ipv6: check for ip6_null_entry in __ip6_del_rt_siblings()

Andrey reported a NULL pointer deref bug in ipv6_route_ioctl()
-> ip6_route_del() -> __ip6_del_rt_siblings() code path. This is
because ip6_null_entry is returned in this path since ip6_null_entry
is kinda default for a ipv6 route table root node. Quote from
David Ahern:

 ip6_null_entry is the root of all ipv6 fib tables making it integrated
 into the table ...

We should ignore any attempt of trying to delete it, like we do in
__ip6_del_rt() path and several others.
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Fixes: 0ae81335 ("net: ipv6: Allow shorthand delete of all nexthops in multipath route")
Cc: David Ahern <dsa@cumulusnetworks.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -2169,10 +2169,13 @@ int ip6_del_rt(struct rt6_info *rt)
 static int __ip6_del_rt_siblings(struct rt6_info *rt, struct fib6_config *cfg)
 {
 	struct nl_info *info = &cfg->fc_nlinfo;
+	struct net *net = info->nl_net;
 	struct sk_buff *skb = NULL;
 	struct fib6_table *table;
-	int err;
+	int err = -ENOENT;
 
+	if (rt == net->ipv6.ip6_null_entry)
+		goto out_put;
 	table = rt->rt6i_table;
 	write_lock_bh(&table->tb6_lock);
 
@@ -2184,7 +2187,7 @@ static int __ip6_del_rt_siblings(struct rt6_info *rt, struct fib6_config *cfg)
 		if (skb) {
 			u32 seq = info->nlh ? info->nlh->nlmsg_seq : 0;
 
-			if (rt6_fill_node(info->nl_net, skb, rt,
+			if (rt6_fill_node(net, skb, rt,
 					  NULL, NULL, 0, RTM_DELROUTE,
 					  info->portid, seq, 0) < 0) {
 				kfree_skb(skb);
@@ -2198,17 +2201,18 @@ static int __ip6_del_rt_siblings(struct rt6_info *rt, struct fib6_config *cfg)
 					 rt6i_siblings) {
 			err = fib6_del(sibling, info);
 			if (err)
-				goto out;
+				goto out_unlock;
 		}
 	}
 
 	err = fib6_del(rt, info);
-out:
+out_unlock:
 	write_unlock_bh(&table->tb6_lock);
+out_put:
 	ip6_rt_put(rt);
 
 	if (skb) {
-		rtnl_notify(skb, info->nl_net, info->portid, RTNLGRP_IPV6_ROUTE,
+		rtnl_notify(skb, net, info->portid, RTNLGRP_IPV6_ROUTE,
 			    info->nlh, gfp_any());
 	}
 	return err;