Commit 37561f68 authored Oct 22, 2012 by Jerry Chu Committed by David S. Miller Oct 23, 2012

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: H.K. Jerry Chu <hkchu@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David 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

Admin message