Merge: CVE-2021-47459: can: j1939: j1939_netdev_start(): fix UAF for rx_kref of j1939_priv

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9/-/merge_requests/4399

```
JIRA: https://issues.redhat.com/browse/RHEL-38415
CVE: CVE-2021-47459

Commits:
    can: j1939: j1939_netdev_start(): fix UAF for rx_kref of j1939_priv

Signed-off-by: Ken Cox <jkc@redhat.com>
```

Approved-by: Radu Rendec <rrendec@redhat.com>
Approved-by: Herton R. Krzesinski <herton@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: Lucas Zampieri <lzampier@redhat.com>

Author: zampierilucas

net/can/j1939/main.c

@@ -253,11 +253,14 @@ struct j1939_priv *j1939_netdev_start(struct net_device *ndev)
 	struct j1939_priv *priv, *priv_new;
 	int ret;
 
-	priv = j1939_priv_get_by_ndev(ndev);
+	spin_lock(&j1939_netdev_lock);
+	priv = j1939_priv_get_by_ndev_locked(ndev);
 	if (priv) {
 		kref_get(&priv->rx_kref);
+		spin_unlock(&j1939_netdev_lock);
 		return priv;
 	}
+	spin_unlock(&j1939_netdev_lock);
 
 	priv = j1939_priv_create(ndev);
 	if (!priv)
@@ -273,10 +276,10 @@ struct j1939_priv *j1939_netdev_start(struct net_device *ndev)
 		/* Someone was faster than us, use their priv and roll
 		 * back our's.
 		 */
+		kref_get(&priv_new->rx_kref);
 		spin_unlock(&j1939_netdev_lock);
 		dev_put(ndev);
 		kfree(priv);
-		kref_get(&priv_new->rx_kref);
 		return priv_new;
 	}
 	j1939_priv_set(ndev, priv);