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

This reverts commit f83a9b8.

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.

v3:
- cc stable

Signed-off-by: Thomas Zimmermann <[email protected]>
Reviewed-by: Simona Vetter <[email protected]>
Acked-by: Christian König <[email protected]>
Acked-by: Zack Rusin <[email protected]>
Cc: <[email protected]> # v6.15+
Link: https://lore.kernel.org/r/[email protected]

Author: Thomas Zimmermann

drivers/gpu/drm/drm_prime.c

@@ -453,7 +453,13 @@ struct dma_buf *drm_gem_prime_handle_to_dmabuf(struct drm_device *dev,
 	}
 
 	mutex_lock(&dev->object_name_lock);
-	/* re-export the original imported/exported object */
+	/* re-export the original imported object */
+	if (obj->import_attach) {
+		dmabuf = obj->import_attach->dmabuf;
+		get_dma_buf(dmabuf);
+		goto out_have_obj;
+	}
+
 	if (obj->dma_buf) {
 		get_dma_buf(obj->dma_buf);
 		dmabuf = obj->dma_buf;