cw1200: Fix concurrency use-after-free bugs in cw1200_hw_scan()

The function cw1200_bss_info_changed() and cw1200_hw_scan() can be
concurrently executed.
The two functions both access a possible shared variable "frame.skb".

This shared variable is freed by dev_kfree_skb() in cw1200_upload_beacon(),
which is called by cw1200_bss_info_changed(). The free operation is
protected by a mutex lock "priv->conf_mutex" in cw1200_bss_info_changed().

In cw1200_hw_scan(), this shared variable is accessed without the
protection of the mutex lock "priv->conf_mutex".
Thus, concurrency use-after-free bugs may occur.

To fix these bugs, the original calls to mutex_lock(&priv->conf_mutex) and
mutex_unlock(&priv->conf_mutex) are moved to the places, which can
protect the accesses to the shared variable.

Signed-off-by: Jia-Ju Bai <[e-mail]>
Signed-off-by: Kalle Valo <[e-mail]>

drivers/net/wireless/st/cw1200/scan.c

···
 	if (req->n_ssids > WSM_SCAN_MAX_NUM_OF_SSIDS)
 		return -EINVAL;
 
+	/* will be unlocked in cw1200_scan_work() */
+	down(&priv->scan.lock);
+	mutex_lock(&priv->conf_mutex);
+
 	frame.skb = ieee80211_probereq_get(hw, priv->vif->addr, NULL, 0,
 		req->ie_len);
 	if (!frame.skb)
···
 	if (req->ie_len)
 		skb_put_data(frame.skb, req->ie, req->ie_len);
 
-	/* will be unlocked in cw1200_scan_work() */
-	down(&priv->scan.lock);
-	mutex_lock(&priv->conf_mutex);
-
 	ret = wsm_set_template_frame(priv, &frame);
 	if (!ret) {
 		/* Host want to be the probe responder. */
 		ret = wsm_set_probe_responder(priv, true);
 	}
 	if (ret) {
+		dev_kfree_skb(frame.skb);
 		mutex_unlock(&priv->conf_mutex);
 		up(&priv->scan.lock);
-		dev_kfree_skb(frame.skb);
 		return ret;
 	}
 
···
 		++priv->scan.n_ssids;
 	}
 
-	mutex_unlock(&priv->conf_mutex);
-
 	if (frame.skb)
 		dev_kfree_skb(frame.skb);
+	mutex_unlock(&priv->conf_mutex);
 	queue_work(priv->workqueue, &priv->scan.work);
 	return 0;
 }