tcp: make challenge acks less predictable

Yue Cao claims that current host rate limiting of challenge ACKS
(RFC 5961) could leak enough information to allow a patient attacker
to hijack TCP sessions. He will soon provide details in an academic
paper.

This patch increases the default limit from 100 to 1000, and adds
some randomization so that the attacker can no longer hijack
sessions without spending a considerable amount of probes.

Based on initial analysis and patch from Linus.

Note that we also have per socket rate limiting, so it is tempting
to remove the host limit in the future.

v2: randomize the count of challenge acks per second, not the period.

Fixes: 282f23c6ee34 ("tcp: implement RFC 5961 3.2")
Reported-by: Yue Cao <ycao009@ucr.edu>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);
	bool fragstolen = false;
	int eaten = -1;

	if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
		__kfree_skb(skb);
		return;
	}
	skb_dst_drop(skb);
	__skb_pull(skb, tcp_hdr(skb)->doff * 4);

	tcp_ecn_accept_cwr(tp, skb);

	tp->rx_opt.dsack = 0;

	/*  Queue data for delivery to the user.
	 *  Packets in sequence go to the receive queue.
	 *  Out of sequence packets to the out_of_order_queue.
	 */
	if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
		if (tcp_receive_window(tp) == 0)
			goto out_of_window;

		/* Ok. In sequence. In window. */
		if (tp->ucopy.task == current &&
		    tp->copied_seq == tp->rcv_nxt && tp->ucopy.len &&
		    sock_owned_by_user(sk) && !tp->urg_data) {
			int chunk = min_t(unsigned int, skb->len,
					  tp->ucopy.len);

			__set_current_state(TASK_RUNNING);

			if (!skb_copy_datagram_msg(skb, 0, tp->ucopy.msg, chunk)) {
				tp->ucopy.len -= chunk;
				tp->copied_seq += chunk;
				eaten = (chunk == skb->len);
				tcp_rcv_space_adjust(sk);
			}
		}

		if (eaten <= 0) {
queue_and_out:
			if (eaten < 0) {
				if (skb_queue_len(&sk->sk_receive_queue) == 0)
					sk_forced_mem_schedule(sk, skb->truesize);
				else if (tcp_try_rmem_schedule(sk, skb, skb->truesize))
					goto drop;
			}
			eaten = tcp_queue_rcv(sk, skb, 0, &fragstolen);
		}
		tcp_rcv_nxt_update(tp, TCP_SKB_CB(skb)->end_seq);
		if (skb->len)
			tcp_event_data_recv(sk, skb);
		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
			tcp_fin(sk);

		if (!skb_queue_empty(&tp->out_of_order_queue)) {
			tcp_ofo_queue(sk);

			/* RFC2581. 4.2. SHOULD send immediate ACK, when
			 * gap in queue is filled.
			 */
			if (skb_queue_empty(&tp->out_of_order_queue))
				inet_csk(sk)->icsk_ack.pingpong = 0;
		}

		if (tp->rx_opt.num_sacks)
			tcp_sack_remove(tp);

		tcp_fast_path_check(sk);

		if (eaten > 0)
			kfree_skb_partial(skb, fragstolen);
		if (!sock_flag(sk, SOCK_DEAD))
			sk->sk_data_ready(sk);
		return;
	}

	if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
		/* A retransmit, 2nd most common case.  Force an immediate ack. */
		NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
		tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);

out_of_window:
		tcp_enter_quickack_mode(sk);
		inet_csk_schedule_ack(sk);
drop:
		tcp_drop(sk, skb);
		return;
	}

	/* Out of window. F.e. zero window probe. */
	if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt + tcp_receive_window(tp)))
		goto out_of_window;

	tcp_enter_quickack_mode(sk);

	if (before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
		/* Partial packet, seq < rcv_next < end_seq */
		SOCK_DEBUG(sk, "partial packet: rcv_next %X seq %X - %X\n",
			   tp->rcv_nxt, TCP_SKB_CB(skb)->seq,
			   TCP_SKB_CB(skb)->end_seq);

		tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, tp->rcv_nxt);

		/* If window is closed, drop tail of packet. But after
		 * remembering D-SACK for its head made in previous line.
		 */
		if (!tcp_receive_window(tp))
			goto out_of_window;
		goto queue_and_out;
	}

	tcp_data_queue_ofo(sk, skb);
}

static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);
	bool fragstolen = false;
	int eaten = -1;

	if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
		__kfree_skb(skb);
		return;
	}
	skb_dst_drop(skb);
	__skb_pull(skb, tcp_hdr(skb)->doff * 4);

	tcp_ecn_accept_cwr(tp, skb);

	tp->rx_opt.dsack = 0;

	/*  Queue data for delivery to the user.
	 *  Packets in sequence go to the receive queue.
	 *  Out of sequence packets to the out_of_order_queue.
	 */
	if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
		if (tcp_receive_window(tp) == 0)
			goto out_of_window;

		/* Ok. In sequence. In window. */
		if (tp->ucopy.task == current &&
		    tp->copied_seq == tp->rcv_nxt && tp->ucopy.len &&
		    sock_owned_by_user(sk) && !tp->urg_data) {
			int chunk = min_t(unsigned int, skb->len,
					  tp->ucopy.len);

			__set_current_state(TASK_RUNNING);

			if (!skb_copy_datagram_msg(skb, 0, tp->ucopy.msg, chunk)) {
				tp->ucopy.len -= chunk;
				tp->copied_seq += chunk;
				eaten = (chunk == skb->len);
				tcp_rcv_space_adjust(sk);
			}
		}

		if (eaten <= 0) {
queue_and_out:
			if (eaten < 0) {
				if (skb_queue_len(&sk->sk_receive_queue) == 0)
					sk_forced_mem_schedule(sk, skb->truesize);
				else if (tcp_try_rmem_schedule(sk, skb, skb->truesize))
					goto drop;
			}
			eaten = tcp_queue_rcv(sk, skb, 0, &fragstolen);
		}
		tcp_rcv_nxt_update(tp, TCP_SKB_CB(skb)->end_seq);
		if (skb->len)
			tcp_event_data_recv(sk, skb);
		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
			tcp_fin(sk);

		if (!skb_queue_empty(&tp->out_of_order_queue)) {
			tcp_ofo_queue(sk);

			/* RFC2581. 4.2. SHOULD send immediate ACK, when
			 * gap in queue is filled.
			 */
			if (skb_queue_empty(&tp->out_of_order_queue))
				inet_csk(sk)->icsk_ack.pingpong = 0;
		}

		if (tp->rx_opt.num_sacks)
			tcp_sack_remove(tp);

		tcp_fast_path_check(sk);

		if (eaten > 0)
			kfree_skb_partial(skb, fragstolen);
		if (!sock_flag(sk, SOCK_DEAD))
			sk->sk_data_ready(sk);
		return;
	}

	if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
		/* A retransmit, 2nd most common case.  Force an immediate ack. */
		NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
		tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);

out_of_window:
		tcp_enter_quickack_mode(sk);
		inet_csk_schedule_ack(sk);
drop:
		tcp_drop(sk, skb);
		return;
	}

	/* Out of window. F.e. zero window probe. */
	if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt + tcp_receive_window(tp)))
		goto out_of_window;

	tcp_enter_quickack_mode(sk);

	if (before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
		/* Partial packet, seq < rcv_next < end_seq */
		SOCK_DEBUG(sk, "partial packet: rcv_next %X seq %X - %X\n",
			   tp->rcv_nxt, TCP_SKB_CB(skb)->seq,
			   TCP_SKB_CB(skb)->end_seq);

		tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, tp->rcv_nxt);

		/* If window is closed, drop tail of packet. But after
		 * remembering D-SACK for its head made in previous line.
		 */
		if (!tcp_receive_window(tp))
			goto out_of_window;
		goto queue_and_out;
	}

	tcp_data_queue_ofo(sk, skb);
}
