udp: Call inet6_destroy_sock() in setsockopt(IPV6_ADDRFORM).

Commit 4b340ae20d0e ("IPv6: Complete IPV6_DONTFRAG support") forgot
to add a change to free inet6_sk(sk)->rxpmtu while converting an IPv6
socket into IPv4 with IPV6_ADDRFORM. After conversion, sk_prot is
changed to udp_prot and ->destroy() never cleans it up, resulting in
a memory leak.

This is due to the discrepancy between inet6_destroy_sock() and
IPV6_ADDRFORM, so let's call inet6_destroy_sock() from IPV6_ADDRFORM
to remove the difference.

However, this is not enough for now because rxpmtu can be changed
without lock_sock() after commit 03485f2adcde ("udpv6: Add lockless
sendmsg() support"). We will fix this case in the following patch.

Note we will rename inet6_destroy_sock() to inet6_cleanup_sock() and
remove unnecessary inet6_destroy_sock() calls in sk_prot->destroy()
in the future.

Fixes: 4b340ae20d0e ("IPv6: Complete IPV6_DONTFRAG support")
Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/ipv6.h

···
 void ipv6_local_error(struct sock *sk, int err, struct flowi6 *fl6, u32 info);
 void ipv6_local_rxpmtu(struct sock *sk, struct flowi6 *fl6, u32 mtu);
 
+void inet6_cleanup_sock(struct sock *sk);
 int inet6_release(struct socket *sock);
 int inet6_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len);
 int inet6_getname(struct socket *sock, struct sockaddr *uaddr,

net/ipv6/af_inet6.c

···
 }
 EXPORT_SYMBOL_GPL(inet6_destroy_sock);
 
+void inet6_cleanup_sock(struct sock *sk)
+{
+	inet6_destroy_sock(sk);
+}
+EXPORT_SYMBOL_GPL(inet6_cleanup_sock);
+
 /*
  *	This does both peername and sockname.
  */

net/ipv6/ipv6_sockglue.c

···
 		if (optlen < sizeof(int))
 			goto e_inval;
 		if (val == PF_INET) {
-			struct ipv6_txoptions *opt;
-			struct sk_buff *pktopt;
-
 			if (sk->sk_type == SOCK_RAW)
 				break;
 
···
 				break;
 			}
 
-			fl6_free_socklist(sk);
 			__ipv6_sock_mc_close(sk);
 			__ipv6_sock_ac_close(sk);
 
···
 				sk->sk_socket->ops = &inet_dgram_ops;
 				sk->sk_family = PF_INET;
 			}
-			opt = xchg((__force struct ipv6_txoptions **)&np->opt,
-				   NULL);
-			if (opt) {
-				atomic_sub(opt->tot_len, &sk->sk_omem_alloc);
-				txopt_put(opt);
-			}
-			pktopt = xchg(&np->pktoptions, NULL);
-			kfree_skb(pktopt);
+
+			/* Disable all options not to allocate memory anymore,
+			 * but there is still a race.  See the lockless path
+			 * in udpv6_sendmsg() and ipv6_local_rxpmtu().
+			 */
+			np->rxopt.all = 0;
+
+			inet6_cleanup_sock(sk);
 
 			/*
 			 * ... and add it to the refcnt debug socks count