[PATCH v1] wifi: mt76: mt792x: fix memory leak in USB TX path

[PATCH v1] wifi: mt76: mt792x: fix memory leak in USB TX path

[Eason Lai]

When tx_prepare_skb() returns an error in the USB TX path, the
skb is not freed, leading to a memory leak. This can occur when
zero-length frames (such as WNM NULL frames) are dropped to prevent
potential hardware TX hangs.

Fix this by properly releasing the skb with ieee80211_tx_status_ext()
when tx_prepare_skb() fails.

Fixes: b40b15e1521f ("mt76: add usb support to mt76 layer")
Signed-off-by: Eason Lai <eason.lai@mediatek.com>
---
 drivers/net/wireless/mediatek/mt76/usb.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wireless/mediatek/mt76/usb.c b/drivers/net/wireless/mediatek/mt76/usb.c
index a9af3aa6b80a..e266509a7873 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,
 	};
@@ -906,11 +909,11 @@ mt76u_tx_queue_skb(struct mt76_phy *phy, struct mt76_queue *q,
 	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;
+		goto err_free_skb;
 
 	mt76u_fill_bulk_urb(dev, USB_DIR_OUT, q->ep, q->entry[idx].urb,
 			    mt76u_complete_tx, &q->entry[idx]);
@@ -921,6 +924,13 @@ 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);
+	return err;
 }
 
 static void mt76u_tx_kick(struct mt76_dev *dev, struct mt76_queue *q)
-- 
2.45.2

Re: [PATCH v1] wifi: mt76: mt792x: fix memory leak in USB TX path

[Devin Wittmayer]

> 	err = mt76u_tx_setup_buffers(dev, tx_info.skb, q->entry[idx].urb);
> 	if (err < 0)
> -		return err;
> +		goto err_free_skb;

The first hunk looks right, that is what mt76s_tx_queue_skb already does.

The second one I think is a problem. By then tx_prepare_skb has succeeded, so
the skb can already be in wcid->pktid, and nothing takes it out before the
free. mt76_tx_status_skb_get walks that idr on every tx status event and reads
the cb off each entry, so this looks like it turns the leak into a
use-after-free. mt7925 asks for tx status every quarter second per station, so
there is usually an entry there.

mt76_tx_complete_skb would cover it instead. It marks the skb DMA_DONE and
leaves it in the idr for the existing timeout to reap.