wifi: zd1211rw: Fix potential NULL pointer dereference in zd_mac_tx_to_dev()

In the Linux kernel, the following vulnerability has been resolved:

wifi: zd1211rw: Fix potential NULL pointer dereference in zd_mac_tx_to_dev()

There is a potential NULL pointer dereference in zd_mac_tx_to_dev(). For example, the following is possible:

T0 T1 zd_mac_tx_to_dev() /* len == skb_queue_len(q) */ while (len > ZD_MAC_MAX_ACK_WAITERS) {

filter_ack() spin_lock_irqsave(&q->lock, flags); /* position == skb_queue_len(q) */ for (i=1; i<position; i++) skb = __skb_dequeue(q)

if (mac->type == NL80211_IFTYPE_AP) skb = __skb_dequeue(q); spin_unlock_irqrestore(&q->lock, flags);

skb_dequeue() -> NULL

Since there is a small gap between checking skb queue length and skb being unconditionally dequeued in zd_mac_tx_to_dev(), skb_dequeue() can return NULL. Then the pointer is passed to zd_mac_tx_status() where it is dereferenced.

In order to avoid potential NULL pointer dereference due to situations like above, check if skb is not NULL before passing it to zd_mac_tx_status().

Found by Linux Verification Center (linuxtesting.org) with SVACE.