ath9k_htc: fix NULL pointer dereference at ath9k_htc_rxep()

syzbot is reporting lockdep warning followed by kernel panic at
ath9k_htc_rxep() [1], for ath9k_htc_rxep() depends on ath9k_rx_init()
being already completed.

Since ath9k_htc_rxep() is set by ath9k_htc_connect_svc(WMI_BEACON_SVC)
 from ath9k_init_htc_services(), it is possible that ath9k_htc_rxep() is
called via timer interrupt before ath9k_rx_init() from ath9k_init_device()
is called.

Since we can't call ath9k_init_device() before ath9k_init_htc_services(),
let's hold ath9k_htc_rxep() no-op until ath9k_rx_init() completes.

Link: https://syzkaller.appspot.com/bug?extid=4d2d56175b934b9a7bf9 [1]
Reported-by: syzbot <syzbot+4d2d56175b934b9a7bf9@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: syzbot <syzbot+4d2d56175b934b9a7bf9@syzkaller.appspotmail.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/2b88f416-b2cb-7a18-d688-951e6dc3fe92@i-love.sakura.ne.jp

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

@@ -281,6 +281,7 @@ struct ath9k_htc_rxbuf {
 struct ath9k_htc_rx {
 	struct list_head rxbuf;
 	spinlock_t rxbuflock;
+	bool initialized;
 };
 
 #define ATH9K_HTC_TX_CLEANUP_INTERVAL 50 /* ms */

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

@@ -1130,6 +1130,10 @@ void ath9k_htc_rxep(void *drv_priv, struct sk_buff *skb,
 	struct ath9k_htc_rxbuf *rxbuf = NULL, *tmp_buf = NULL;
 	unsigned long flags;
 
+	/* Check if ath9k_rx_init() completed. */
+	if (!data_race(priv->rx.initialized))
+		goto err;
+
 	spin_lock_irqsave(&priv->rx.rxbuflock, flags);
 	list_for_each_entry(tmp_buf, &priv->rx.rxbuf, list) {
 		if (!tmp_buf->in_process) {
@@ -1185,6 +1189,10 @@ int ath9k_rx_init(struct ath9k_htc_priv *priv)
 		list_add_tail(&rxbuf->list, &priv->rx.rxbuf);
 	}
 
+	/* Allow ath9k_htc_rxep() to operate. */
+	smp_wmb();
+	priv->rx.initialized = true;
+
 	return 0;
 
 err: