Code: static int dccp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
			   int addr_len)
{
	struct sockaddr_in6 *usin = (struct sockaddr_in6 *)uaddr;
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct inet_sock *inet = inet_sk(sk);
 	struct ipv6_pinfo *np = inet6_sk(sk);
 	struct dccp_sock *dp = dccp_sk(sk);
 	struct in6_addr *saddr = NULL, *final_p, final;
 	struct flowi6 fl6;
 	struct dst_entry *dst;
 	int addr_type;
	int err;

	dp->dccps_role = DCCP_ROLE_CLIENT;

	if (addr_len < SIN6_LEN_RFC2133)
		return -EINVAL;

	if (usin->sin6_family != AF_INET6)
		return -EAFNOSUPPORT;

	memset(&fl6, 0, sizeof(fl6));

	if (np->sndflow) {
		fl6.flowlabel = usin->sin6_flowinfo & IPV6_FLOWINFO_MASK;
		IP6_ECN_flow_init(fl6.flowlabel);
		if (fl6.flowlabel & IPV6_FLOWLABEL_MASK) {
			struct ip6_flowlabel *flowlabel;
			flowlabel = fl6_sock_lookup(sk, fl6.flowlabel);
			if (flowlabel == NULL)
				return -EINVAL;
			fl6_sock_release(flowlabel);
		}
	}
	/*
	 * connect() to INADDR_ANY means loopback (BSD'ism).
	 */
	if (ipv6_addr_any(&usin->sin6_addr))
		usin->sin6_addr.s6_addr[15] = 1;

	addr_type = ipv6_addr_type(&usin->sin6_addr);

	if (addr_type & IPV6_ADDR_MULTICAST)
		return -ENETUNREACH;

	if (addr_type & IPV6_ADDR_LINKLOCAL) {
		if (addr_len >= sizeof(struct sockaddr_in6) &&
		    usin->sin6_scope_id) {
			/* If interface is set while binding, indices
			 * must coincide.
			 */
			if (sk->sk_bound_dev_if &&
			    sk->sk_bound_dev_if != usin->sin6_scope_id)
				return -EINVAL;

			sk->sk_bound_dev_if = usin->sin6_scope_id;
		}

		/* Connect to link-local address requires an interface */
		if (!sk->sk_bound_dev_if)
			return -EINVAL;
	}

	sk->sk_v6_daddr = usin->sin6_addr;
	np->flow_label = fl6.flowlabel;

	/*
	 * DCCP over IPv4
	 */
	if (addr_type == IPV6_ADDR_MAPPED) {
		u32 exthdrlen = icsk->icsk_ext_hdr_len;
		struct sockaddr_in sin;

		SOCK_DEBUG(sk, "connect: ipv4 mapped\n");

		if (__ipv6_only_sock(sk))
			return -ENETUNREACH;

		sin.sin_family = AF_INET;
		sin.sin_port = usin->sin6_port;
		sin.sin_addr.s_addr = usin->sin6_addr.s6_addr32[3];

		icsk->icsk_af_ops = &dccp_ipv6_mapped;
		sk->sk_backlog_rcv = dccp_v4_do_rcv;

		err = dccp_v4_connect(sk, (struct sockaddr *)&sin, sizeof(sin));
		if (err) {
			icsk->icsk_ext_hdr_len = exthdrlen;
			icsk->icsk_af_ops = &dccp_ipv6_af_ops;
			sk->sk_backlog_rcv = dccp_v6_do_rcv;
			goto failure;
		}
		np->saddr = sk->sk_v6_rcv_saddr;
		return err;
	}

	if (!ipv6_addr_any(&sk->sk_v6_rcv_saddr))
		saddr = &sk->sk_v6_rcv_saddr;

	fl6.flowi6_proto = IPPROTO_DCCP;
	fl6.daddr = sk->sk_v6_daddr;
	fl6.saddr = saddr ? *saddr : np->saddr;
	fl6.flowi6_oif = sk->sk_bound_dev_if;
	fl6.fl6_dport = usin->sin6_port;
 	fl6.fl6_sport = inet->inet_sport;
 	security_sk_classify_flow(sk, 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);
		goto failure;
	}

	if (saddr == NULL) {
		saddr = &fl6.saddr;
		sk->sk_v6_rcv_saddr = *saddr;
	}

	/* set the source address */
	np->saddr = *saddr;
	inet->inet_rcv_saddr = LOOPBACK4_IPV6;

 	__ip6_dst_store(sk, dst, NULL, NULL);
 
 	icsk->icsk_ext_hdr_len = 0;
	if (np->opt != NULL)
		icsk->icsk_ext_hdr_len = (np->opt->opt_flen +
					  np->opt->opt_nflen);
 
 	inet->inet_dport = usin->sin6_port;
 
	dccp_set_state(sk, DCCP_REQUESTING);
	err = inet6_hash_connect(&dccp_death_row, sk);
	if (err)
		goto late_failure;

	dp->dccps_iss = secure_dccpv6_sequence_number(np->saddr.s6_addr32,
						      sk->sk_v6_daddr.s6_addr32,
						      inet->inet_sport,
						      inet->inet_dport);
	err = dccp_connect(sk);
	if (err)
		goto late_failure;

	return 0;

late_failure:
	dccp_set_state(sk, DCCP_CLOSED);
	__sk_dst_reset(sk);
failure:
	inet->inet_dport = 0;
	sk->sk_route_caps = 0;
	return err;
}

Vulnerability Type: DoS +Priv 
CWE ID: CWE-416
Summary: The IPv6 stack in the Linux kernel before 4.3.3 mishandles options data, which allows local users to gain privileges or cause a denial of service (use-after-free and system crash) via a crafted sendmsg system call.
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>