wifi: mac80211: always free skb on ieee80211_tx_prepare_skb() failure

ieee80211_tx_prepare_skb() has three error paths, but only two of them
free the skb. The first error path (ieee80211_tx_prepare() returning
TX_DROP) does not free it, while invoke_tx_handlers() failure and the
fragmentation check both do.

Add kfree_skb() to the first error path so all three are consistent,
and remove the now-redundant frees in callers (ath9k, mt76,
mac80211_hwsim) to avoid double-free.

Document the skb ownership guarantee in the function's kdoc.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://patch.msgid.link/20260314065455.2462900-1-nbd@nbd.name
Fixes: 06be6b1 ("mac80211: add ieee80211_tx_prepare_skb() helper function")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: nbd168

drivers/net/wireless/ath/ath9k/channel.c

@@ -1006,7 +1006,7 @@ static void ath_scan_send_probe(struct ath_softc *sc,
 	skb_set_queue_mapping(skb, IEEE80211_AC_VO);
 
 	if (!ieee80211_tx_prepare_skb(sc->hw, vif, skb, band, NULL))
-		goto error;
+		return;
 
 	txctl.txq = sc->tx.txq_map[IEEE80211_AC_VO];
 	if (ath_tx_start(sc->hw, skb, &txctl))
@@ -1119,10 +1119,8 @@ ath_chanctx_send_vif_ps_frame(struct ath_softc *sc, struct ath_vif *avp,
 
 		skb->priority = 7;
 		skb_set_queue_mapping(skb, IEEE80211_AC_VO);
-		if (!ieee80211_tx_prepare_skb(sc->hw, vif, skb, band, &sta)) {
-			dev_kfree_skb_any(skb);
+		if (!ieee80211_tx_prepare_skb(sc->hw, vif, skb, band, &sta))
 			return false;
-		}
 		break;
 	default:
 		return false;

drivers/net/wireless/mediatek/mt76/scan.c

@@ -63,10 +63,8 @@ mt76_scan_send_probe(struct mt76_dev *dev, struct cfg80211_ssid *ssid)
 
 	rcu_read_lock();
 
-	if (!ieee80211_tx_prepare_skb(phy->hw, vif, skb, band, NULL)) {
-		ieee80211_free_txskb(phy->hw, skb);
+	if (!ieee80211_tx_prepare_skb(phy->hw, vif, skb, band, NULL))
 		goto out;
-	}
 
 	info = IEEE80211_SKB_CB(skb);
 	if (req->no_cck)

drivers/net/wireless/virtual/mac80211_hwsim.c

@@ -3021,7 +3021,6 @@ static void hw_scan_work(struct work_struct *work)
 						      hwsim->tmp_chan->band,
 						      NULL)) {
 				rcu_read_unlock();
-				kfree_skb(probe);
 				continue;
 			}
 

include/net/mac80211.h

@@ -7407,7 +7407,9 @@ void ieee80211_report_wowlan_wakeup(struct ieee80211_vif *vif,
  * @band: the band to transmit on
  * @sta: optional pointer to get the station to send the frame to
  *
- * Return: %true if the skb was prepared, %false otherwise
+ * Return: %true if the skb was prepared, %false otherwise.
+ * On failure, the skb is freed by this function; callers must not
+ * free it again.
  *
  * Note: must be called under RCU lock
  */

net/mac80211/tx.c

@@ -1899,8 +1899,10 @@ bool ieee80211_tx_prepare_skb(struct ieee80211_hw *hw,
 	struct ieee80211_tx_data tx;
 	struct sk_buff *skb2;
 
-	if (ieee80211_tx_prepare(sdata, &tx, NULL, skb) == TX_DROP)
+	if (ieee80211_tx_prepare(sdata, &tx, NULL, skb) == TX_DROP) {
+		kfree_skb(skb);
 		return false;
+	}
 
 	info->band = band;
 	info->control.vif = vif;