ipv6: fix a potential deadlock in do_ipv6_setsockopt()

Baozeng reported this deadlock case:

       CPU0                    CPU1
       ----                    ----
  lock([  165.136033] sk_lock-AF_INET6);
                               lock([  165.136033] rtnl_mutex);
                               lock([  165.136033] sk_lock-AF_INET6);
  lock([  165.136033] rtnl_mutex);

Similar to commit 87e9f03159
("ipv4: fix a potential deadlock in mcast getsockopt() path")
this is due to we still have a case, ipv6_sock_mc_close(),
where we acquire sk_lock before rtnl_lock. Close this deadlock
with the similar solution, that is always acquire rtnl lock first.

Fixes: baf606d9c9 ("ipv4,ipv6: grab rtnl before locking the socket")
Reported-by: Baozeng Ding <sploving1@gmail.com>
Tested-by: Baozeng Ding <sploving1@gmail.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/addrconf.h

@@ -174,6 +174,7 @@ int ipv6_sock_mc_join(struct sock *sk, int ifindex,
 		      const struct in6_addr *addr);
 int ipv6_sock_mc_drop(struct sock *sk, int ifindex,
 		      const struct in6_addr *addr);
+void __ipv6_sock_mc_close(struct sock *sk);
 void ipv6_sock_mc_close(struct sock *sk);
 bool inet6_mc_check(struct sock *sk, const struct in6_addr *mc_addr,
 		    const struct in6_addr *src_addr);

net/ipv6/ipv6_sockglue.c

@@ -120,6 +120,7 @@ struct ipv6_txoptions *ipv6_update_options(struct sock *sk,
 static bool setsockopt_needs_rtnl(int optname)
 {
 	switch (optname) {
+	case IPV6_ADDRFORM:
 	case IPV6_ADD_MEMBERSHIP:
 	case IPV6_DROP_MEMBERSHIP:
 	case IPV6_JOIN_ANYCAST:
@@ -198,7 +199,7 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 			}
 
 			fl6_free_socklist(sk);
-			ipv6_sock_mc_close(sk);
+			__ipv6_sock_mc_close(sk);
 
 			/*
 			 * Sock is moving from IPv6 to IPv4 (sk_prot), so

net/ipv6/mcast.c

@@ -276,16 +276,14 @@ static struct inet6_dev *ip6_mc_find_dev_rcu(struct net *net,
 	return idev;
 }
 
-void ipv6_sock_mc_close(struct sock *sk)
+void __ipv6_sock_mc_close(struct sock *sk)
 {
 	struct ipv6_pinfo *np = inet6_sk(sk);
 	struct ipv6_mc_socklist *mc_lst;
 	struct net *net = sock_net(sk);
 
-	if (!rcu_access_pointer(np->ipv6_mc_list))
-		return;
+	ASSERT_RTNL();
 
-	rtnl_lock();
 	while ((mc_lst = rtnl_dereference(np->ipv6_mc_list)) != NULL) {
 		struct net_device *dev;
 
@@ -303,8 +301,17 @@ void ipv6_sock_mc_close(struct sock *sk)
 
 		atomic_sub(sizeof(*mc_lst), &sk->sk_omem_alloc);
 		kfree_rcu(mc_lst, rcu);
-
 	}
+}
+
+void ipv6_sock_mc_close(struct sock *sk)
+{
+	struct ipv6_pinfo *np = inet6_sk(sk);
+
+	if (!rcu_access_pointer(np->ipv6_mc_list))
+		return;
+	rtnl_lock();
+	__ipv6_sock_mc_close(sk);
 	rtnl_unlock();
 }