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 <redacted>
Signed-off-by: Cong Wang <redacted>
---
 net/ipv6/route.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index f54f426..9da77e9 100644
--- a/net/ipv6/route.c
+++ b/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;
 
+	if (rt == net->ipv6.ip6_null_entry)
+		return -ENOENT;