iwlwifi: fix skb usage after free

commit b2502698 upstream.

Since

commit a120e912
Author: Stanislaw Gruszka <sgruszka@redhat.com>
Date:   Fri Feb 19 15:47:33 2010 -0800

    iwlwifi: sanity check before counting number of tfds can be free

we use skb->data after calling ieee80211_tx_status_irqsafe(), which
could free skb instantly.

On current kernels I do not observe practical problems related with
bug, but on 2.6.35.y it cause random system hangs when stressing
wireless link.
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

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

@@ -1088,11 +1088,15 @@ int iwl_tx_queue_reclaim(struct iwl_priv *priv, int txq_id, int index)
 	     q->read_ptr = iwl_queue_inc_wrap(q->read_ptr, q->n_bd)) {
 
 		tx_info = &txq->txb[txq->q.read_ptr];
-		ieee80211_tx_status_irqsafe(priv->hw, tx_info->skb[0]);
+
+		if (WARN_ON_ONCE(tx_info->skb == NULL))
+			continue;
 
 		hdr = (struct ieee80211_hdr *)tx_info->skb[0]->data;
-		if (hdr && ieee80211_is_data_qos(hdr->frame_control))
+		if (ieee80211_is_data_qos(hdr->frame_control))
 			nfreed++;
+
+		ieee80211_tx_status_irqsafe(priv->hw, tx_info->skb[0]);
 		tx_info->skb[0] = NULL;
 
 		if (priv->cfg->ops->lib->txq_inval_byte_cnt_tbl)