[PATCH mt76-next 23/29] wifi: mt76: mt7615: fix NULL deref in mt7615_mac_set_rates

[Felix Fietkau <nbd@nbd.name>]

mt7615_tx() falls back to the vif BSS wcid when mac80211 hands a frame
over without a station, e.g. while a station is being torn down, and to
the global wcid when there is no vif either. Both tx_prepare_skb
implementations derive a mt7615_sta from that wcid unconditionally and,
if the rate control probe flag is set, pass it to mt7615_mac_set_rates(),
which dereferences the NULL vif backpointer of the per-vif embedded sta:

  Unable to handle kernel read from unreadable memory at virtual
  address 0000000000000002
  ...
  pc : mt7615_mac_set_rates
  lr : mt7615_tx_prepare_skb

Neither entry is rate controlled by mac80211: the embedded sta has an
empty rate set, and for the global wcid the container_of does not yield a
valid mt7615_sta at all. Only resolve the sta for wcids that belong to a
station.

Reported-by: Chad Monroe <chad@monroe.io>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
---
 drivers/net/wireless/mediatek/mt76/mt7615/pci_mac.c  | 5 +++--
 drivers/net/wireless/mediatek/mt76/mt7615/usb_sdio.c | 5 +++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/net/wireless/mediatek/mt76/mt7615/pci_mac.c b/drivers/net/wireless/mediatek/mt76/mt7615/pci_mac.c
index 53cb1eed1e4f..0aca590f2bae 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7615/pci_mac.c
+++ b/drivers/net/wireless/mediatek/mt76/mt7615/pci_mac.c
@@ -68,10 +68,11 @@ int mt7615_tx_prepare_skb(struct mt76_dev *mdev, void *txwi_ptr,
 	int pid, id;
 	u8 *txwi = (u8 *)txwi_ptr;
 	struct mt76_txwi_cache *t;
-	struct mt7615_sta *msta;
+	struct mt7615_sta *msta = NULL;
 	void *txp;
 
-	msta = wcid ? container_of(wcid, struct mt7615_sta, wcid) : NULL;
+	if (wcid && wcid->sta)
+		msta = container_of(wcid, struct mt7615_sta, wcid);
 	if (!wcid)
 		wcid = &dev->mt76.global_wcid;
 
diff --git a/drivers/net/wireless/mediatek/mt76/mt7615/usb_sdio.c b/drivers/net/wireless/mediatek/mt76/mt7615/usb_sdio.c
index f4169de939c4..9525d8062b44 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7615/usb_sdio.c
+++ b/drivers/net/wireless/mediatek/mt76/mt7615/usb_sdio.c
@@ -187,10 +187,11 @@ int mt7663_usb_sdio_tx_prepare_skb(struct mt76_dev *mdev, void *txwi_ptr,
 	struct sk_buff *skb = tx_info->skb;
 	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
 	struct ieee80211_key_conf *key = info->control.hw_key;
-	struct mt7615_sta *msta;
+	struct mt7615_sta *msta = NULL;
 	int pad, err, pktid;
 
-	msta = wcid ? container_of(wcid, struct mt7615_sta, wcid) : NULL;
+	if (wcid && wcid->sta)
+		msta = container_of(wcid, struct mt7615_sta, wcid);
 	if (!wcid)
 		wcid = &dev->mt76.global_wcid;
 
-- 
2.53.0