drm/xe: Fix oops in xe_gem_fault when running core_hotunplug test.

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: drm/xe: Fix oops in xe_gem_fault when running core_hotunplug test. I saw an oops in xe_gem_fault when running the xe-fast-feedback testlist against the realtime kernel without debug options enabled. The panic happens after core_hotunplug unbind-rebind finishes. Presumably what happens is that a process mmaps, unlocks because of the FAULT_FLAG_RETRY_NOWAIT logic, has no process memory left, causing ttm_bo_vm_dummy_page() to return VM_FAULT_NOPAGE, since there was nothing left to populate, and then oopses in "mem_type_is_vram(tbo->resource->mem_type)" because tbo->resource is NULL. It's convoluted, but fits the data and explains the oops after the test exits.

References

• https://git.kernel.org/stable/c/99428bd6123d5676209dfb1d7a8f176cc830b665
• https://git.kernel.org/stable/c/29a3064f9c5a908aaf0b39cd6ed30374db11840d
• https://git.kernel.org/stable/c/1cda3c755bb7770be07d75949bb0f45fb88651f6