Synopsys DWC2 USB Controller: Hardware Flaw Enabling usbliter8

The Synopsys DWC2 USB controller contains a hardware flaw that allows a DMA buffer underflow. The controller stores incoming USB Setup packets via DMA, buffers up to three, then resets its write pointer on the fourth by decrementing it by a fixed 24 bytes. It also accepts smaller-than-standard packets, incrementing the pointer only by the actual bytes written, leading to a repeatable buffer underflow. This flaw is exploited by usbliter8 on Apple A12 and A13 chips.