ip6_gre: fix null-ptr-deref in ip6gre_init_net()

KASAN report null-ptr-deref error when register_netdev() failed:

KASAN: null-ptr-deref in range [0x00000000000003c0-0x00000000000003c7]
CPU: 2 PID: 422 Comm: ip Not tainted 5.8.0-rc4+ MiCode#12
Call Trace:
 ip6gre_init_net+0x4ab/0x580
 ? ip6gre_tunnel_uninit+0x3f0/0x3f0
 ops_init+0xa8/0x3c0
 setup_net+0x2de/0x7e0
 ? rcu_read_lock_bh_held+0xb0/0xb0
 ? ops_init+0x3c0/0x3c0
 ? kasan_unpoison_shadow+0x33/0x40
 ? __kasan_kmalloc.constprop.0+0xc2/0xd0
 copy_net_ns+0x27d/0x530
 create_new_namespaces+0x382/0xa30
 unshare_nsproxy_namespaces+0xa1/0x1d0
 ksys_unshare+0x39c/0x780
 ? walk_process_tree+0x2a0/0x2a0
 ? trace_hardirqs_on+0x4a/0x1b0
 ? _raw_spin_unlock_irq+0x1f/0x30
 ? syscall_trace_enter+0x1a7/0x330
 ? do_syscall_64+0x1c/0xa0
 __x64_sys_unshare+0x2d/0x40
 do_syscall_64+0x56/0xa0
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

ip6gre_tunnel_uninit() has set 'ign->fb_tunnel_dev' to NULL, later
access to ign->fb_tunnel_dev cause null-ptr-deref. Fix it by saving
'ign->fb_tunnel_dev' to local variable ndev.

Fixes: dafabb6 ("ip6_gre: fix use-after-free in ip6gre_tunnel_lookup()")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -1562,17 +1562,18 @@ static void ip6gre_destroy_tunnels(struct net *net, struct list_head *head)
 static int __net_init ip6gre_init_net(struct net *net)
 {
 	struct ip6gre_net *ign = net_generic(net, ip6gre_net_id);
+	struct net_device *ndev;
 	int err;
 
 	if (!net_has_fallback_tunnels(net))
 		return 0;
-	ign->fb_tunnel_dev = alloc_netdev(sizeof(struct ip6_tnl), "ip6gre0",
-					  NET_NAME_UNKNOWN,
-					  ip6gre_tunnel_setup);
-	if (!ign->fb_tunnel_dev) {
+	ndev = alloc_netdev(sizeof(struct ip6_tnl), "ip6gre0",
+			    NET_NAME_UNKNOWN, ip6gre_tunnel_setup);
+	if (!ndev) {
 		err = -ENOMEM;
 		goto err_alloc_dev;
 	}
+	ign->fb_tunnel_dev = ndev;
 	dev_net_set(ign->fb_tunnel_dev, net);
 	/* FB netdevice is special: we have one, and only one per netns.
 	 * Allowing to move it to another netns is clearly unsafe.
@@ -1592,7 +1593,7 @@ static int __net_init ip6gre_init_net(struct net *net)
 	return 0;
 
 err_reg_dev:
-	free_netdev(ign->fb_tunnel_dev);
+	free_netdev(ndev);
 err_alloc_dev:
 	return err;
 }