iwlagn: verify flow id in compressed BA packet

The flow id (scd_flow) in a compressed BA packet should match the txq_id
of the queue from which the aggregated packets were sent. However, in
some hardware like the 1000 series, sometimes the flow id is 0 for the
txq_id (10 to 19). This can cause the annoying message:
[ 2213.306191] iwlagn 0000:01:00.0: Received BA when not expected
[ 2213.310178] iwlagn 0000:01:00.0: Read index for DMA queue txq id (0),
index 5, is out of range [0-256] 7 7.

And even worse, if agg->wait_for_ba is true when the bad BA is arriving,
this can cause system hang due to NULL pointer dereference because the
code is operating in a wrong tx queue!

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Pradeep Kulkarni <pradeepx.kulkarni@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>

drivers/net/wireless/iwlwifi/iwl-agn-tx.c

@@ -1299,6 +1299,11 @@ void iwlagn_rx_reply_compressed_ba(struct iwl_priv *priv,
 	sta_id = ba_resp->sta_id;
 	tid = ba_resp->tid;
 	agg = &priv->stations[sta_id].tid[tid].agg;
+	if (unlikely(agg->txq_id != scd_flow)) {
+		IWL_ERR(priv, "BA scd_flow %d does not match txq_id %d\n",
+			scd_flow, agg->txq_id);
+		return;
+	}
 
 	/* Find index just before block-ack window */
 	index = iwl_queue_dec_wrap(ba_resp_scd_ssn & 0xff, txq->q.n_bd);