ipv6: add complete rcu protection around np->opt

This patch addresses multiple problems :

UDP/RAW sendmsg() need to get a stable struct ipv6_txoptions
while socket is not locked : Other threads can change np->opt
concurrently. Dmitry posted a syzkaller
(http://github.com/google/syzkaller) program desmonstrating
use-after-free.

Starting with TCP/DCCP lockless listeners, tcp_v6_syn_recv_sock()
and dccp_v6_request_recv_sock() also need to use RCU protection
to dereference np->opt once (before calling ipv6_dup_options())

This patch adds full RCU protection to np->opt

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
int inet6_csk_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl_unused)
{
	struct ipv6_pinfo *np = inet6_sk(sk);
	struct flowi6 fl6;
	struct dst_entry *dst;
	int res;

	dst = inet6_csk_route_socket(sk, &fl6);
	if (IS_ERR(dst)) {
		sk->sk_err_soft = -PTR_ERR(dst);
		sk->sk_route_caps = 0;
		kfree_skb(skb);
		return PTR_ERR(dst);
	}

	rcu_read_lock();
	skb_dst_set_noref(skb, dst);

 	/* Restore final destination back after routing done */
 	fl6.daddr = sk->sk_v6_daddr;
 
	res = ip6_xmit(sk, skb, &fl6, rcu_dereference(np->opt),
		       np->tclass);
 	rcu_read_unlock();
 	return res;
 }

int inet6_csk_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl_unused)
{
	struct ipv6_pinfo *np = inet6_sk(sk);
	struct flowi6 fl6;
	struct dst_entry *dst;
	int res;

	dst = inet6_csk_route_socket(sk, &fl6);
	if (IS_ERR(dst)) {
		sk->sk_err_soft = -PTR_ERR(dst);
		sk->sk_route_caps = 0;
		kfree_skb(skb);
		return PTR_ERR(dst);
	}

	rcu_read_lock();
	skb_dst_set_noref(skb, dst);

 	/* Restore final destination back after routing done */
 	fl6.daddr = sk->sk_v6_daddr;
 
	res = ip6_xmit(sk, skb, &fl6, np->opt, np->tclass);
 	rcu_read_unlock();
 	return res;
 }
