Return RST to unexpected ACK in SYN-SENT state.

Before this commit, the socket got stuck in an unusable state.

src/socket/tcp.rs

@@ -995,6 +995,15 @@ impl<'a> TcpSocket<'a> {
  self.meta.handle, self.local_endpoint, self.remote_endpoint);
  return Err(Error::Dropped)
  }
+ // Any ACK in the SYN-SENT state must have the SYN flag set.
+ (State::SynSent, &TcpRepr {
+ control: TcpControl::None, ack_number: Some(_), ..
+ }) => {
+ net_debug!("{}:{}:{}: expecting a SYN|ACK",
+ self.meta.handle, self.local_endpoint, self.remote_endpoint);
+ self.abort();
+ return Err(Error::Dropped)
+ }
  // Every acknowledgement must be for transmitted but unacknowledged data.
  (_, &TcpRepr { ack_number: Some(ack_number), .. }) => {
  let unacknowledged = self.tx_buffer.len() + control_len;
@@ -2490,6 +2499,17 @@ mod test {
  assert_eq!(s.state, State::SynSent);
  }
 
+ #[test]
+ fn test_syn_sent_bad_ack() {
+ let mut s = socket_syn_sent();
+ send!(s, TcpRepr {
+ control: TcpControl::None,
+ ack_number: Some(TcpSeqNumber(1)),
+ ..SEND_TEMPL
+ }, Err(Error::Dropped));
+ assert_eq!(s.state, State::Closed);
+ }
+
  #[test]
  fn test_syn_sent_close() {
  let mut s = socket();