Fix #6: bound formula evaluation to stop untrusted-profile DoS

The AST sandbox whitelisted ** and << with no magnitude bound, so a hostile
profile formula (9**9**9, 1<<10**9) computed a multi-hundred-MB integer on the
scheduler thread -> CPU pin + OOM. The scheduler except clause never catches a
runaway/OOM (not a raised exception), and a derived PID with empty deps fires
every tick on connect.

- _apply() guards each BinOp: shift amount <= 256, exponent <= 64, and any int
  result bit_length > 512 raises FormulaError (caught by the scheduler -> sample
  dropped, thread survives).
- compile-time caps: expr length <= 500, AST depth <= 60; parse also catches
  RecursionError.
- test_formula_dos_bounded: giant-int payloads rejected in <0.5s; legit bit ops
  and scaling still work.

Closes #6

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016yT89n4zR4qbrySoSiEyZs

obdcore/formula.py

@@ -1,8 +1,9 @@
 """Safe formula evaluator for vehicle-profile PID scaling.
 
 Profiles are community-contributed data, so decode formulas must NOT be able to
-execute arbitrary code. Formulas are arithmetic expressions over named
-variables -- the de-facto OBD convention used by Torque / FORScan / ScanGauge:
+execute arbitrary code -- OR exhaust CPU/memory. Formulas are arithmetic
+expressions over named variables -- the de-facto OBD convention used by Torque /
+FORScan / ScanGauge:
 
     raw-mode PIDs: variables A, B, C, ... = response data bytes 0, 1, 2, ...
         e.g.  "(A*256+B)*0.57"   "A-40"   "(A>>1)&1"   "A//2"
@@ -10,9 +11,11 @@ variables -- the de-facto OBD convention used by Torque / FORScan / ScanGauge:
         e.g.  "MAP - BARO"
 
 Only numeric literals, the named variables, arithmetic/bitwise operators, and a
-small whitelist of functions are allowed. No names, attributes, subscripts,
-comprehensions, or calls outside the whitelist -- anything else raises
-FormulaError at compile time, so a bad/hostile profile fails loudly on load.
+small whitelist of functions are allowed. Anything else raises FormulaError at
+compile time. To stop a hostile profile from freezing the acquisition thread
+with a giant-integer expression (e.g. `9**9**9`, `1<<10**9`), evaluation also
+BOUNDS magnitude: shift/exponent amounts and integer result bit-lengths are
+capped, and expression length + nesting depth are limited at compile.
 """
 import ast
 import operator
@@ -28,24 +31,33 @@ _UNARY = {ast.USub: operator.neg, ast.UAdd: operator.pos, ast.Invert: operator.i
 _FUNCS = {"min": min, "max": max, "abs": abs, "round": round,
           "int": int, "float": float}
 
+# magnitude / complexity limits (far above any real OBD byte arithmetic)
+MAX_RESULT_BITS = 512      # ~155 decimal digits; real decode stays < 32 bits
+MAX_SHIFT = 256            # bit-field decode never shifts more than a few bytes
+MAX_POW_EXP = 64
+MAX_EXPR_LEN = 500
+MAX_DEPTH = 60
+
 
 class FormulaError(ValueError):
     pass
 
 
-def _validate(node, allowed):
+def _validate(node, allowed, depth=0):
+    if depth > MAX_DEPTH:
+        raise FormulaError("formula too deeply nested")
     if isinstance(node, ast.Expression):
-        return _validate(node.body, allowed)
+        return _validate(node.body, allowed, depth + 1)
     if isinstance(node, ast.BinOp):
         if type(node.op) not in _BIN:
             raise FormulaError(f"operator not allowed: {type(node.op).__name__}")
-        _validate(node.left, allowed)
-        _validate(node.right, allowed)
+        _validate(node.left, allowed, depth + 1)
+        _validate(node.right, allowed, depth + 1)
         return
     if isinstance(node, ast.UnaryOp):
         if type(node.op) not in _UNARY:
             raise FormulaError(f"unary op not allowed: {type(node.op).__name__}")
-        _validate(node.operand, allowed)
+        _validate(node.operand, allowed, depth + 1)
         return
     if isinstance(node, ast.Constant):
         if not isinstance(node.value, (int, float)) or isinstance(node.value, bool):
@@ -61,16 +73,42 @@ def _validate(node, allowed):
         if node.keywords:
             raise FormulaError("keyword args not allowed")
         for a in node.args:
-            _validate(a, allowed)
+            _validate(a, allowed, depth + 1)
         return
     raise FormulaError(f"expression not allowed: {type(node).__name__}")
 
 
+def _apply(op_type, left, right):
+    """Apply a binary op with magnitude guards so an untrusted formula can't
+    allocate a giant integer (Pow / shift amplification)."""
+    if op_type in (ast.LShift, ast.RShift):
+        try:
+            r = operator.index(right)
+        except TypeError:
+            raise FormulaError("shift amount must be an integer")
+        if not 0 <= r <= MAX_SHIFT:
+            raise FormulaError("shift amount out of range")
+        if op_type is ast.LShift and isinstance(left, int) and \
+                left.bit_length() + r > MAX_RESULT_BITS:
+            raise FormulaError("shift result too large")
+    elif op_type is ast.Pow:
+        if isinstance(right, int):
+            if right > MAX_POW_EXP:
+                raise FormulaError("exponent too large")
+            if isinstance(left, int) and right > 0 and \
+                    left.bit_length() * right > MAX_RESULT_BITS:
+                raise FormulaError("power result too large")
+    res = _BIN[op_type](left, right)
+    if isinstance(res, int) and res.bit_length() > MAX_RESULT_BITS:
+        raise FormulaError("result magnitude too large")
+    return res
+
+
 def _eval(node, names):
     if isinstance(node, ast.Expression):
         return _eval(node.body, names)
     if isinstance(node, ast.BinOp):
-        return _BIN[type(node.op)](_eval(node.left, names), _eval(node.right, names))
+        return _apply(type(node.op), _eval(node.left, names), _eval(node.right, names))
     if isinstance(node, ast.UnaryOp):
         return _UNARY[type(node.op)](_eval(node.operand, names))
     if isinstance(node, ast.Constant):
@@ -84,9 +122,11 @@ def _eval(node, names):
 
 def compile_formula(expr, allowed_names):
     """Return fn(names_dict) -> number. Raises FormulaError on disallowed input."""
+    if len(expr) > MAX_EXPR_LEN:
+        raise FormulaError("formula too long")
     try:
         tree = ast.parse(expr, mode="eval")
-    except SyntaxError as e:
+    except (SyntaxError, ValueError, RecursionError) as e:
         raise FormulaError(f"bad formula {expr!r}: {e}")
     allowed = set(allowed_names)
     _validate(tree, allowed)