neighbour: Move neigh_find_table() to neigh_get().

neigh_valid_get_req() calls neigh_find_table() to fetch neigh_tables[].

neigh_find_table() uses rcu_dereference_rtnl(), but RTNL actually does
not protect it at all; neigh_table_clear() can be called without RTNL
and only waits for RCU readers by synchronize_rcu().

Fortunately, there is no bug because IPv4 is built-in, IPv6 cannot be
unloaded, and DECNET was removed.

To fetch neigh_tables[] by rcu_dereference() later, let's move
neigh_find_table() from neigh_valid_get_req() to neigh_get().

Signed-off-by: Kuniyuki Iwashima <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: q2ven

net/core/neighbour.c

@@ -2911,10 +2911,9 @@ static int neigh_dump_info(struct sk_buff *skb, struct netlink_callback *cb)
 }
 
 static struct ndmsg *neigh_valid_get_req(const struct nlmsghdr *nlh,
-					 struct neigh_table **tbl, void **dst,
+					 struct nlattr **tb,
 					 struct netlink_ext_ack *extack)
 {
-	struct nlattr *tb[NDA_MAX + 1];
 	struct ndmsg *ndm;
 	int err, i;
 
@@ -2945,25 +2944,13 @@ static struct ndmsg *neigh_valid_get_req(const struct nlmsghdr *nlh,
 	if (err < 0)
 		return ERR_PTR(err);
 
-	*tbl = neigh_find_table(ndm->ndm_family);
-	if (!*tbl) {
-		NL_SET_ERR_MSG(extack, "Unsupported family in header for neighbor get request");
-		return ERR_PTR(-EAFNOSUPPORT);
-	}
-
 	for (i = 0; i <= NDA_MAX; ++i) {
 		switch (i) {
 		case NDA_DST:
 			if (!tb[i]) {
 				NL_SET_ERR_ATTR_MISS(extack, NULL, NDA_DST);
 				return ERR_PTR(-EINVAL);
 			}
-
-			if (nla_len(tb[i]) != (int)(*tbl)->key_len) {
-				NL_SET_ERR_MSG(extack, "Invalid network address in neighbor get request");
-				return ERR_PTR(-EINVAL);
-			}
-			*dst = nla_data(tb[i]);
 			break;
 		default:
 			if (!tb[i])
@@ -3001,16 +2988,17 @@ static int neigh_get(struct sk_buff *in_skb, struct nlmsghdr *nlh,
 {
 	struct net *net = sock_net(in_skb->sk);
 	u32 pid = NETLINK_CB(in_skb).portid;
+	struct nlattr *tb[NDA_MAX + 1];
 	struct net_device *dev = NULL;
-	struct neigh_table *tbl = NULL;
 	u32 seq = nlh->nlmsg_seq;
+	struct neigh_table *tbl;
 	struct neighbour *neigh;
 	struct sk_buff *skb;
 	struct ndmsg *ndm;
-	void *dst = NULL;
+	void *dst;
 	int err;
 
-	ndm = neigh_valid_get_req(nlh, &tbl, &dst, extack);
+	ndm = neigh_valid_get_req(nlh, tb, extack);
 	if (IS_ERR(ndm))
 		return PTR_ERR(ndm);
 
@@ -3021,6 +3009,21 @@ static int neigh_get(struct sk_buff *in_skb, struct nlmsghdr *nlh,
 	if (!skb)
 		return -ENOBUFS;
 
+	tbl = neigh_find_table(ndm->ndm_family);
+	if (!tbl) {
+		NL_SET_ERR_MSG(extack, "Unsupported family in header for neighbor get request");
+		err = -EAFNOSUPPORT;
+		goto err_free_skb;
+	}
+
+	if (nla_len(tb[NDA_DST]) != (int)tbl->key_len) {
+		NL_SET_ERR_MSG(extack, "Invalid network address in neighbor get request");
+		err = -EINVAL;
+		goto err_free_skb;
+	}
+
+	dst = nla_data(tb[NDA_DST]);
+
 	if (ndm->ndm_ifindex) {
 		dev = __dev_get_by_index(net, ndm->ndm_ifindex);
 		if (!dev) {