wifi: ath9k: hif_usb: Fix use-after-free in ath9k_hif_usb_reg_in_cb()

It is possible that skb is freed in ath9k_htc_rx_msg(), then
usb_submit_urb() fails and we try to free skb again. It causes
use-after-free bug. Moreover, if alloc_skb() fails, urb->context becomes
NULL but rx_buf is not freed and there can be a memory leak.

The patch removes unnecessary nskb and makes skb processing more clear: it
is supposed that ath9k_htc_rx_msg() either frees old skb or passes its
managing to another callback function.

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

Fixes: 3deff760 ("ath9k_htc: Increase URB count for REG_IN pipe")
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20221008114917.21404-1-pchelkin@ispras.ru

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

@@ -708,14 +708,13 @@ static void ath9k_hif_usb_reg_in_cb(struct urb *urb)
 	struct rx_buf *rx_buf = (struct rx_buf *)urb->context;
 	struct hif_device_usb *hif_dev = rx_buf->hif_dev;
 	struct sk_buff *skb = rx_buf->skb;
-	struct sk_buff *nskb;
 	int ret;
 
 	if (!skb)
 		return;
 
 	if (!hif_dev)
-		goto free;
+		goto free_skb;
 
 	switch (urb->status) {
 	case 0:
@@ -724,7 +723,7 @@ static void ath9k_hif_usb_reg_in_cb(struct urb *urb)
 	case -ECONNRESET:
 	case -ENODEV:
 	case -ESHUTDOWN:
-		goto free;
+		goto free_skb;
 	default:
 		skb_reset_tail_pointer(skb);
 		skb_trim(skb, 0);
@@ -735,25 +734,27 @@ static void ath9k_hif_usb_reg_in_cb(struct urb *urb)
 	if (likely(urb->actual_length != 0)) {
 		skb_put(skb, urb->actual_length);
 
-		/* Process the command first */
+		/*
+		 * Process the command first.
+		 * skb is either freed here or passed to be
+		 * managed to another callback function.
+		 */
 		ath9k_htc_rx_msg(hif_dev->htc_handle, skb,
 				 skb->len, USB_REG_IN_PIPE);
 
-
-		nskb = alloc_skb(MAX_REG_IN_BUF_SIZE, GFP_ATOMIC);
-		if (!nskb) {
+		skb = alloc_skb(MAX_REG_IN_BUF_SIZE, GFP_ATOMIC);
+		if (!skb) {
 			dev_err(&hif_dev->udev->dev,
 				"ath9k_htc: REG_IN memory allocation failure\n");
-			urb->context = NULL;
-			return;
+			goto free_rx_buf;
 		}
 
-		rx_buf->skb = nskb;
+		rx_buf->skb = skb;
 
 		usb_fill_int_urb(urb, hif_dev->udev,
 				 usb_rcvintpipe(hif_dev->udev,
 						 USB_REG_IN_PIPE),
-				 nskb->data, MAX_REG_IN_BUF_SIZE,
+				 skb->data, MAX_REG_IN_BUF_SIZE,
 				 ath9k_hif_usb_reg_in_cb, rx_buf, 1);
 	}
 
@@ -762,12 +763,13 @@ static void ath9k_hif_usb_reg_in_cb(struct urb *urb)
 	ret = usb_submit_urb(urb, GFP_ATOMIC);
 	if (ret) {
 		usb_unanchor_urb(urb);
-		goto free;
+		goto free_skb;
 	}
 
 	return;
-free:
+free_skb:
 	kfree_skb(skb);
+free_rx_buf:
 	kfree(rx_buf);
 	urb->context = NULL;
 }