iwlwifi: fix skb usage after free

Since

commit a120e912eb51e347f36c71b60a1d13af74d30e83
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.

Cc: stable@kernel.org # 2.6.32+
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>

1 files changed, 7 insertions, 3 deletions

diff --git a/drivers/net/wireless/iwlwifi/iwl-agn-tx.c b/drivers/net/wireless/iwlwifi/iwl-agn-tx.c
index a709d05c5868..2dd7d54a796f 100644
--- a/drivers/net/wireless/iwlwifi/iwl-agn-tx.c
+++ b/drivers/net/wireless/iwlwifi/iwl-agn-tx.c
@@ -1224,12 +1224,16 @@ int iwlagn_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];
-                iwlagn_tx_status(priv, tx_info,
-                                 txq_id >= IWLAGN_FIRST_AMPDU_QUEUE);
+
+                if (WARN_ON_ONCE(tx_info->skb == NULL))
+                        continue;
 
                 hdr = (struct ieee80211_hdr *)tx_info->skb->data;
-                if (hdr && ieee80211_is_data_qos(hdr->frame_control))
+                if (ieee80211_is_data_qos(hdr->frame_control))
                         nfreed++;
+
+                iwlagn_tx_status(priv, tx_info,
+                                 txq_id >= IWLAGN_FIRST_AMPDU_QUEUE);
                 tx_info->skb = NULL;
 
                 if (priv->cfg->ops->lib->txq_inval_byte_cnt_tbl)