ipv6: Convert inet6_rtm_newaddr() to per-netns RTNL.

Let's register inet6_rtm_newaddr() with RTNL_FLAG_DOIT_PERNET
and hold rtnl_net_lock() before __dev_get_by_index().

Now that inet6_addr_add() and inet6_addr_modify() are always
called under per-netns RTNL.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250115080608.28127-11-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: q2ven

net/ipv6/addrconf.c

@@ -3014,7 +3014,7 @@ static int inet6_addr_add(struct net *net, struct net_device *dev,
 	struct inet6_ifaddr *ifp;
 	struct inet6_dev *idev;
 
-	ASSERT_RTNL();
+	ASSERT_RTNL_NET(net);
 
 	if (cfg->plen > 128) {
 		NL_SET_ERR_MSG_MOD(extack, "Invalid prefix length");
@@ -4849,7 +4849,7 @@ static int inet6_addr_modify(struct net *net, struct inet6_ifaddr *ifp,
 	bool new_peer = false;
 	bool had_prefixroute;
 
-	ASSERT_RTNL();
+	ASSERT_RTNL_NET(net);
 
 	if (cfg->ifa_flags & IFA_F_MANAGETEMPADDR &&
 	    (ifp->flags & IFA_F_TEMPORARY || ifp->prefix_len != 64))
@@ -5016,23 +5016,29 @@ inet6_rtm_newaddr(struct sk_buff *skb, struct nlmsghdr *nlh,
 		}
 	}
 
+	rtnl_net_lock(net);
+
 	dev =  __dev_get_by_index(net, ifm->ifa_index);
 	if (!dev) {
 		NL_SET_ERR_MSG_MOD(extack, "Unable to find the interface");
-		return -ENODEV;
+		err = -ENODEV;
+		goto unlock;
 	}
 
 	idev = ipv6_find_idev(dev);
-	if (IS_ERR(idev))
-		return PTR_ERR(idev);
+	if (IS_ERR(idev)) {
+		err = PTR_ERR(idev);
+		goto unlock;
+	}
 
 	if (!ipv6_allow_optimistic_dad(net, idev))
 		cfg.ifa_flags &= ~IFA_F_OPTIMISTIC;
 
 	if (cfg.ifa_flags & IFA_F_NODAD &&
 	    cfg.ifa_flags & IFA_F_OPTIMISTIC) {
 		NL_SET_ERR_MSG(extack, "IFA_F_NODAD and IFA_F_OPTIMISTIC are mutually exclusive");
-		return -EINVAL;
+		err = -EINVAL;
+		goto unlock;
 	}
 
 	ifa = ipv6_get_ifaddr(net, cfg.pfx, dev, 1);
@@ -5041,7 +5047,8 @@ inet6_rtm_newaddr(struct sk_buff *skb, struct nlmsghdr *nlh,
 		 * It would be best to check for !NLM_F_CREATE here but
 		 * userspace already relies on not having to provide this.
 		 */
-		return inet6_addr_add(net, dev, &cfg, expires, flags, extack);
+		err = inet6_addr_add(net, dev, &cfg, expires, flags, extack);
+		goto unlock;
 	}
 
 	if (nlh->nlmsg_flags & NLM_F_EXCL ||
@@ -5053,6 +5060,8 @@ inet6_rtm_newaddr(struct sk_buff *skb, struct nlmsghdr *nlh,
 	}
 
 	in6_ifa_put(ifa);
+unlock:
+	rtnl_net_unlock(net);
 
 	return err;
 }
@@ -7393,7 +7402,7 @@ static const struct rtnl_msg_handler addrconf_rtnl_msg_handlers[] __initconst_or
 	{.owner = THIS_MODULE, .protocol = PF_INET6, .msgtype = RTM_GETLINK,
 	 .dumpit = inet6_dump_ifinfo, .flags = RTNL_FLAG_DUMP_UNLOCKED},
 	{.owner = THIS_MODULE, .protocol = PF_INET6, .msgtype = RTM_NEWADDR,
-	 .doit = inet6_rtm_newaddr},
+	 .doit = inet6_rtm_newaddr, .flags = RTNL_FLAG_DOIT_PERNET},
 	{.owner = THIS_MODULE, .protocol = PF_INET6, .msgtype = RTM_DELADDR,
 	 .doit = inet6_rtm_deladdr},
 	{.owner = THIS_MODULE, .protocol = PF_INET6, .msgtype = RTM_GETADDR,