mt76: usb: fix possible memory leak in mt76u_buf_free

Move q->ndesc initialization before the for loop in mt76u_alloc_rx
since otherwise allocated urbs will not be freed in mt76u_buf_free
Double-check scatterlist pointer in mt76u_buf_free

Fixes: b40b15e1 ("mt76: add usb support to mt76 layer")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

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

@@ -330,10 +330,16 @@ int mt76u_buf_alloc(struct mt76_dev *dev, struct mt76u_buf *buf,
 void mt76u_buf_free(struct mt76u_buf *buf)
 {
 	struct urb *urb = buf->urb;
+	struct scatterlist *sg;
 	int i;
 
-	for (i = 0; i < urb->num_sgs; i++)
-		skb_free_frag(sg_virt(&urb->sg[i]));
+	for (i = 0; i < urb->num_sgs; i++) {
+		sg = &urb->sg[i];
+		if (!sg)
+			continue;
+
+		skb_free_frag(sg_virt(sg));
+	}
 	usb_free_urb(buf->urb);
 }
 EXPORT_SYMBOL_GPL(mt76u_buf_free);
@@ -537,7 +543,8 @@ static int mt76u_alloc_rx(struct mt76_dev *dev)
 		nsgs = 1;
 	}
 
-	for (i = 0; i < MT_NUM_RX_ENTRIES; i++) {
+	q->ndesc = MT_NUM_RX_ENTRIES;
+	for (i = 0; i < q->ndesc; i++) {
 		err = mt76u_buf_alloc(dev, &q->entry[i].ubuf,
 				      nsgs, q->buf_size,
 				      SKB_WITH_OVERHEAD(q->buf_size),
@@ -545,7 +552,6 @@ static int mt76u_alloc_rx(struct mt76_dev *dev)
 		if (err < 0)
 			return err;
 	}
-	q->ndesc = MT_NUM_RX_ENTRIES;
 
 	return mt76u_submit_rx_buffers(dev);
 }