wifi: ath9k_htc: Use __skb_set_length() for resetting urb before resubmit

Syzbot points out that skb_trim() has a sanity check on the existing length of
the skb, which can be uninitialised in some error paths. The intent here is
clearly just to reset the length to zero before resubmitting, so switch to
calling __skb_set_length(skb, 0) directly. In addition, __skb_set_length()
already contains a call to skb_reset_tail_pointer(), so remove the redundant
call.

The syzbot report came from ath9k_hif_usb_reg_in_cb(), but there's a similar
usage of skb_trim() in ath9k_hif_usb_rx_cb(), change both while we're at it.

Reported-by: [email protected]
Signed-off-by: Toke Høiland-Jørgensen <[email protected]>
Signed-off-by: Kalle Valo <[email protected]>
Link: https://patch.msgid.link/[email protected]

Author: tohojo

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

@@ -716,8 +716,7 @@ static void ath9k_hif_usb_rx_cb(struct urb *urb)
 	}
 
 resubmit:
-	skb_reset_tail_pointer(skb);
-	skb_trim(skb, 0);
+	__skb_set_length(skb, 0);
 
 	usb_anchor_urb(urb, &hif_dev->rx_submitted);
 	ret = usb_submit_urb(urb, GFP_ATOMIC);
@@ -754,8 +753,7 @@ static void ath9k_hif_usb_reg_in_cb(struct urb *urb)
 	case -ESHUTDOWN:
 		goto free_skb;
 	default:
-		skb_reset_tail_pointer(skb);
-		skb_trim(skb, 0);
+		__skb_set_length(skb, 0);
 
 		goto resubmit;
 	}