CSAW Quals 2017 FuntimeJS

| pernicious


This challenge was formally Part 2 of LittleQuery (Web). Description:

JavaScript is memory safe, right? So you can’t read the flag at physical address 0xdeadbeeeef, right? Right?

This was a very interesting challenge from CSAW Quals 2017 (although whether a funtime was had is still questionable…). We are given a web page where we can submit javascript, and a link to the open source project that will run it, runtime.js, an “operating system…that runs JavaScript.” Because running javascript in ring 0 is just what this world needs… This writeup is a bit long, skimming is not discouraged.

Step 1: Arbitrary Read/Write

Finding a bug was a lengthy process of going through the source and trying things out. The syscalls seemed a good place to start, especially a few:

// runtime.js syscalls: Low level system access
DECLARE_NATIVE(BufferAddress);       // Get buffer physical address
DECLARE_NATIVE(GetSystemResources);  // Get low-level system resources

console.log(__SYSCALL.bufferAddress(new Uint8Array(17))) => [ 17, 510626304, 0, 0, 0, 0 ]

Huh, that second entry looks suspicously like a memory address… Looking a bit into the source for getSystemResources (some stuff is cut out):

NATIVE_FUNCTION(NativesObject, GetSystemResources) {
  LOCAL_V8STRING(s_memory_range, "memoryRange");
                                       //     vvvvv   memoryRanges's type
  obj->Set(context, s_memory_range, (new ResourceMemoryRangeObject(Range<size_t>(0, 0xffffffff)))

and following the bread crumbs…

NATIVE_FUNCTION(ResourceMemoryRangeObject, Block) {
  auto base = static_cast<uint64_t>(arg0->NumberValue(context).FromJust());
  auto size = static_cast<uint32_t>(arg1->Uint32Value(context).FromJust());
  Range<size_t> subrange(base, base + size);
  if (!subrange.IsSubrangeOf(that->memory_range_)) {
    THROW_RANGE_ERROR("block: out of bounds");
  }                             //    vvvvv  return type of memoryRange.block()
  args.GetReturnValue().Set((new ResourceMemoryBlockObject(
                               MemoryBlock<uint32_t>(reinterpret_cast<void*>(base), size)))

NATIVE_FUNCTION(ResourceMemoryBlockObject, Buffer) {
  void* ptr = that->memory_block_.base(); // <---- uses raw void* ???
  auto length = that->memory_block_.size();
  RT_ASSERT(length > 0);
  auto abv8 = v8::ArrayBuffer::New …
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