[PATCH v3 2/2] wifi: mt76: fix memory leak in USB TX path

[Eason Lai <eason.lai@mediatek.com>]

When errors occur in mt76u_tx_queue_skb(), the skb is not properly
freed, leading to memory leaks. This happens when:
- The queue is full
- tx_prepare_skb() drops zero-length frames (such as WMM NULL frames)
  to prevent hardware TX hangs
- Buffer setup fails

Fix by properly releasing the skb with ieee80211_tx_status_ext() when
the queue is full or tx_prepare_skb() fails, and using
mt76_tx_complete_skb() when mt76u_tx_setup_buffers() fails (since
tx_prepare_skb() has already succeeded and may have allocated resources
that need cleanup)

Fixes: b40b15e1521f ("mt76: add usb support to mt76 layer")

Cc: stable@vger.kernel.org
Tested-by: Devin Wittmayer <lucid_duck@justthetip.ca>
Signed-off-by: Eason Lai <eason.lai@mediatek.com>
---
v2:
  - add error handling for queue full and tx_prepare_skb()
  - correct error handling for mt76u_tx_setup_buffers()
v3: add Tested-by: Devin Wittmayer and cc stable
---
 drivers/net/wireless/mediatek/mt76/usb.c | 31 ++++++++++++++++++++----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/drivers/net/wireless/mediatek/mt76/usb.c b/drivers/net/wireless/mediatek/mt76/usb.c
index a9af3aa6b80a..dfc12f6944a7 100644
--- a/drivers/net/wireless/mediatek/mt76/usb.c
+++ b/drivers/net/wireless/mediatek/mt76/usb.c
@@ -893,6 +893,9 @@ mt76u_tx_queue_skb(struct mt76_phy *phy, struct mt76_queue *q,
 		   enum mt76_txq_id qid, struct sk_buff *skb,
 		   struct mt76_wcid *wcid, struct ieee80211_sta *sta)
 {
+	struct ieee80211_tx_status status = {
+		.sta = sta,
+	};
 	struct mt76_tx_info tx_info = {
 		.skb = skb,
 	};
@@ -900,17 +903,27 @@ mt76u_tx_queue_skb(struct mt76_phy *phy, struct mt76_queue *q,
 	u16 idx = q->head;
 	int err;
 
-	if (q->queued == q->ndesc)
-		return -ENOSPC;
+	if (q->queued == q->ndesc) {
+		err = -ENOSPC;
+		goto err_free_skb;
+	}
 
 	skb->prev = skb->next = NULL;
 	err = dev->drv->tx_prepare_skb(dev, NULL, qid, wcid, sta, &tx_info);
 	if (err < 0)
-		return err;
+		goto err_free_skb;
 
 	err = mt76u_tx_setup_buffers(dev, tx_info.skb, q->entry[idx].urb);
-	if (err < 0)
-		return err;
+	if (err < 0) {
+		/*
+		 * mt76_tx_status_skb_get() walks the idr and dereferences
+		 * a freed skb. This SKB is not counted in non-AQL counter
+		 * due to error return, so using 0xffff as wcid to keep
+		 * balanced.
+		 */
+		mt76_tx_complete_skb(dev, 0xffff, tx_info.skb);
+		goto err_ret;
+	}
 
 	mt76u_fill_bulk_urb(dev, USB_DIR_OUT, q->ep, q->entry[idx].urb,
 			    mt76u_complete_tx, &q->entry[idx]);
@@ -921,6 +934,14 @@ mt76u_tx_queue_skb(struct mt76_phy *phy, struct mt76_queue *q,
 	q->queued++;
 
 	return idx;
+
+err_free_skb:
+	status.skb = tx_info.skb;
+	spin_lock_bh(&dev->rx_lock);
+	ieee80211_tx_status_ext(dev->hw, &status);
+	spin_unlock_bh(&dev->rx_lock);
+err_ret:
+	return err;
 }
 
 static void mt76u_tx_kick(struct mt76_dev *dev, struct mt76_queue *q)
-- 
2.45.2