gtp: fix an use-before-init in gtp_newlink()

*_pdp_find() from gtp_encap_recv() would trigger a crash when a peer
sends GTP packets while creating new GTP device.

RIP: 0010:gtp1_pdp_find.isra.0+0x68/0x90 [gtp]
<SNIP>
Call Trace:
 <IRQ>
 gtp_encap_recv+0xc2/0x2e0 [gtp]
 ? gtp1_pdp_find.isra.0+0x90/0x90 [gtp]
 udp_queue_rcv_one_skb+0x1fe/0x530
 udp_queue_rcv_skb+0x40/0x1b0
 udp_unicast_rcv_skb.isra.0+0x78/0x90
 __udp4_lib_rcv+0x5af/0xc70
 udp_rcv+0x1a/0x20
 ip_protocol_deliver_rcu+0xc5/0x1b0
 ip_local_deliver_finish+0x48/0x50
 ip_local_deliver+0xe5/0xf0
 ? ip_protocol_deliver_rcu+0x1b0/0x1b0

gtp_encap_enable() should be called after gtp_hastable_new() otherwise
*_pdp_find() will access the uninitialized hash table.

Fixes: 1e3a3abd ("gtp: make GTP sockets in gtp_newlink optional")
Signed-off-by: Masahiro Fujiwara <fujiwara.masahiro@gmail.com>
Link: https://lore.kernel.org/r/20201027114846.3924-1-fujiwara.masahiro@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/gtp.c

@@ -657,10 +657,6 @@ static int gtp_newlink(struct net *src_net, struct net_device *dev,
 
 	gtp = netdev_priv(dev);
 
-	err = gtp_encap_enable(gtp, data);
-	if (err < 0)
-		return err;
-
 	if (!data[IFLA_GTP_PDP_HASHSIZE]) {
 		hashsize = 1024;
 	} else {
@@ -671,12 +667,16 @@ static int gtp_newlink(struct net *src_net, struct net_device *dev,
 
 	err = gtp_hashtable_new(gtp, hashsize);
 	if (err < 0)
-		goto out_encap;
+		return err;
+
+	err = gtp_encap_enable(gtp, data);
+	if (err < 0)
+		goto out_hashtable;
 
 	err = register_netdevice(dev);
 	if (err < 0) {
 		netdev_dbg(dev, "failed to register new netdev %d\n", err);
-		goto out_hashtable;
+		goto out_encap;
 	}
 
 	gn = net_generic(dev_net(dev), gtp_net_id);
@@ -687,11 +687,11 @@ static int gtp_newlink(struct net *src_net, struct net_device *dev,
 
 	return 0;
 
+out_encap:
+	gtp_encap_disable(gtp);
 out_hashtable:
 	kfree(gtp->addr_hash);
 	kfree(gtp->tid_hash);
-out_encap:
-	gtp_encap_disable(gtp);
 	return err;
 }