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Infinite loop in xhci_ring_chain_length() in hw/usb/hcd-xhci.c (CVE-2020-14394)
Description of problem:
An infinite loop issue was found in the USB xHCI controller emulation of QEMU. Specifically, function `xhci_ring_chain_length()` in hw/usb/hcd-xhci.c may get stuck while fetching empty TRBs from guest memory, since the exit conditions of the loop depend on values that are fully controlled by guest. A privileged guest user may exploit this issue to hang the QEMU process on the host, resulting in a denial of service.
Steps to reproduce:
Build and load `xhci.ko` from within the guest:

1) make
2) insmod xhci.ko

[Makefile](/uploads/98dbf7b4facc9b100817b3c8f63b5cb2/Makefile)

[usb-xhci.h](/uploads/f225524b1553d8cf6c1dfa89369b6edc/usb-xhci.h)

[xhci.c](/uploads/c635f742d12a2bba6ea472ddfe006d56/xhci.c)
Additional information:
This issue was reported by Gaoning Pan (Zhejiang University) and Xingwei Li (Ant Security Light-Year Lab).

RH bug: https://bugzilla.redhat.com/show_bug.cgi?id=1908004.