inet: add RCU protection to inet->opt

We lack proper synchronization to manipulate inet->opt ip_options

Problem is ip_make_skb() calls ip_setup_cork() and
ip_setup_cork() possibly makes a copy of ipc->opt (struct ip_options),
without any protection against another thread manipulating inet->opt.

Another thread can change inet->opt pointer and free old one under us.

Use RCU to protect inet->opt (changed to inet->inet_opt).

Instead of handling atomic refcounts, just copy ip_options when
necessary, to avoid cache line dirtying.

We cant insert an rcu_head in struct ip_options since its included in
skb->cb[], so this patch is large because I had to introduce a new
ip_options_rcu structure.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
struct dst_entry *inet_csk_route_req(struct sock *sk,
				     const struct request_sock *req)
 {
 	struct rtable *rt;
 	const struct inet_request_sock *ireq = inet_rsk(req);
	struct ip_options_rcu *opt = inet_rsk(req)->opt;
 	struct net *net = sock_net(sk);
 	struct flowi4 fl4;
 
 	flowi4_init_output(&fl4, sk->sk_bound_dev_if, sk->sk_mark,
 			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE,
 			   sk->sk_protocol, inet_sk_flowi_flags(sk),
			   (opt && opt->opt.srr) ? opt->opt.faddr : ireq->rmt_addr,
 			   ireq->loc_addr, ireq->rmt_port, inet_sk(sk)->inet_sport);
 	security_req_classify_flow(req, flowi4_to_flowi(&fl4));
 	rt = ip_route_output_flow(net, &fl4, sk);
 	if (IS_ERR(rt))
 		goto no_route;
	if (opt && opt->opt.is_strictroute && rt->rt_dst != rt->rt_gateway)
 		goto route_err;
 	return &rt->dst;
 
route_err:
	ip_rt_put(rt);
no_route:
	IP_INC_STATS_BH(net, IPSTATS_MIB_OUTNOROUTES);
	return NULL;
}

struct dst_entry *inet_csk_route_req(struct sock *sk,
				     const struct request_sock *req)
 {
 	struct rtable *rt;
 	const struct inet_request_sock *ireq = inet_rsk(req);
	struct ip_options *opt = inet_rsk(req)->opt;
 	struct net *net = sock_net(sk);
 	struct flowi4 fl4;
 
 	flowi4_init_output(&fl4, sk->sk_bound_dev_if, sk->sk_mark,
 			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE,
 			   sk->sk_protocol, inet_sk_flowi_flags(sk),
			   (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
 			   ireq->loc_addr, ireq->rmt_port, inet_sk(sk)->inet_sport);
 	security_req_classify_flow(req, flowi4_to_flowi(&fl4));
 	rt = ip_route_output_flow(net, &fl4, sk);
 	if (IS_ERR(rt))
 		goto no_route;
	if (opt && opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
 		goto route_err;
 	return &rt->dst;
 
route_err:
	ip_rt_put(rt);
no_route:
	IP_INC_STATS_BH(net, IPSTATS_MIB_OUTNOROUTES);
	return NULL;
}
