usb: xhci: Fix invalid pointer dereference in Etron workaround

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: usb: xhci: Fix invalid pointer dereference in Etron workaround This check is performed before prepare_transfer() and prepare_ring(), so enqueue can already point at the final link TRB of a segment. And indeed it will, some 0.4% of times this code is called. Then enqueue + 1 is an invalid pointer. It will crash the kernel right away or load some junk which may look like a link TRB and cause the real link TRB to be replaced with a NOOP. This wouldn't end well. Use a functionally equivalent test which doesn't dereference the pointer and always gives correct result. Something has crashed my machine twice in recent days while playing with an Etron HC, and a control transfer stress test ran for confirmation has just crashed it again. The same test passes with this patch applied.

References

• https://git.kernel.org/stable/c/142273a49f2c315eabdbdf5a71c15e479b75ca91
• https://git.kernel.org/stable/c/bce3055b08e303e28a8751f6073066f5c33a0744
• https://git.kernel.org/stable/c/0624e29c595b05e7a0e6d1c368f0a05799928e30
• https://git.kernel.org/stable/c/1ea050da5562af9b930d17cbbe9632d30f5df43a