Commit message: net: ipv4: fix for a race condition in raw_sendmsg

inet->hdrincl is racy, and could lead to uninitialized stack pointer
usage, so its value should be read only once.

Fixes: c008ba5bdc9f ("ipv4: Avoid reading user iov twice after raw_probe_proto_opt")
Signed-off-by: Mohamed Ghannam <simo.ghannam@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
CVE: CVE-2017-17712
CWE: CWE-362
Vulnerability description: The raw_sendmsg() function in net/ipv4/raw.c in the Linux kernel through 4.14.6 has a race condition in inet->hdrincl that leads to uninitialized stack pointer usage; this allows a local user to execute code and gain privileges.

Project: linux
File: raw.c

static int raw_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
{
	struct inet_sock *inet = inet_sk(sk);
	struct net *net = sock_net(sk);
	struct ipcm_cookie ipc;
	struct rtable *rt = NULL;
	struct flowi4 fl4;
	int free = 0;
	__be32 daddr;
	__be32 saddr;
	u8  tos;
 	int err;
 	struct ip_options_data opt_copy;
 	struct raw_frag_vec rfv;
	int hdrincl;
 
 	err = -EMSGSIZE;
 	if (len > 0xFFFF)
 		goto out;
 
	/* hdrincl should be READ_ONCE(inet->hdrincl)
	 * but READ_ONCE() doesn't work with bit fields
	 */
	hdrincl = inet->hdrincl;
 	/*
 	 *	Check the flags.
 	 */

	err = -EOPNOTSUPP;
	if (msg->msg_flags & MSG_OOB)	/* Mirror BSD error message */
		goto out;               /* compatibility */

	/*
	 *	Get and verify the address.
	 */

	if (msg->msg_namelen) {
		DECLARE_SOCKADDR(struct sockaddr_in *, usin, msg->msg_name);
		err = -EINVAL;
		if (msg->msg_namelen < sizeof(*usin))
			goto out;
		if (usin->sin_family != AF_INET) {
			pr_info_once("%s: %s forgot to set AF_INET. Fix it!\n",
				     __func__, current->comm);
			err = -EAFNOSUPPORT;
			if (usin->sin_family)
				goto out;
		}
		daddr = usin->sin_addr.s_addr;
		/* ANK: I did not forget to get protocol from port field.
		 * I just do not know, who uses this weirdness.
		 * IP_HDRINCL is much more convenient.
		 */
	} else {
		err = -EDESTADDRREQ;
		if (sk->sk_state != TCP_ESTABLISHED)
			goto out;
		daddr = inet->inet_daddr;
	}

	ipc.sockc.tsflags = sk->sk_tsflags;
	ipc.addr = inet->inet_saddr;
	ipc.opt = NULL;
	ipc.tx_flags = 0;
	ipc.ttl = 0;
	ipc.tos = -1;
	ipc.oif = sk->sk_bound_dev_if;

	if (msg->msg_controllen) {
		err = ip_cmsg_send(sk, msg, &ipc, false);
		if (unlikely(err)) {
			kfree(ipc.opt);
			goto out;
		}
		if (ipc.opt)
			free = 1;
	}

	saddr = ipc.addr;
	ipc.addr = daddr;

	if (!ipc.opt) {
		struct ip_options_rcu *inet_opt;

		rcu_read_lock();
		inet_opt = rcu_dereference(inet->inet_opt);
		if (inet_opt) {
			memcpy(&opt_copy, inet_opt,
			       sizeof(*inet_opt) + inet_opt->opt.optlen);
			ipc.opt = &opt_copy.opt;
		}
		rcu_read_unlock();
	}

	if (ipc.opt) {
		err = -EINVAL;
 		/* Linux does not mangle headers on raw sockets,
 		 * so that IP options + IP_HDRINCL is non-sense.
 		 */
		if (hdrincl)
 			goto done;
 		if (ipc.opt->opt.srr) {
 			if (!daddr)
				goto done;
			daddr = ipc.opt->opt.faddr;
		}
	}
	tos = get_rtconn_flags(&ipc, sk);
	if (msg->msg_flags & MSG_DONTROUTE)
		tos |= RTO_ONLINK;

	if (ipv4_is_multicast(daddr)) {
		if (!ipc.oif)
			ipc.oif = inet->mc_index;
		if (!saddr)
			saddr = inet->mc_addr;
	} else if (!ipc.oif)
		ipc.oif = inet->uc_index;
 
 	flowi4_init_output(&fl4, ipc.oif, sk->sk_mark, tos,
 			   RT_SCOPE_UNIVERSE,
			   hdrincl ? IPPROTO_RAW : sk->sk_protocol,
 			   inet_sk_flowi_flags(sk) |
			    (hdrincl ? FLOWI_FLAG_KNOWN_NH : 0),
 			   daddr, saddr, 0, 0, sk->sk_uid);
 
	if (!hdrincl) {
 		rfv.msg = msg;
 		rfv.hlen = 0;
 
		err = raw_probe_proto_opt(&rfv, &fl4);
		if (err)
			goto done;
	}

	security_sk_classify_flow(sk, flowi4_to_flowi(&fl4));
	rt = ip_route_output_flow(net, &fl4, sk);
	if (IS_ERR(rt)) {
		err = PTR_ERR(rt);
		rt = NULL;
		goto done;
	}

	err = -EACCES;
	if (rt->rt_flags & RTCF_BROADCAST && !sock_flag(sk, SOCK_BROADCAST))
		goto done;

	if (msg->msg_flags & MSG_CONFIRM)
 		goto do_confirm;
 back_from_confirm:
 
	if (hdrincl)
 		err = raw_send_hdrinc(sk, &fl4, msg, len,
 				      &rt, msg->msg_flags, &ipc.sockc);
 
	 else {
		sock_tx_timestamp(sk, ipc.sockc.tsflags, &ipc.tx_flags);

		if (!ipc.addr)
			ipc.addr = fl4.daddr;
		lock_sock(sk);
		err = ip_append_data(sk, &fl4, raw_getfrag,
				     &rfv, len, 0,
				     &ipc, &rt, msg->msg_flags);
		if (err)
			ip_flush_pending_frames(sk);
		else if (!(msg->msg_flags & MSG_MORE)) {
			err = ip_push_pending_frames(sk, &fl4);
			if (err == -ENOBUFS && !inet->recverr)
				err = 0;
		}
		release_sock(sk);
	}
done:
	if (free)
		kfree(ipc.opt);
	ip_rt_put(rt);

out:
	if (err < 0)
		return err;
	return len;

do_confirm:
	if (msg->msg_flags & MSG_PROBE)
		dst_confirm_neigh(&rt->dst, &fl4.daddr);
	if (!(msg->msg_flags & MSG_PROBE) || len)
		goto back_from_confirm;
	err = 0;
	goto done;
}