In the Linux kernel, the following vulnerability has been resolved:

drm/amdkfd: Avoid double-unpin of DOORBELL/MMIO BOs on free

amdgpu_amdkfd_gpuvm_free_memory_of_gpu() unpinned DOORBELL and MMIO
remap BOs (which are pinned at allocation time) before checking whether
the BO is still mapped to the GPU. When the BO is still mapped, the
function returns -EBUSY and leaves the BO alive, but it has already
been unpinned. The BO is then unpinned again when it is finally freed
during process teardown, triggering a ttm_bo_unpin() underflow warning:

  WARNING: CPU: 18 PID: 15066 at ttm/ttm_bo.c:650 amdttm_bo_unpin+0x6d/0x80 [amdttm]
  Workqueue: kfd_process_wq kfd_process_wq_release [amdgpu]
  RIP: 0010:amdttm_bo_unpin+0x6d/0x80 [amdttm]
  Call Trace:
   amdgpu_bo_unpin+0x1a/0x90 [amdgpu]
   amdgpu_amdkfd_gpuvm_unpin_bo+0x31/0xb0 [amdgpu]
   amdgpu_amdkfd_gpuvm_free_memory_of_gpu+0x3bf/0x460 [amdgpu]
   kfd_process_free_outstanding_kfd_bos+0xd4/0x170 [amdgpu]
   kfd_process_wq_release+0x109/0x1b0 [amdgpu]
   process_one_work+0x1e2/0x3b0
   worker_thread+0x50/0x3a0
   kthread+0xdd/0x100
   ret_from_fork+0x29/0x50

Move the unpin after the mapped_to_gpu_memory check so it only happens
once we are committed to freeing the BO.

(cherry picked from commit 927c5b2defb9b09856444d94bebfd056a002bd75)