Revert "drm/amdgpu: Use dma_buf from GEM object instance"

This reverts commit 5159861.

The dma_buf field in struct drm_gem_object is not stable over the
object instance's lifetime. The field becomes NULL when user space
releases the final GEM handle on the buffer object. This resulted
in a NULL-pointer deref.

Workarounds in commit 5307dce ("drm/gem: Acquire references on
GEM handles for framebuffers") and commit f6bfc9a ("drm/framebuffer:
Acquire internal references on GEM handles") only solved the problem
partially. They especially don't work for buffer objects without a DRM
framebuffer associated.

Hence, this revert to going back to using .import_attach->dmabuf.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://lore.kernel.org/r/20250715082635.34974-1-tzimmermann@suse.de

Author: tdz

drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c

@@ -514,7 +514,7 @@ bool amdgpu_dmabuf_is_xgmi_accessible(struct amdgpu_device *adev,
 		return false;
 
 	if (drm_gem_is_imported(obj)) {
-		struct dma_buf *dma_buf = obj->dma_buf;
+		struct dma_buf *dma_buf = obj->import_attach->dmabuf;
 
 		if (dma_buf->ops != &amdgpu_dmabuf_ops)
 			/* No XGMI with non AMD GPUs */

drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c

@@ -317,7 +317,8 @@ static int amdgpu_gem_object_open(struct drm_gem_object *obj,
 	 */
 	if (!vm->is_compute_context || !vm->process_info)
 		return 0;
-	if (!drm_gem_is_imported(obj) || !dma_buf_is_dynamic(obj->dma_buf))
+	if (!drm_gem_is_imported(obj) ||
+	    !dma_buf_is_dynamic(obj->import_attach->dmabuf))
 		return 0;
 	mutex_lock_nested(&vm->process_info->lock, 1);
 	if (!WARN_ON(!vm->process_info->eviction_fence)) {

drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c

@@ -1276,7 +1276,7 @@ int amdgpu_vm_bo_update(struct amdgpu_device *adev, struct amdgpu_bo_va *bo_va,
 		struct drm_gem_object *obj = &bo->tbo.base;
 
 		if (drm_gem_is_imported(obj) && bo_va->is_xgmi) {
-			struct dma_buf *dma_buf = obj->dma_buf;
+			struct dma_buf *dma_buf = obj->import_attach->dmabuf;
 			struct drm_gem_object *gobj = dma_buf->priv;
 			struct amdgpu_bo *abo = gem_to_amdgpu_bo(gobj);