ath9k: fix use-after-free in ath9k_hif_usb_rx_cb

Syzbot reported use-after-free Read in ath9k_hif_usb_rx_cb() [0]. The
problem was in incorrect htc_handle->drv_priv initialization.

Probable call trace which can trigger use-after-free:

ath9k_htc_probe_device()
  /* htc_handle->drv_priv = priv; */
  ath9k_htc_wait_for_target()      <--- Failed
  ieee80211_free_hw()		   <--- priv pointer is freed

<IRQ>
...
ath9k_hif_usb_rx_cb()
  ath9k_hif_usb_rx_stream()
   RX_STAT_INC()		<--- htc_handle->drv_priv access

In order to not add fancy protection for drv_priv we can move
htc_handle->drv_priv initialization at the end of the
ath9k_htc_probe_device() and add helper macro to make
all *_STAT_* macros NULL safe, since syzbot has reported related NULL
deref in that macros [1]

Link: https://syzkaller.appspot.com/bug?id=6ead44e37afb6866ac0c7dd121b4ce07cb665f60 [0]
Link: https://syzkaller.appspot.com/bug?id=b8101ffcec107c0567a0cd8acbbacec91e9ee8de [1]
Fixes: fb9987d0 ("ath9k_htc: Support for AR9271 chipset.")
Reported-and-tested-by: syzbot+03110230a11411024147@syzkaller.appspotmail.com
Reported-and-tested-by: syzbot+c6dde1f690b60e0b9fbe@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/d57bbedc857950659bfacac0ab48790c1eda00c8.1655145743.git.paskripkin@gmail.com

drivers/net/wireless/ath/ath9k/htc.h

@@ -327,11 +327,11 @@ static inline struct ath9k_htc_tx_ctl *HTC_SKB_CB(struct sk_buff *skb)
 }
 
 #ifdef CONFIG_ATH9K_HTC_DEBUGFS
-
-#define TX_STAT_INC(c) (hif_dev->htc_handle->drv_priv->debug.tx_stats.c++)
-#define TX_STAT_ADD(c, a) (hif_dev->htc_handle->drv_priv->debug.tx_stats.c += a)
-#define RX_STAT_INC(c) (hif_dev->htc_handle->drv_priv->debug.skbrx_stats.c++)
-#define RX_STAT_ADD(c, a) (hif_dev->htc_handle->drv_priv->debug.skbrx_stats.c += a)
+#define __STAT_SAFE(expr) (hif_dev->htc_handle->drv_priv ? (expr) : 0)
+#define TX_STAT_INC(c) __STAT_SAFE(hif_dev->htc_handle->drv_priv->debug.tx_stats.c++)
+#define TX_STAT_ADD(c, a) __STAT_SAFE(hif_dev->htc_handle->drv_priv->debug.tx_stats.c += a)
+#define RX_STAT_INC(c) __STAT_SAFE(hif_dev->htc_handle->drv_priv->debug.skbrx_stats.c++)
+#define RX_STAT_ADD(c, a) __STAT_SAFE(hif_dev->htc_handle->drv_priv->debug.skbrx_stats.c += a)
 #define CAB_STAT_INC   priv->debug.tx_stats.cab_queued++
 
 #define TX_QSTAT_INC(q) (priv->debug.tx_stats.queue_stats[q]++)

drivers/net/wireless/ath/ath9k/htc_drv_init.c

@@ -944,7 +944,6 @@ int ath9k_htc_probe_device(struct htc_target *htc_handle, struct device *dev,
 	priv->hw = hw;
 	priv->htc = htc_handle;
 	priv->dev = dev;
-	htc_handle->drv_priv = priv;
 	SET_IEEE80211_DEV(hw, priv->dev);
 
 	ret = ath9k_htc_wait_for_target(priv);
@@ -965,6 +964,8 @@ int ath9k_htc_probe_device(struct htc_target *htc_handle, struct device *dev,
 	if (ret)
 		goto err_init;
 
+	htc_handle->drv_priv = priv;
+
 	return 0;
 
 err_init: