fix(expr): bound runtime parser and native recursion depth (#2124, #2125)

eval, Function and the generator/async function constructors now check the nesting of their source like the expression text. join, toLocaleString, flat and JSON.stringify check the run's stack use per level, results nested past the detached levels become [object], and Proxy/Reflect are no longer available to expressions. Each of these overflowed the stack and aborted the app.

crates/expr/src/prelude.js

@@ -670,14 +670,17 @@ var __groupHandler = {
   },
 };
 
-function __mkGroup(c, l, info) {
-  var f = function (k) { return f.__get(k); };
-  delete f.name;
-  delete f.length;
-  Object.setPrototypeOf(f, __GroupProto);
-  f.__c = c; f.__l = l; f.__p = info[0]; f.__info = info;
-  return new Proxy(f, __groupHandler);
-}
+// `Proxy` is private to the object model (see "Stack safety" at the end).
+var __mkGroup = (function (P) {
+  return function (c, l, info) {
+    var f = function (k) { return f.__get(k); };
+    delete f.name;
+    delete f.length;
+    Object.setPrototypeOf(f, __GroupProto);
+    f.__c = c; f.__l = l; f.__p = info[0]; f.__info = info;
+    return new P(f, __groupHandler);
+  };
+})(Proxy);
 
 // Properties ------------------------------------------------------------------------------------
 
@@ -852,9 +855,11 @@ var __propHandler = {
   },
 };
 
-function __mkProp(c, l, path) {
-  return new Proxy(new Prop(c, l, path), __propHandler);
-}
+var __mkProp = (function (P) {
+  return function (c, l, path) {
+    return new P(new Prop(c, l, path), __propHandler);
+  };
+})(Proxy);
 
 // Text styles (the Source Text style API) ------------------------------------------------------
 // A TextStyle is the source document (as JSON from the host), the character index it reads its
@@ -1069,3 +1074,92 @@ function velocityAtTime(t) { return thisProperty.velocityAtTime(t); }
 function speedAtTime(t) { return thisProperty.speedAtTime(t); }
 function key(n) { return thisProperty.key(n); }
 function nearestKey(t) { return thisProperty.nearestKey(t); }
+
+// ---------------------------------------------------------------------------------------------
+// Stack safety (#2124, #2125). A stack overflow aborts the whole app, so nothing a script does
+// may nest native code without bound. The expression text is checked for nesting before it is
+// compiled; source handed to `eval` and the function constructors at run time is checked by
+// `__nest`. The natives that walk nested values recursively (`join`, which `toString` and string
+// conversion use, `toLocaleString`, `JSON.stringify`, `flat`) call `__stack` once per level,
+// which throws once the run has used its share of the stack. `Proxy` (chains of proxies nest
+// without any script frame) and `Reflect` are not available to expressions.
+(function () {
+  var nest = __nest, stack = __stack, apply = Reflect.apply, construct = Reflect.construct;
+  var define = Object.defineProperty, describe = Object.getOwnPropertyDescriptor, proto = Object.getPrototypeOf;
+  // Replace `obj[name]` keeping its attributes.
+  function replace(obj, name, f) {
+    var d = describe(obj, name);
+    define(obj, name, { value: f, writable: d.writable, enumerable: d.enumerable, configurable: true });
+  }
+  function guard(obj, name) {
+    var f = obj[name];
+    replace(obj, name, function () { stack(); return apply(f, this, arguments); });
+  }
+
+  var evalJs = eval;
+  eval = function (x) {
+    if (typeof x === 'string') {
+      stack();
+      nest(x);
+    }
+    return evalJs(x);
+  };
+  // `Function` and the generator / async variants (reachable as `(function* () {}).constructor`).
+  function dynamic(F) {
+    var w = function () {
+      var args = [];
+      for (var i = 0; i < arguments.length; i++) args.push(String(arguments[i]));
+      stack();
+      nest(args.slice(0, -1).join(','));
+      nest(args.length ? args[args.length - 1] : '');
+      return construct(F, args);
+    };
+    define(w, 'name', { value: F.name, configurable: true });
+    w.prototype = F.prototype;
+    replace(F.prototype, 'constructor', w);
+    return w;
+  }
+  Function = dynamic(Function);
+  dynamic(proto(function* () {}).constructor);
+  dynamic(proto(async function () {}).constructor);
+  dynamic(proto(async function* () {}).constructor);
+
+  guard(Array.prototype, 'join');
+  guard(Array.prototype, 'toLocaleString');
+  var flat = Array.prototype.flat;
+  replace(Array.prototype, 'flat', function (depth) {
+    stack();
+    var d = depth === undefined ? 1 : Number(depth);
+    return apply(flat, this, [d > 64 ? 64 : d]);
+  });
+
+  // A replacer function sees every value before it is serialized: the per-level check. An array
+  // replacer (a list of keys) becomes one that copies each object's listed keys, in list order.
+  var stringify = JSON.stringify;
+  replace(JSON, 'stringify', function (value, replacer, space) {
+    var fn = typeof replacer === 'function' ? replacer : null;
+    var keys = null;
+    if (Array.isArray(replacer)) {
+      keys = [];
+      for (var i = 0; i < replacer.length; i++) {
+        var k = replacer[i];
+        if (typeof k === 'number' || k instanceof Number || k instanceof String) k = String(k);
+        if (typeof k === 'string' && keys.indexOf(k) < 0) keys.push(k);
+      }
+    }
+    return apply(stringify, JSON, [value, function (key, v) {
+      stack();
+      if (fn !== null) v = apply(fn, this, [key, v]);
+      if (keys !== null && v !== null && typeof v === 'object' && !Array.isArray(v) &&
+          !(v instanceof Number || v instanceof String || v instanceof Boolean)) {
+        var o = {};
+        for (var j = 0; j < keys.length; j++) if (keys[j] in v) o[keys[j]] = v[keys[j]];
+        v = o;
+      }
+      return v;
+    }, space]);
+  });
+
+  delete globalThis.Proxy;
+  delete globalThis.Reflect;
+})();

crates/expr/src/rewrite.rs

@@ -758,6 +758,12 @@ pub fn check_nesting(src: &str) -> Result<(), String> {
     Ok(())
 }
 
+/// An error when plain JavaScript `src` (not rewritten: what `eval` and `Function` get at run
+/// time) nests deeper than [`MAX_NESTING`] levels.
+pub fn check_plain_nesting(src: &str) -> Result<(), String> {
+    if too_nested(&tokenize(src).0) { Err(too_deep()) } else { Ok(()) }
+}
+
 fn too_deep() -> String {
     format!("Error: expression is nested too deeply (more than {MAX_NESTING} levels of brackets, or too long a chain of operators)")
 }

crates/expr/src/runtime.rs

@@ -51,6 +51,8 @@ thread_local! {
     pub static FRAMES: RefCell<Vec<Frame>> = const { RefCell::new(Vec::new()) };
     /// Script runs since the outermost evaluation on this thread began (nested ones included).
     pub static RUNS: std::cell::Cell<usize> = const { std::cell::Cell::new(0) };
+    /// Stack address where the current script run began (0: none; see [`native_stack`]).
+    static STACK_BASE: std::cell::Cell<usize> = const { std::cell::Cell::new(0) };
 }
 
 #[cfg(not(target_arch = "wasm32"))]
@@ -232,17 +234,49 @@ fn native_random(_: &JsValue, args: &[JsValue], ctx: &mut Context) -> JsResult<J
     Ok(JsValue::from(noise::unit(noise::mix(&parts))))
 }
 
+/// Stack a script run may use below [`run`] for natives that recurse once per level of a nested
+/// value (`join`, `JSON.stringify`, …; the prelude calls [`native_stack`] per level). Render
+/// threads have 2 MiB; the parser's own nesting limit ([`crate::rewrite::MAX_NESTING`]) needs
+/// most of the rest.
+const NATIVE_STACK: usize = 256 << 10;
+
+/// An address on the current stack.
+fn stack_here() -> usize {
+    let marker = 0u8;
+    std::hint::black_box(&marker) as *const u8 as usize
+}
+
+/// `__stack()`: throw once the current run has used [`NATIVE_STACK`] (values nested so deeply
+/// would overflow the stack, which aborts the app).
+fn native_stack(_: &JsValue, _: &[JsValue], _: &mut Context) -> JsResult<JsValue> {
+    let base = STACK_BASE.with(|b| b.get());
+    if base != 0 && base.abs_diff(stack_here()) > NATIVE_STACK {
+        return Err(JsNativeError::range().with_message("value is nested too deeply").into());
+    }
+    Ok(JsValue::undefined())
+}
+
+/// `__nest(src)`: throw when source handed to `eval` / `Function` at run time nests deeper than
+/// the parser's stack allows (the expression text itself is checked before it is compiled).
+fn native_nesting(_: &JsValue, args: &[JsValue], ctx: &mut Context) -> JsResult<JsValue> {
+    let src = string(args, 0, ctx)?;
+    crate::rewrite::check_plain_nesting(&src).map_err(|e| JsNativeError::range().with_message(e.trim_start_matches("Error: ").to_string()))?;
+    Ok(JsValue::undefined())
+}
+
 impl Runtime {
     fn new() -> Result<Runtime, String> {
         let mut ctx = Context::default();
         let limits = ctx.runtime_limits_mut();
         limits.set_loop_iteration_limit(LOOP_LIMIT);
         limits.set_recursion_limit(400);
-        let natives: [(JsString, usize, fn(&JsValue, &[JsValue], &mut Context) -> JsResult<JsValue>); 4] = [
+        let natives: [(JsString, usize, fn(&JsValue, &[JsValue], &mut Context) -> JsResult<JsValue>); 6] = [
+            (js_string!("__stack"), 0, native_stack),
             (js_string!("__h"), 6, native_host),
             (js_string!("__wig"), 8, native_wiggle),
             (js_string!("__noise"), 3, native_noise),
             (js_string!("__rnd"), 6, native_random),
+            (js_string!("__nest"), 1, native_nesting),
         ];
         for (name, len, f) in natives {
             ctx.register_global_callable(name, len, NativeFunction::from_fn_ptr(f)).map_err(|e| e.to_string())?;
@@ -361,6 +395,9 @@ pub fn out_of_time() -> bool {
     }
 }
 
+/// Levels of nested arrays a script result keeps.
+const MAX_DETACH: usize = 4;
+
 /// Copy a JS value out of the heap.
 fn detach(v: &JsValue, ctx: &mut Context, depth: usize) -> Result<Out, String> {
     if v.is_undefined() {
@@ -380,7 +417,7 @@ fn detach(v: &JsValue, ctx: &mut Context, depth: usize) -> Result<Out, String> {
     }
     let err = |e: boa_engine::JsError| format_error(&e.to_string(), usize::MAX);
     if let Some(o) = v.as_object() {
-        if o.is_array() && depth < 4 {
+        if o.is_array() && depth < MAX_DETACH {
             let arr = JsArray::from_object(o).map_err(err)?;
             let n = arr.length(ctx).map_err(err)?.min(1 << 16);
             let mut out = Vec::with_capacity(n as usize);
@@ -408,6 +445,11 @@ fn detach(v: &JsValue, ctx: &mut Context, depth: usize) -> Result<Out, String> {
             return Ok(Out::Path { points, ins, outs, closed });
         }
     }
+    // Below the levels detached above, an object's text could be its whole nested content
+    // (`String([[[…]]])`), converted by natives that recurse once per level.
+    if depth >= MAX_DETACH && v.is_object() {
+        return Ok(Out::Object("[object]".into()));
+    }
     Ok(Out::Object(v.to_string(ctx).map(|s| s.to_std_string_escaped()).unwrap_or_default()))
 }
 
@@ -484,6 +526,8 @@ fn position_line(msg: &str) -> Option<usize> {
 /// `__begin` with memoized answers, so nothing carries over. A script that really overflows the
 /// stack fails again on the fresh context and reports the error.
 pub fn run(text: &str, b: &Begin) -> Run {
+    // Runs don't nest (other expressions are evaluated between runs), so one base suffices.
+    STACK_BASE.with(|s| s.set(stack_here()));
     RUNTIME.with(|r| {
         let mut r = r.borrow_mut();
         for fresh in [false, true] {

crates/expr/src/tests.rs

@@ -1,5 +1,6 @@
 use std::time::Instant;
 
+mod deep_nesting;
 mod fuzz;
 
 use effectcraft_color::Label;

crates/expr/src/tests/deep_nesting.rs

@@ -0,0 +1,104 @@
+//! Deep nesting the expression text check can't see (#2124, #2125): source handed to `eval` /
+//! `Function` at run time, and deeply nested values that boa's natives (`join`, `JSON.stringify`,
+//! proxies…) walk recursively. Each used to overflow the stack, which aborts the whole app
+//! (`catch_unwind` can't catch it); now each is an expression error.
+
+use super::*;
+
+/// The render threads' stack size (rayon's default): what the app gets.
+const RENDER_STACK: usize = 2 << 20;
+
+/// Evaluate `text` on the title's source text (any result converts) on a render-sized stack.
+fn eval_small_stack(text: &str) -> Result<Value, String> {
+    let text = text.to_string();
+    std::thread::Builder::new()
+        .stack_size(RENDER_STACK)
+        .spawn(move || {
+            let f = fx();
+            f.ev(f.title, "text/sourceText", &text, 0.0)
+        })
+        .unwrap()
+        .join()
+        .unwrap()
+}
+
+fn text_of(r: Result<Value, String>) -> String {
+    match r {
+        Ok(Value::Text(d)) => d.text,
+        other => panic!("not text: {other:?}"),
+    }
+}
+
+/// A 100 000-deep array: `var a = []; … a = [a]` in a loop (under the loop limit).
+const DEEP_ARRAY: &str = "var a = []; for (var i = 0; i < 100000; i++) a = [a];";
+
+#[test]
+fn eval_and_function_constructors_check_nesting() {
+    let deep = r#""(".repeat(1000) + "1" + ")".repeat(1000)"#;
+    for call in [
+        format!("eval({deep})"),
+        format!("Function({deep})()"),
+        format!("new Function('a', {deep})(1)"),
+        format!("Function('a = ' + {deep}, 'return a')()"),
+        "Function.apply(null, '('.repeat(1000).split('').concat(['1']))".to_string(),
+        format!("(function () {{}}).constructor({deep})()"),
+        format!("Object.getPrototypeOf(function* () {{}}).constructor({deep})"),
+        format!("Object.getPrototypeOf(async function () {{}}).constructor({deep})"),
+        format!("Object.getPrototypeOf(async function* () {{}}).constructor({deep})"),
+        format!("Reflect.construct(Function, [{deep}])"),
+    ] {
+        let e = eval_small_stack(&call).expect_err(&call);
+        assert!(!e.is_empty(), "{call}");
+    }
+}
+
+#[test]
+fn eval_and_function_still_work() {
+    let f = fx();
+    assert_eq!(f.num("eval('1 + 2')", 0.0), 3.0);
+    assert_eq!(f.num("eval('[1, [2, (3)]]')[1][1]", 0.0), 3.0);
+    assert_eq!(f.num("new Function('a', 'b', 'return a * b')(3, 4)", 0.0), 12.0);
+    assert_eq!(f.num("Function('return 5')()", 0.0), 5.0);
+    assert_eq!(f.num("(function () {}).constructor('return 6')()", 0.0), 6.0);
+    assert_eq!(f.num("(function () {}) instanceof Function ? 1 : 0", 0.0), 1.0);
+    assert_eq!(f.num("(function () {}).constructor === Function ? 1 : 0", 0.0), 1.0);
+    assert_eq!(f.num("Object.getPrototypeOf(function* () { yield 7; }).constructor('yield 7')().next().value", 0.0), 7.0);
+}
+
+#[test]
+fn deep_array_result_is_an_error_or_text_not_a_crash() {
+    // The result itself: detaching it used to `toString` it past depth 4.
+    let _ = eval_small_stack(&format!("{DEEP_ARRAY} a"));
+    // Explicit conversions inside the script.
+    for tail in ["a.join()", "a.toString()", "String(a)", "a + ''", "`${a}`", "a.toLocaleString()", "JSON.stringify(a)", "JSON.stringify({ a: a })"] {
+        let text = format!("{DEEP_ARRAY} {tail}");
+        assert!(eval_small_stack(&text).is_err(), "{text}");
+    }
+    // A deep array as a text style value goes through JSON.stringify in the prelude.
+    let _ = eval_small_stack(&format!("{DEEP_ARRAY} text.sourceText.style.setFontSize(a)"));
+}
+
+#[test]
+fn deep_proxy_chain_is_an_error_not_a_crash() {
+    let text = "var p = {}; for (var i = 0; i < 100000; i++) p = new Proxy(p, {}); p.x";
+    assert!(eval_small_stack(text).is_err(), "{text}");
+    // Neither is available to expressions; the object model's own proxies still work.
+    let f = fx();
+    assert_eq!(f.text("typeof Proxy + ' ' + typeof Reflect", 0.0), "undefined undefined");
+    assert_eq!(f.num("thisComp.layer('A').effect('Slider Control')('Slider')", 0.0), 42.0);
+}
+
+#[test]
+fn nested_values_still_convert() {
+    let f = fx();
+    assert_eq!(f.text("[[1, 2], [3, [4, 5]]].join(';')", 0.0), "1,2;3,4,5");
+    assert_eq!(f.text("String([[1, 2], 3])", 0.0), "1,2,3");
+    assert_eq!(f.text("JSON.stringify({ a: [1, { b: [2] }] })", 0.0), r#"{"a":[1,{"b":[2]}]}"#);
+    assert_eq!(f.text("JSON.stringify({ a: 1, b: 2 }, ['b'])", 0.0), r#"{"b":2}"#);
+    assert_eq!(f.text("JSON.stringify([1, [2]], null, 1)", 0.0), "[\n 1,\n [\n  2\n ]\n]");
+    assert_eq!(f.text("JSON.stringify({ a: 1 }, function (k, v) { return k === 'a' ? v + 1 : v; })", 0.0), r#"{"a":2}"#);
+    // Nested deeper than the detached levels: still converts to its text.
+    assert_eq!(text_of(eval_small_stack("[[[[[[1, 2]]]]]]")), "");
+    assert_eq!(f.text("String([[[[[[[[1]]]]]]]])", 0.0), "1");
+    assert_eq!(f.num("[1, [2, [3, [4]]]].flat(Infinity).length", 0.0), 4.0);
+}