[PATCH v2 1/7] Revert "drm/virtio: Use dma_buf from GEM object instance"

[Thomas Zimmermann]

This reverts commit 415cb45895f43015515473fbc40563ca5eec9a7c.

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 5307dce878d4 ("drm/gem: Acquire references on
GEM handles for framebuffers") and commit f6bfc9afc751 ("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 <tzimmerm...@suse.de>
Reviewed-by: Simona Vetter <simona.vet...@ffwll.ch>
---
 drivers/gpu/drm/virtio/virtgpu_prime.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_prime.c 
b/drivers/gpu/drm/virtio/virtgpu_prime.c
index 722cde5e2d86..97aaee26cb02 100644
--- a/drivers/gpu/drm/virtio/virtgpu_prime.c
+++ b/drivers/gpu/drm/virtio/virtgpu_prime.c
@@ -204,15 +204,16 @@ static void virtgpu_dma_buf_free_obj(struct 
drm_gem_object *obj)
 {
        struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(obj);
        struct virtio_gpu_device *vgdev = obj->dev->dev_private;
+       struct dma_buf_attachment *attach = obj->import_attach;
 
        if (drm_gem_is_imported(obj)) {
-               struct dma_buf *dmabuf = bo->dma_buf;
+               struct dma_buf *dmabuf = attach->dmabuf;
 
                dma_resv_lock(dmabuf->resv, NULL);
                virtgpu_dma_buf_unmap(bo);
                dma_resv_unlock(dmabuf->resv);
 
-               dma_buf_detach(dmabuf, obj->import_attach);
+               dma_buf_detach(dmabuf, attach);
                dma_buf_put(dmabuf);
        }
 
-- 
2.50.0