Forwarded: Don't allow device double-register

[syzbot <syzbot+bb9127e278fa198e110c@syzkaller.appspotmail.com>]

For archival purposes, forwarding an incoming command email to
linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com.

***

Subject: Don't allow device double-register
Author: fw@strlen.de

#syz test:  https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git main

diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
index fd7f7e4e2a43..b0fd94427a5a 100644
--- a/net/netfilter/nf_tables_api.c
+++ b/net/netfilter/nf_tables_api.c
@@ -9677,8 +9677,11 @@ static int nft_flowtable_event(unsigned long event, struct net_device *dev,
 			kfree_rcu(ops, rcu);
 			break;
 		case NETDEV_REGISTER:
+			if (ops)
+				pr_warn("reg %px for %s, again, hook ifname %s (%d) cn %d\n", ops, dev->name, hook->ifname, hook->ifnamelen, changename);
+
 			/* NOP if not matching or already registered */
-			if (!match || (changename && ops))
+			if (!match || ops)
 				continue;
 
 			ops = kzalloc_obj(struct nf_hook_ops,
@@ -9701,7 +9704,6 @@ static int nft_flowtable_event(unsigned long event, struct net_device *dev,
 			list_add_tail_rcu(&ops->list, &hook->ops_list);
 			break;
 		}
-		break;
 	}
 	return 0;
 }