xdp: fix potential deadlock on socket mutex

There are 2 call chains:

  a) xsk_bind --> xdp_umem_assign_dev
  b) unregister_netdevice_queue --> xsk_notifier

with the following locking order:

  a) xs->mutex --> rtnl_lock
  b) rtnl_lock --> xdp.lock --> xs->mutex

Different order of taking 'xs->mutex' and 'rtnl_lock' could produce a
deadlock here. Fix that by moving the 'rtnl_lock' before 'xs->lock' in
the bind call chain (a).

Reported-by: syzbot+bf64ec93de836d7f4c2c@syzkaller.appspotmail.com
Fixes: 455302d1c9 ("xdp: fix hang while unregistering device bound to xdp socket")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/xdp/xdp_umem.c

@@ -87,21 +87,20 @@ int xdp_umem_assign_dev(struct xdp_umem *umem, struct net_device *dev,
 	struct netdev_bpf bpf;
 	int err = 0;
 
+	ASSERT_RTNL();
+
 	force_zc = flags & XDP_ZEROCOPY;
 	force_copy = flags & XDP_COPY;
 
 	if (force_zc && force_copy)
 		return -EINVAL;
 
-	rtnl_lock();
-	if (xdp_get_umem_from_qid(dev, queue_id)) {
-		err = -EBUSY;
-		goto out_rtnl_unlock;
-	}
+	if (xdp_get_umem_from_qid(dev, queue_id))
+		return -EBUSY;
 
 	err = xdp_reg_umem_at_qid(dev, umem, queue_id);
 	if (err)
-		goto out_rtnl_unlock;
+		return err;
 
 	umem->dev = dev;
 	umem->queue_id = queue_id;
@@ -110,7 +109,7 @@ int xdp_umem_assign_dev(struct xdp_umem *umem, struct net_device *dev,
 
 	if (force_copy)
 		/* For copy-mode, we are done. */
-		goto out_rtnl_unlock;
+		return 0;
 
 	if (!dev->netdev_ops->ndo_bpf ||
 	    !dev->netdev_ops->ndo_xsk_async_xmit) {
@@ -125,7 +124,6 @@ int xdp_umem_assign_dev(struct xdp_umem *umem, struct net_device *dev,
 	err = dev->netdev_ops->ndo_bpf(dev, &bpf);
 	if (err)
 		goto err_unreg_umem;
-	rtnl_unlock();
 
 	umem->zc = true;
 	return 0;
@@ -135,8 +133,6 @@ err_unreg_umem:
 		err = 0; /* fallback to copy mode */
 	if (err)
 		xdp_clear_umem_at_qid(dev, queue_id);
-out_rtnl_unlock:
-	rtnl_unlock();
 	return err;
 }
 

net/xdp/xsk.c

@@ -430,6 +430,7 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
 	if (flags & ~(XDP_SHARED_UMEM | XDP_COPY | XDP_ZEROCOPY))
 		return -EINVAL;
 
+	rtnl_lock();
 	mutex_lock(&xs->mutex);
 	if (xs->state != XSK_READY) {
 		err = -EBUSY;
@@ -515,6 +516,7 @@ out_unlock:
 		xs->state = XSK_BOUND;
 out_release:
 	mutex_unlock(&xs->mutex);
+	rtnl_unlock();
 	return err;
 }