net: wwan: mhi_wwan_mbim: fix dangling links on registration failure

mhi_mbim_newlink() adds the link to mbim->link_list before calling register_netdevice(). If registration fails, the caller frees the netdev without removing the link, leaving a dangling list entry that RCU readers such as mhi_mbim_rx() can access after free.

Register the netdev before adding the link to the list so that failed registrations do not publish a link.

Found while reviewing newlink error paths. A UML/KASAN test module using driver wrappers and a fake MHI context reproduced the use-after-free in the driver's RCU lookup. The fix passed the registration-failure and create/delete tests. Both UML builds passed with W=1.

---
 drivers/net/wwan/mhi_wwan_mbim.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

index 336f897..846b317 100644
@@ -561,6 +561,7 @@ static int mhi_mbim_newlink(void *ctxt, struct net_device *ndev, u32 if_id,
 {
 	struct mhi_mbim_link *link = wwan_netdev_drvpriv(ndev);
 	struct mhi_mbim_context *mbim = ctxt;
+	int ret;
 
 	link->mbim = mbim;
 	link->session = mhi_mbim_get_link_mux_id(link->mbim->mdev->mhi_cntrl) + if_id;

@@ -575,10 +576,14 @@ static int mhi_mbim_newlink(void *ctxt, struct net_device *ndev, u32 if_id,
 	}
 	rcu_read_unlock();
 
+	ret = register_netdevice(ndev);
+	if (ret)
+		return ret;
+
 	/* Already protected by RTNL lock */
 	hlist_add_head_rcu(&link->hlnode, &mbim->link_list[LINK_HASH(if_id)]);
 
-	return register_netdevice(ndev);
+	return 0;
 }
 
 static void mhi_mbim_dellink(void *ctxt, struct net_device *ndev,

base-commit: 8d5ce4751fad9499ea408163af7f410f18a85782