GPU DDK - Write UAF of sync checkpoint in GPU kick function after export fence file descriptor is prematurely closed

Software installed and run as a non-privileged user may conduct improper GPU system calls to manipulate the lifetimes of synchronisation objects in the kernel, leading to read/write UAFs.

During workload submission involving a fence exported by the GPU driver, the reference count of the underlying synchronisation primitive is not properly incremented. This can be exploited, by destroying the exported fence and prematurely release the underlying primitive, resulting in a potential use-after-free condition.