tcp: accept RST for rcv_nxt - 1 after receiving a FIN

Using a Mac OSX box as a client connecting to a Linux server, we have found
that when certain applications (such as 'ab'), are abruptly terminated
(via ^C), a FIN is sent followed by a RST packet on tcp connections. The
FIN is accepted by the Linux stack but the RST is sent with the same
sequence number as the FIN, and Linux responds with a challenge ACK per
RFC 5961. The OSX client then sometimes (they are rate-limited) does not
reply with any RST as would be expected on a closed socket.

This results in sockets accumulating on the Linux server left mostly in
the CLOSE_WAIT state, although LAST_ACK and CLOSING are also possible.
This sequence of events can tie up a lot of resources on the Linux server
since there may be a lot of data in write buffers at the time of the RST.
Accepting a RST equal to rcv_nxt - 1, after we have already successfully
processed a FIN, has made a significant difference for us in practice, by
freeing up unneeded resources in a more expedient fashion.

A packetdrill test demonstrating the behavior:

// testing mac osx rst behavior

// Establish a connection
0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
0.000 bind(3, ..., ...) = 0
0.000 listen(3, 1) = 0

0.100 < S 0:0(0) win 32768 <mss 1460,nop,wscale 10>
0.100 > S. 0:0(0) ack 1 <mss 1460,nop,wscale 5>
0.200 < . 1:1(0) ack 1 win 32768
0.200 accept(3, ..., ...) = 4

// Client closes the connection
0.300 < F. 1:1(0) ack 1 win 32768

// now send rst with same sequence
0.300 < R. 1:1(0) ack 1 win 32768

// make sure we are in TCP_CLOSE
0.400 %{
assert tcpi_state == 7
}%
Signed-off-by: Jason Baron <jbaron@akamai.com>
Cc: Eric Dumazet <edumazet@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -5199,6 +5199,23 @@ static int tcp_copy_to_iovec(struct sock *sk, struct sk_buff *skb, int hlen)
 	return err;
 }
 
+/* Accept RST for rcv_nxt - 1 after a FIN.
+ * When tcp connections are abruptly terminated from Mac OSX (via ^C), a
+ * FIN is sent followed by a RST packet. The RST is sent with the same
+ * sequence number as the FIN, and thus according to RFC 5961 a challenge
+ * ACK should be sent. However, Mac OSX rate limits replies to challenge
+ * ACKs on the closed socket. In addition middleboxes can drop either the
+ * challenge ACK or a subsequent RST.
+ */
+static bool tcp_reset_check(const struct sock *sk, const struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	return unlikely(TCP_SKB_CB(skb)->seq == (tp->rcv_nxt - 1) &&
+			(1 << sk->sk_state) & (TCPF_CLOSE_WAIT | TCPF_LAST_ACK |
+					       TCPF_CLOSING));
+}
+
 /* Does PAWS and seqno based validation of an incoming segment, flags will
  * play significant role here.
  */
@@ -5237,20 +5254,25 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 						  LINUX_MIB_TCPACKSKIPPEDSEQ,
 						  &tp->last_oow_ack_time))
 				tcp_send_dupack(sk, skb);
+		} else if (tcp_reset_check(sk, skb)) {
+			tcp_reset(sk);
 		}
 		goto discard;
 	}
 
 	/* Step 2: check RST bit */
 	if (th->rst) {
-		/* RFC 5961 3.2 (extend to match against SACK too if available):
-		 * If seq num matches RCV.NXT or the right-most SACK block,
+		/* RFC 5961 3.2 (extend to match against (RCV.NXT - 1) after a
+		 * FIN and SACK too if available):
+		 * If seq num matches RCV.NXT or (RCV.NXT - 1) after a FIN, or
+		 * the right-most SACK block,
 		 * then
 		 *     RESET the connection
 		 * else
 		 *     Send a challenge ACK
 		 */
-		if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
+		if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt ||
+		    tcp_reset_check(sk, skb)) {
 			rst_seq_match = true;
 		} else if (tcp_is_sack(tp) && tp->rx_opt.num_sacks > 0) {
 			struct tcp_sack_block *sp = &tp->selective_acks[0];