ipv6: Validate RTA_GATEWAY of RTA_MULTIPATH in rtm_to_fib6_config().

We will perform RTM_NEWROUTE and RTM_DELROUTE under RCU, and then
we want to perform some validation out of the RCU scope.

When creating / removing an IPv6 route with RTA_MULTIPATH,
inet6_rtm_newroute() / inet6_rtm_delroute() validates RTA_GATEWAY
in each multipath entry.

Let's do that in rtm_to_fib6_config().

Note that now RTM_DELROUTE returns an error for RTA_MULTIPATH with
0 entries, which was accepted but should result in -EINVAL as
RTM_NEWROUTE.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250418000443.43734-2-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: q2ven

net/ipv6/route.c

@@ -5051,6 +5051,44 @@ static const struct nla_policy rtm_ipv6_policy[RTA_MAX+1] = {
 	[RTA_FLOWLABEL]		= { .type = NLA_BE32 },
 };
 
+static int rtm_to_fib6_multipath_config(struct fib6_config *cfg,
+					struct netlink_ext_ack *extack)
+{
+	struct rtnexthop *rtnh;
+	int remaining;
+
+	remaining = cfg->fc_mp_len;
+	rtnh = (struct rtnexthop *)cfg->fc_mp;
+
+	if (!rtnh_ok(rtnh, remaining)) {
+		NL_SET_ERR_MSG(extack, "Invalid nexthop configuration - no valid nexthops");
+		return -EINVAL;
+	}
+
+	do {
+		int attrlen = rtnh_attrlen(rtnh);
+
+		if (attrlen > 0) {
+			struct nlattr *nla, *attrs;
+
+			attrs = rtnh_attrs(rtnh);
+			nla = nla_find(attrs, attrlen, RTA_GATEWAY);
+			if (nla) {
+				if (nla_len(nla) < sizeof(cfg->fc_gateway)) {
+					NL_SET_ERR_MSG(extack,
+						       "Invalid IPv6 address in RTA_GATEWAY");
+					return -EINVAL;
+				}
+			}
+		}
+
+		rtnh = rtnh_next(rtnh, &remaining);
+	} while (rtnh_ok(rtnh, remaining));
+
+	return lwtunnel_valid_encap_type_attr(cfg->fc_mp, cfg->fc_mp_len,
+					      extack, true);
+}
+
 static int rtm_to_fib6_config(struct sk_buff *skb, struct nlmsghdr *nlh,
 			      struct fib6_config *cfg,
 			      struct netlink_ext_ack *extack)
@@ -5165,9 +5203,7 @@ static int rtm_to_fib6_config(struct sk_buff *skb, struct nlmsghdr *nlh,
 		cfg->fc_mp = nla_data(tb[RTA_MULTIPATH]);
 		cfg->fc_mp_len = nla_len(tb[RTA_MULTIPATH]);
 
-		err = lwtunnel_valid_encap_type_attr(cfg->fc_mp,
-						     cfg->fc_mp_len,
-						     extack, true);
+		err = rtm_to_fib6_multipath_config(cfg, extack);
 		if (err < 0)
 			goto errout;
 	}
@@ -5287,19 +5323,6 @@ static bool ip6_route_mpath_should_notify(const struct fib6_info *rt)
 	return should_notify;
 }
 
-static int fib6_gw_from_attr(struct in6_addr *gw, struct nlattr *nla,
-			     struct netlink_ext_ack *extack)
-{
-	if (nla_len(nla) < sizeof(*gw)) {
-		NL_SET_ERR_MSG(extack, "Invalid IPv6 address in RTA_GATEWAY");
-		return -EINVAL;
-	}
-
-	*gw = nla_get_in6_addr(nla);
-
-	return 0;
-}
-
 static int ip6_route_multipath_add(struct fib6_config *cfg,
 				   struct netlink_ext_ack *extack)
 {
@@ -5340,18 +5363,11 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
 
 			nla = nla_find(attrs, attrlen, RTA_GATEWAY);
 			if (nla) {
-				err = fib6_gw_from_attr(&r_cfg.fc_gateway, nla,
-							extack);
-				if (err)
-					goto cleanup;
-
+				r_cfg.fc_gateway = nla_get_in6_addr(nla);
 				r_cfg.fc_flags |= RTF_GATEWAY;
 			}
-			r_cfg.fc_encap = nla_find(attrs, attrlen, RTA_ENCAP);
 
-			/* RTA_ENCAP_TYPE length checked in
-			 * lwtunnel_valid_encap_type_attr
-			 */
+			r_cfg.fc_encap = nla_find(attrs, attrlen, RTA_ENCAP);
 			nla = nla_find(attrs, attrlen, RTA_ENCAP_TYPE);
 			if (nla)
 				r_cfg.fc_encap_type = nla_get_u16(nla);
@@ -5384,12 +5400,6 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
 		rtnh = rtnh_next(rtnh, &remaining);
 	}
 
-	if (list_empty(&rt6_nh_list)) {
-		NL_SET_ERR_MSG(extack,
-			       "Invalid nexthop configuration - no valid nexthops");
-		return -EINVAL;
-	}
-
 	/* for add and replace send one notification with all nexthops.
 	 * Skip the notification in fib6_add_rt2node and send one with
 	 * the full route when done
@@ -5511,21 +5521,15 @@ static int ip6_route_multipath_del(struct fib6_config *cfg,
 
 			nla = nla_find(attrs, attrlen, RTA_GATEWAY);
 			if (nla) {
-				err = fib6_gw_from_attr(&r_cfg.fc_gateway, nla,
-							extack);
-				if (err) {
-					last_err = err;
-					goto next_rtnh;
-				}
-
+				r_cfg.fc_gateway = nla_get_in6_addr(nla);
 				r_cfg.fc_flags |= RTF_GATEWAY;
 			}
 		}
+
 		err = ip6_route_del(&r_cfg, extack);
 		if (err)
 			last_err = err;
 
-next_rtnh:
 		rtnh = rtnh_next(rtnh, &remaining);
 	}