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Commit 37561f68 authored by Jerry Chu's avatar Jerry Chu Committed by David S. Miller
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tcp: Reject invalid ack_seq to Fast Open sockets 



A packet with an invalid ack_seq may cause a TCP Fast Open socket to switch
to the unexpected TCP_CLOSING state, triggering a BUG_ON kernel panic.

When a FIN packet with an invalid ack_seq# arrives at a socket in
the TCP_FIN_WAIT1 state, rather than discarding the packet, the current
code will accept the FIN, causing state transition to TCP_CLOSING.

This may be a small deviation from RFC793, which seems to say that the
packet should be dropped. Unfortunately I did not expect this case for
Fast Open hence it will trigger a BUG_ON panic.

It turns out there is really nothing bad about a TFO socket going into
TCP_CLOSING state so I could just remove the BUG_ON statements. But after
some thought I think it's better to treat this case like TCP_SYN_RECV
and return a RST to the confused peer who caused the unacceptable ack_seq
to be generated in the first place.

Signed-off-by: default avatarH.K. Jerry Chu <hkchu@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: default avatarYuchung Cheng <ycheng@google.com>
Acked-by: default avatarEric Dumazet <edumazet@google.com>
Acked-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 0b63bf1f

net/ipv4/tcp_input.c

+10 −2
Original line number Diff line number Diff line
@@ -5964,7 +5964,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb, 


	req = tp->fastopen_rsk;

	if (req != NULL) {

		BUG_ON(sk->sk_state != TCP_SYN_RECV &&

		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&

		    sk->sk_state != TCP_FIN_WAIT1);



		if (tcp_check_req(sk, skb, req, NULL, true) == NULL)
@@ -6053,7 +6053,15 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb, 
			 * ACK we have received, this would have acknowledged

			 * our SYNACK so stop the SYNACK timer.

			 */

			if (acceptable && req != NULL) {

			if (req != NULL) {

				/* Return RST if ack_seq is invalid.

				 * Note that RFC793 only says to generate a

				 * DUPACK for it but for TCP Fast Open it seems

				 * better to treat this case like TCP_SYN_RECV

				 * above.

				 */

				if (!acceptable)

					return 1;

				/* We no longer need the request sock. */

				reqsk_fastopen_remove(sk, req, false);

				tcp_rearm_rto(sk);

net/ipv4/tcp_timer.c

+2 −2
Original line number Diff line number Diff line
@@ -347,7 +347,7 @@ void tcp_retransmit_timer(struct sock *sk) 
		return;

	}

	if (tp->fastopen_rsk) {

		BUG_ON(sk->sk_state != TCP_SYN_RECV &&

		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&

			     sk->sk_state != TCP_FIN_WAIT1);

		tcp_fastopen_synack_timer(sk);

		/* Before we receive ACK to our SYN-ACK don't retransmit