I picked a terrible example which doesn't demonstrate what I intended to. Any USB link requires the host to maintain some persistent state in data structures m

I picked a terrible example which doesn't demonstrate what I intended to. Any USB link requires the host to maintain some persistent state in data structures mirroring what it thinks the state of the device is. There's no "DMA" in the sense that the device has direct access to the host - but that doesn't preclude something as mundane as a buffer overflow, use-after-free and so on. "DMA", with appropriate IOMMU, is just fancy shared memory communication. You're just as likely to mess that up as a serial link. It's happened. A lot.