drm/ttm: Avoid NULL pointer deref for evicted BOs

It is possible for a BO to exist that is not currently associated with a
resource, e.g. because it has been evicted.

When devcoredump tries to read the contents of all BOs for dumping, we need
to expect this as well -- in this case, ENODATA is recorded instead of the
buffer contents.

Fixes: 7d08df5 ("drm/ttm: Add ttm_bo_access")
Fixes: 09ac4fc ("drm/ttm: Implement vm_operations_struct.access v2")
Cc: stable <stable@kernel.org>
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6271
Signed-off-by: Simon Richter <Simon.Richter@hogyros.de>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Shuicheng Lin <shuicheng.lin@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20251013161241.709916-1-Simon.Richter@hogyros.de

Author: GyrosGeier

drivers/gpu/drm/ttm/ttm_bo_vm.c

@@ -434,6 +434,11 @@ int ttm_bo_access(struct ttm_buffer_object *bo, unsigned long offset,
 	if (ret)
 		return ret;
 
+	if (!bo->resource) {
+		ret = -ENODATA;
+		goto unlock;
+	}
+
 	switch (bo->resource->mem_type) {
 	case TTM_PL_SYSTEM:
 		fallthrough;
@@ -448,6 +453,7 @@ int ttm_bo_access(struct ttm_buffer_object *bo, unsigned long offset,
 			ret = -EIO;
 	}
 
+unlock:
 	ttm_bo_unreserve(bo);
 
 	return ret;