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

JIRA: https://issues.redhat.com/browse/RHEL-106712

commit fb4ef4a
Author: Thomas Zimmermann <tzimmermann@suse.de>
Date:   Tue Jul 15 17:58:14 2025 +0200

    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 <tzimmermann@suse.de>
    Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
    Acked-by: Christian König <christian.koenig@amd.com>
    Acked-by: Zack Rusin <zack.rusin@broadcom.com>
    Cc: <stable@vger.kernel.org> # v6.15+
    Link: https://lore.kernel.org/r/20250715155934.150656-5-tzimmermann@suse.de

Signed-off-by: José Expósito <jexposit@redhat.com>

Author: jexposit

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;