mt76: mt7615: fix NULL pointer dereference in tx_prepare_skb()

Fix theoretical NULL pointer dereference in mt7615_tx_prepare_skb and
mt7663_usb_sdio_tx_prepare_skb routines. This issue has been identified
by code analysis.

Fixes: 6aa4ed79 ("mt76: mt7615: implement DMA support for MT7622")
Fixes: 4bb586bc ("mt76: mt7663u: sync probe sampling with rate configuration")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7615/pci_mac.c

@@ -131,20 +131,21 @@ int mt7615_tx_prepare_skb(struct mt76_dev *mdev, void *txwi_ptr,
 			  struct mt76_tx_info *tx_info)
 {
 	struct mt7615_dev *dev = container_of(mdev, struct mt7615_dev, mt76);
-	struct mt7615_sta *msta = container_of(wcid, struct mt7615_sta, wcid);
 	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(tx_info->skb);
 	struct ieee80211_key_conf *key = info->control.hw_key;
 	int pid, id;
 	u8 *txwi = (u8 *)txwi_ptr;
 	struct mt76_txwi_cache *t;
+	struct mt7615_sta *msta;
 	void *txp;
 
+	msta = wcid ? container_of(wcid, struct mt7615_sta, wcid) : NULL;
 	if (!wcid)
 		wcid = &dev->mt76.global_wcid;
 
 	pid = mt76_tx_status_skb_add(mdev, wcid, tx_info->skb);
 
-	if (info->flags & IEEE80211_TX_CTL_RATE_CTRL_PROBE) {
+	if ((info->flags & IEEE80211_TX_CTL_RATE_CTRL_PROBE) && msta) {
 		struct mt7615_phy *phy = &dev->phy;
 
 		if ((info->hw_queue & MT_TX_HW_QUEUE_EXT_PHY) && mdev->phy2)

drivers/net/wireless/mediatek/mt76/mt7615/usb_sdio.c

@@ -191,14 +191,15 @@ int mt7663_usb_sdio_tx_prepare_skb(struct mt76_dev *mdev, void *txwi_ptr,
 				   struct ieee80211_sta *sta,
 				   struct mt76_tx_info *tx_info)
 {
-	struct mt7615_sta *msta = container_of(wcid, struct mt7615_sta, wcid);
 	struct mt7615_dev *dev = container_of(mdev, struct mt7615_dev, mt76);
 	struct sk_buff *skb = tx_info->skb;
 	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
+	struct mt7615_sta *msta;
 	int pad;
 
+	msta = wcid ? container_of(wcid, struct mt7615_sta, wcid) : NULL;
 	if ((info->flags & IEEE80211_TX_CTL_RATE_CTRL_PROBE) &&
-	    !msta->rate_probe) {
+	    msta && !msta->rate_probe) {
 		/* request to configure sampling rate */
 		spin_lock_bh(&dev->mt76.lock);
 		mt7615_mac_set_rates(&dev->phy, msta, &info->control.rates[0],