Commit message: dccp: fix freeing skb too early for IPV6_RECVPKTINFO

In the current DCCP implementation an skb for a DCCP_PKT_REQUEST packet
is forcibly freed via __kfree_skb in dccp_rcv_state_process if
dccp_v6_conn_request successfully returns.

However, if IPV6_RECVPKTINFO is set on a socket, the address of the skb
is saved to ireq->pktopts and the ref count for skb is incremented in
dccp_v6_conn_request, so skb is still in use. Nevertheless, it gets freed
in dccp_rcv_state_process.

Fix by calling consume_skb instead of doing goto discard and therefore
calling __kfree_skb.

Similar fixes for TCP:

fb7e2399ec17f1004c0e0ccfd17439f8759ede01 [TCP]: skb is unexpectedly freed.
0aea76d35c9651d55bbaf746e7914e5f9ae5a25d tcp: SYN packets are now
simply consumed

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
CVE: CVE-2017-6074
CWE: CWE-415
Vulnerability description: The dccp_rcv_state_process function in net/dccp/input.c in the Linux kernel through 4.9.11 mishandles DCCP_PKT_REQUEST packet data structures in the LISTEN state, which allows local users to obtain root privileges or cause a denial of service (double free) via an application that makes an IPV6_RECVPKTINFO setsockopt system call.

Project: linux
File: input.c

int dccp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
			   struct dccp_hdr *dh, unsigned int len)
{
	struct dccp_sock *dp = dccp_sk(sk);
	struct dccp_skb_cb *dcb = DCCP_SKB_CB(skb);
	const int old_state = sk->sk_state;
	int queued = 0;

	/*
	 *  Step 3: Process LISTEN state
	 *
	 *     If S.state == LISTEN,
	 *	 If P.type == Request or P contains a valid Init Cookie option,
	 *	      (* Must scan the packet's options to check for Init
	 *		 Cookies.  Only Init Cookies are processed here,
	 *		 however; other options are processed in Step 8.  This
	 *		 scan need only be performed if the endpoint uses Init
	 *		 Cookies *)
	 *	      (* Generate a new socket and switch to that socket *)
	 *	      Set S := new socket for this port pair
	 *	      S.state = RESPOND
	 *	      Choose S.ISS (initial seqno) or set from Init Cookies
	 *	      Initialize S.GAR := S.ISS
	 *	      Set S.ISR, S.GSR, S.SWL, S.SWH from packet or Init
	 *	      Cookies Continue with S.state == RESPOND
	 *	      (* A Response packet will be generated in Step 11 *)
	 *	 Otherwise,
	 *	      Generate Reset(No Connection) unless P.type == Reset
	 *	      Drop packet and return
	 */
	if (sk->sk_state == DCCP_LISTEN) {
		if (dh->dccph_type == DCCP_PKT_REQUEST) {
 			if (inet_csk(sk)->icsk_af_ops->conn_request(sk,
 								    skb) < 0)
 				return 1;
			goto discard;
 		}
 		if (dh->dccph_type == DCCP_PKT_RESET)
 			goto discard;

		/* Caller (dccp_v4_do_rcv) will send Reset */
		dcb->dccpd_reset_code = DCCP_RESET_CODE_NO_CONNECTION;
		return 1;
	} else if (sk->sk_state == DCCP_CLOSED) {
		dcb->dccpd_reset_code = DCCP_RESET_CODE_NO_CONNECTION;
		return 1;
	}

	/* Step 6: Check sequence numbers (omitted in LISTEN/REQUEST state) */
	if (sk->sk_state != DCCP_REQUESTING && dccp_check_seqno(sk, skb))
		goto discard;

	/*
	 *   Step 7: Check for unexpected packet types
	 *      If (S.is_server and P.type == Response)
	 *	    or (S.is_client and P.type == Request)
	 *	    or (S.state == RESPOND and P.type == Data),
	 *	  Send Sync packet acknowledging P.seqno
	 *	  Drop packet and return
	 */
	if ((dp->dccps_role != DCCP_ROLE_CLIENT &&
	     dh->dccph_type == DCCP_PKT_RESPONSE) ||
	    (dp->dccps_role == DCCP_ROLE_CLIENT &&
	     dh->dccph_type == DCCP_PKT_REQUEST) ||
	    (sk->sk_state == DCCP_RESPOND && dh->dccph_type == DCCP_PKT_DATA)) {
		dccp_send_sync(sk, dcb->dccpd_seq, DCCP_PKT_SYNC);
		goto discard;
	}

	/*  Step 8: Process options */
	if (dccp_parse_options(sk, NULL, skb))
		return 1;

	/*
	 *  Step 9: Process Reset
	 *	If P.type == Reset,
	 *		Tear down connection
	 *		S.state := TIMEWAIT
	 *		Set TIMEWAIT timer
	 *		Drop packet and return
	 */
	if (dh->dccph_type == DCCP_PKT_RESET) {
		dccp_rcv_reset(sk, skb);
		return 0;
	} else if (dh->dccph_type == DCCP_PKT_CLOSEREQ) {	/* Step 13 */
		if (dccp_rcv_closereq(sk, skb))
			return 0;
		goto discard;
	} else if (dh->dccph_type == DCCP_PKT_CLOSE) {		/* Step 14 */
		if (dccp_rcv_close(sk, skb))
			return 0;
		goto discard;
	}

	switch (sk->sk_state) {
	case DCCP_REQUESTING:
		queued = dccp_rcv_request_sent_state_process(sk, skb, dh, len);
		if (queued >= 0)
			return queued;

		__kfree_skb(skb);
		return 0;

	case DCCP_PARTOPEN:
		/* Step 8: if using Ack Vectors, mark packet acknowledgeable */
		dccp_handle_ackvec_processing(sk, skb);
		dccp_deliver_input_to_ccids(sk, skb);
		/* fall through */
	case DCCP_RESPOND:
		queued = dccp_rcv_respond_partopen_state_process(sk, skb,
								 dh, len);
		break;
	}

	if (dh->dccph_type == DCCP_PKT_ACK ||
	    dh->dccph_type == DCCP_PKT_DATAACK) {
		switch (old_state) {
		case DCCP_PARTOPEN:
			sk->sk_state_change(sk);
			sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
			break;
		}
	} else if (unlikely(dh->dccph_type == DCCP_PKT_SYNC)) {
		dccp_send_sync(sk, dcb->dccpd_seq, DCCP_PKT_SYNCACK);
		goto discard;
	}

	if (!queued) {
discard:
		__kfree_skb(skb);
	}
	return 0;
}