Code: static int dccp_v6_send_response(const struct sock *sk, struct request_sock *req)
{
	struct inet_request_sock *ireq = inet_rsk(req);
	struct ipv6_pinfo *np = inet6_sk(sk);
	struct sk_buff *skb;
	struct in6_addr *final_p, final;
	struct flowi6 fl6;
	int err = -1;
	struct dst_entry *dst;

	memset(&fl6, 0, sizeof(fl6));
	fl6.flowi6_proto = IPPROTO_DCCP;
	fl6.daddr = ireq->ir_v6_rmt_addr;
	fl6.saddr = ireq->ir_v6_loc_addr;
	fl6.flowlabel = 0;
	fl6.flowi6_oif = ireq->ir_iif;
	fl6.fl6_dport = ireq->ir_rmt_port;
	fl6.fl6_sport = htons(ireq->ir_num);
 	security_req_classify_flow(req, flowi6_to_flowi(&fl6));
 
 
	final_p = fl6_update_dst(&fl6, np->opt, &final);
 
 	dst = ip6_dst_lookup_flow(sk, &fl6, final_p);
 	if (IS_ERR(dst)) {
		err = PTR_ERR(dst);
		dst = NULL;
		goto done;
	}

	skb = dccp_make_response(sk, dst, req);
	if (skb != NULL) {
		struct dccp_hdr *dh = dccp_hdr(skb);

		dh->dccph_checksum = dccp_v6_csum_finish(skb,
 							 &ireq->ir_v6_loc_addr,
 							 &ireq->ir_v6_rmt_addr);
 		fl6.daddr = ireq->ir_v6_rmt_addr;
		err = ip6_xmit(sk, skb, &fl6, np->opt, np->tclass);
 		err = net_xmit_eval(err);
 	}
 
done:
	dst_release(dst);
	return err;
}

Commit Message: 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>
CWE ID: CWE-416
static int dccp_v6_send_response(const struct sock *sk, struct request_sock *req)
{
	struct inet_request_sock *ireq = inet_rsk(req);
	struct ipv6_pinfo *np = inet6_sk(sk);
	struct sk_buff *skb;
	struct in6_addr *final_p, final;
	struct flowi6 fl6;
	int err = -1;
	struct dst_entry *dst;

	memset(&fl6, 0, sizeof(fl6));
	fl6.flowi6_proto = IPPROTO_DCCP;
	fl6.daddr = ireq->ir_v6_rmt_addr;
	fl6.saddr = ireq->ir_v6_loc_addr;
	fl6.flowlabel = 0;
	fl6.flowi6_oif = ireq->ir_iif;
	fl6.fl6_dport = ireq->ir_rmt_port;
	fl6.fl6_sport = htons(ireq->ir_num);
 	security_req_classify_flow(req, flowi6_to_flowi(&fl6));
 
 
	rcu_read_lock();
	final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
	rcu_read_unlock();
 
 	dst = ip6_dst_lookup_flow(sk, &fl6, final_p);
 	if (IS_ERR(dst)) {
		err = PTR_ERR(dst);
		dst = NULL;
		goto done;
	}

	skb = dccp_make_response(sk, dst, req);
	if (skb != NULL) {
		struct dccp_hdr *dh = dccp_hdr(skb);

		dh->dccph_checksum = dccp_v6_csum_finish(skb,
 							 &ireq->ir_v6_loc_addr,
 							 &ireq->ir_v6_rmt_addr);
 		fl6.daddr = ireq->ir_v6_rmt_addr;
		rcu_read_lock();
		err = ip6_xmit(sk, skb, &fl6, rcu_dereference(np->opt),
			       np->tclass);
		rcu_read_unlock();
 		err = net_xmit_eval(err);
 	}
 
done:
	dst_release(dst);
	return err;
}
