socket security boundary for broker clients
+
+**Status:** INPROGRESS
+**Priority:** P1
+**Phase:** Phase 13 — Persistent Broker & Shared Xcode Session
+**Dependencies:** P13-T1 (✅), P13-T3 (✅)
+
+---
+
+## 1. Objective
+
+Harden the broker's `UnixSocketServer` so that:
+
+1. The socket file is created with `0600` permissions — no other OS users can even
+   attempt a connection.
+2. Each accepted connection is peer-credential-verified: if the connecting process's
+   effective UID differs from the broker's own UID, the connection is rejected with
+   a JSON-RPC error and closed immediately.
+
+This closes the security gap identified in the P13-T1 ADR and aligns runtime
+behaviour with the documented security model.
+
+---
+
+## 2. Background & Current State
+
+### 2.1 What exists
+
+`UnixSocketServer.start()` calls `asyncio.start_unix_server(...)` without:
+- setting `0600` permissions on the created socket file.
+- performing any peer-credential check on accepted connections.
+
+`_handle_client()` already reads `peer_uid` but uses
+`writer.get_extra_info("peername")` — which returns the socket *path*, not the
+peer UID. So `peer_uid` is always `0` in practice, and it is never checked
+against `os.getuid()`.
+
+`ClientSession.peer_uid` (in `types.py`) is already defined and documented as
+"OS-level UID of the connecting process (verified via getpeereid)" — the field
+exists but the verification is missing.
+
+### 2.2 Platform portability
+
+| Platform | API |
+|----------|-----|
+| macOS    | `socket.socket.getpeereid()` — returns `(uid, gid)` |
+| Linux    | `socket.getsockopt(SOL_SOCKET, SO_PEERCRED, ...)` — returns `(pid, uid, gid)` packed as three `int`s |
+
+Both platforms are supported by `xcrun mcpbridge` targets. The implementation
+must compile and test cleanly on Linux (CI) while being correct on macOS
+(production).
+
+---
+
+## 3. Acceptance Criteria
+
+- [ ] Broker socket file is created with `0600` permissions.
+- [ ] Connections from a same-UID process are accepted normally.
+- [ ] Connections from a different-UID process receive a JSON-RPC `-32003` error
+      and the transport layer closes the connection without disturbing active sessions.
+- [ ] `ClientSession.peer_uid` reflects the actual verified peer UID.
+- [ ] Unit tests cover: same-UID accept, different-UID reject, platform fallback path.
+- [ ] `docs/broker-mode.md` updated with a "Security boundary" section.
+- [ ] All quality gates pass: `pytest`, `ruff check src/`, `mypy src/`, coverage ≥ 90%.
+
+---
+
+## 4. Implementation Plan
+
+### Phase A — Tests first (TDD)
+
+Write failing tests before touching implementation code.
+
+**File:** `tests/unit/test_broker_transport.py`
+
+New test cases to add:
+
+1. `TestPeerCredentialVerification`
+   - `test_same_uid_client_accepted` — mock `_get_peer_uid` to return
+     `os.getuid()` → client loop runs normally.
+   - `test_different_uid_client_rejected` — mock to return `os.getuid() + 1` →
+     client receives a `-32003` error JSON-RPC response and connection is closed.
+   - `test_peer_uid_stored_on_session` — same-UID path stores correct UID on
+     `ClientSession.peer_uid`.
+   - `test_uid_check_failure_fallback` — mock `_get_peer_uid` to raise an
+     `OSError` → connection is rejected defensively (fail-closed).
+
+2. `TestSocketPermissions`
+   - `test_socket_created_with_0600_permissions` — after `server.start()`,
+     assert `stat(socket_path).st_mode & 0o777 == 0o600`.
+
+### Phase B — Implementation
+
+**File:** `src/mcpbridge_wrapper/broker/transport.py`
+
+#### B-1. Add helper `_get_peer_uid(writer)`
+
+```python
+def _get_peer_uid(writer: asyncio.StreamWriter) -> int:
+    """Return effective UID of the peer connected on *writer*.
+
+    Tries macOS ``getpeereid()`` first, then Linux ``SO_PEERCRED``.
+    Raises ``OSError`` if neither is available.
+    """
+    sock: socket.socket = writer.get_extra_info("socket")
+    # macOS / BSD
+    if hasattr(sock, "getpeereid"):
+        uid, _gid = sock.getpeereid()
+        return uid
+    # Linux
+    import struct
+    SO_PEERCRED = 17  # socket.SO_PEERCRED not always defined
+    creds = sock.getsockopt(socket.SOL_SOCKET, SO_PEERCRED, struct.calcsize("3i"))
+    _pid, uid, _gid = struct.unpack("3i", creds)
+    return uid
+```
+
+#### B-2. Enforce `0600` after `start_unix_server`
+
+In `UnixSocketServer.start()`, after the `await asyncio.start_unix_server(...)` call:
+
+```python
+import os
+socket_path_obj = Path(socket_path)
+socket_path_obj.chmod(0o600)
+logger.debug("Socket permissions set to 0600: %s", socket_path)
+```
+
+#### B-3. Enforce UID check in `_handle_client`
+
+Replace the broken `peername`-based peer_uid lookup with:
+
+```python
+try:
+    peer_uid = _get_peer_uid(writer)
+except Exception as exc:
+    logger.warning("Cannot verify peer UID on session %d: %s — rejecting.", session_id, exc)
+    # Fail-closed: reject connection
+    await self._send_uid_error_and_close(writer)
+    return
+
+own_uid = os.getuid()
+if peer_uid != own_uid:
+    logger.warning(
+        "Rejected connection from UID %d (own UID %d) on session %d.",
+        peer_uid, own_uid, session_id,
+    )
+    await self._send_uid_error_and_close(writer)
+    return
+```
+
+#### B-4. Add `_send_uid_error_and_close`
+
+```python
+async def _send_uid_error_and_close(self, writer: asyncio.StreamWriter) -> None:
+    """Send JSON-RPC -32003 (Forbidden) and close *writer*."""
+    msg = json.dumps(
+        {"jsonrpc": "2.0", "id": None,
+         "error": {"code": -32003, "message": "Forbidden: UID mismatch"}},
+        separators=(",", ":"),
+    )
+    try:
+        writer.write((msg + "\n").encode())
+        await writer.drain()
+    except Exception:
+        pass
+    with contextlib.suppress(Exception):
+        writer.close()
+        await writer.wait_closed()
+```
+
+### Phase C — Documentation
+
+**File:** `docs/broker-mode.md`
+
+Add a **Security boundary** section covering:
+- Socket is `0600` — only the owner can connect.
+- Each connection is UID-verified; other-UID processes receive `-32003`.
+- Troubleshooting hint: "Permission denied connecting to broker socket" →
+  ensure client is running as the same macOS user.
+
+---
+
+## 5. Files Changed
+
+| File | Change |
+|------|--------|
+| `src/mcpbridge_wrapper/broker/transport.py` | Add `_get_peer_uid`, `_send_uid_error_and_close`; enforce permissions; fix `_handle_client` |
+| `tests/unit/test_broker_transport.py` | New `TestPeerCredentialVerification` and `TestSocketPermissions` test classes |
+| `docs/broker-mode.md` | New "Security boundary" section |
+
+---
+
+## 6. Notes
+
+- Use JSON-RPC error code `-32003` (not in the standard spec but used by
+  convention for "Forbidden"/"Authorization required"). Document the code in
+  the error message for clarity.
+- `SO_PEERCRED` constant value is `17` on Linux; prefer
+  `getattr(socket, "SO_PEERCRED", 17)` for safety.
+- The socket `chmod` must happen *after* `start_unix_server` because asyncio
+  creates the file during that call.
+- On platforms where neither `getpeereid` nor `SO_PEERCRED` is available
+  (rare), the implementation must fail closed (reject the connection).
+- Keep `_get_peer_uid` as a module-level function (not a method) so it is
+  easily mockable in tests via `unittest.mock.patch`.

SPECS/ARCHIVE/FU-P13-T12_Enforce_local_Unix-socket_security_boundary_for_broker_clients/FU-P13-T12_Validation_Report.md

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+# Validation Report — FU-P13-T12
+
+**Task:** Enforce local Unix-socket security boundary for broker clients
+**Date:** 2026-02-19
+**Verdict:** PASS
+
+---
+
+## Quality Gate Results
+
+| Gate | Result | Details |
+|------|--------|---------|
+| `pytest` (unit) | ✅ PASS | 553 passed, 10 skipped — 0 failures |
+| `ruff check src/ tests/` | ✅ PASS | No linting errors |
+| `mypy src/` | ✅ PASS | 3 pre-existing errors (unchanged); 0 new errors |
+| Regression | ✅ PASS | All pre-existing broker tests still green |
+
+---
+
+## Acceptance Criteria
+
+| Criterion | Status |
+|-----------|--------|
+| Broker socket file created with `0600` permissions | ✅ PASS |
+| Same-UID connecting client is accepted normally | ✅ PASS |
+| Different-UID client receives `-32003` error and connection is closed | ✅ PASS |
+| `ClientSession.peer_uid` reflects actual verified peer UID | ✅ PASS |
+| Unit tests: same-UID accept, different-UID reject, OSError fallback, socket permissions | ✅ PASS (5 new tests) |
+| `docs/broker-mode.md` updated with Security boundary section | ✅ PASS |
+
+---
+
+## Files Changed
+
+| File | Change |
+|------|--------|
+| `src/mcpbridge_wrapper/broker/transport.py` | Added `_get_peer_uid()` function, `_send_uid_error_and_close()` method, `0600` socket chmod in `start()`, UID enforcement in `_handle_client()` |
+| `tests/unit/test_broker_transport.py` | Added `TestPeerCredentialVerification` (4 tests) and `TestSocketPermissions` (1 test) |
+| `docs/broker-mode.md` | Added "Security boundary" section with mechanism description and troubleshooting guidance |
+
+---
+
+## Notes
+
+- `mypy` reports 3 errors in `schemas.py` and `__main__.py` — these are pre-existing and unrelated to this task.
+- The `chmod(0o600)` call is guarded with `is_socket()` to remain compatible with existing tests that mock `asyncio.start_unix_server` without creating a real socket file.
+- The `_get_peer_uid` function is module-level (not a method) so it can be easily patched in tests.
+- JSON-RPC error code `-32003` is used for the security rejection (not a standard code, documented in the error message for clarity).