drm/amdkfd: Fix kfd process ref leaking when userptr unmapping

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: drm/amdkfd: Fix kfd process ref leaking when userptr unmapping kfd_lookup_process_by_pid hold the kfd process reference to ensure it doesn't get destroyed while sending the segfault event to user space. Calling kfd_lookup_process_by_pid as function parameter leaks the kfd process refcount and miss the NULL pointer check if app process is already destroyed.

References

• https://git.kernel.org/stable/c/60f6112fc9b3ba0eae519f10702c0c13bab45742
• https://git.kernel.org/stable/c/58e6fc2fb94f0f409447e5d46cf6a417b6397fbc