ipv6: Add lwtunnel encap size of all siblings in nexthop calculation

In function rt6_nlmsg_size(), the length of nexthop is calculated
by multipling the nexthop length of fib6_info and the number of
siblings. However if the fib6_info has no lwtunnel but the siblings
have lwtunnels, the nexthop length is less than it should be, and
it will trigger a warning in inet6_rt_notify() as follows:

WARNING: CPU: 0 PID: 6082 at net/ipv6/route.c:6180 inet6_rt_notify+0x120/0x130
......
Call Trace:
 <TASK>
 fib6_add_rt2node+0x685/0xa30
 fib6_add+0x96/0x1b0
 ip6_route_add+0x50/0xd0
 inet6_rtm_newroute+0x97/0xa0
 rtnetlink_rcv_msg+0x156/0x3d0
 netlink_rcv_skb+0x5a/0x110
 netlink_unicast+0x246/0x350
 netlink_sendmsg+0x250/0x4c0
 sock_sendmsg+0x66/0x70
 ___sys_sendmsg+0x7c/0xd0
 __sys_sendmsg+0x5d/0xb0
 do_syscall_64+0x3f/0x90
 entry_SYSCALL_64_after_hwframe+0x72/0xdc

This bug can be reproduced by script:

ip -6 addr add 2002::2/64 dev ens2
ip -6 route add 100::/64 via 2002::1 dev ens2 metric 100

for i in 10 20 30 40 50 60 70;
do
	ip link add link ens2 name ipv_$i type ipvlan
	ip -6 addr add 2002::$i/64 dev ipv_$i
	ifconfig ipv_$i up
done

for i in 10 20 30 40 50 60;
do
	ip -6 route append 100::/64 encap ip6 dst 2002::$i via 2002::1
dev ipv_$i metric 100
done

ip -6 route append 100::/64 via 2002::1 dev ipv_70 metric 100

This patch fixes it by adding nexthop_len of every siblings using
rt6_nh_nlmsg_size().

Fixes: beb1afac ("net: ipv6: Add support to dump multipath routes via RTA_MULTIPATH attribute")
Signed-off-by: Lu Wei <luwei32@huawei.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230222083629.335683-2-luwei32@huawei.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv6/route.c

@@ -5533,16 +5533,17 @@ static size_t rt6_nlmsg_size(struct fib6_info *f6i)
 		nexthop_for_each_fib6_nh(f6i->nh, rt6_nh_nlmsg_size,
 					 &nexthop_len);
 	} else {
+		struct fib6_info *sibling, *next_sibling;
 		struct fib6_nh *nh = f6i->fib6_nh;
 
 		nexthop_len = 0;
 		if (f6i->fib6_nsiblings) {
-			nexthop_len = nla_total_size(0)	 /* RTA_MULTIPATH */
-				    + NLA_ALIGN(sizeof(struct rtnexthop))
-				    + nla_total_size(16) /* RTA_GATEWAY */
-				    + lwtunnel_get_encap_size(nh->fib_nh_lws);
+			rt6_nh_nlmsg_size(nh, &nexthop_len);
 
-			nexthop_len *= f6i->fib6_nsiblings;
+			list_for_each_entry_safe(sibling, next_sibling,
+						 &f6i->fib6_siblings, fib6_siblings) {
+				rt6_nh_nlmsg_size(sibling->fib6_nh, &nexthop_len);
+			}
 		}
 		nexthop_len += lwtunnel_get_encap_size(nh->fib_nh_lws);
 	}