[PATCH drm] drm/gem: fix race between change_handle and handle_delete

[Zhenghang Xiao <kipreyyy@gmail.com>]

drm_gem_change_handle_ioctl leaves the old handle live in the IDR
during the window between spin_unlock(table_lock) and the final
spin_lock(table_lock). A concurrent drm_gem_handle_delete on the old
handle succeeds in this window, decrements handle_count to 0, and frees
the GEM object while the new handle's IDR entry still references it.

NULL the old handle's IDR entry before dropping table_lock so that any
concurrent GEM_CLOSE on the old handle sees NULL and returns -EINVAL.
Restore the old entry on the prime-bookkeeping error path.

Fixes: 5e28b7b94408 ("drm: Set old handle to NULL before prime swap in change_handle")
Signed-off-by: Zhenghang Xiao <kipreyyy@gmail.com>
---
 drivers/gpu/drm/drm_gem.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 8afab57fc055..189b990cc78e 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -1065,6 +1065,7 @@ int drm_gem_change_handle_ioctl(struct drm_device *dev, void *data,
 	       goto out_unlock;
        }
 
+	idr_replace(&file_priv->object_idr, NULL, args->handle);
 	spin_unlock(&file_priv->table_lock);
 
 	if (obj->dma_buf) {
@@ -1073,6 +1074,7 @@ int drm_gem_change_handle_ioctl(struct drm_device *dev, void *data,
 		if (ret < 0) {
 			spin_lock(&file_priv->table_lock);
 			idr_remove(&file_priv->object_idr, handle);
+			idr_replace(&file_priv->object_idr, obj, args->handle);
 			spin_unlock(&file_priv->table_lock);
 			goto out_unlock;
 		}
-- 
2.50.1 (Apple Git-155)