rtlwifi: rtl_pci: Fix problem of too small skb->len

In commit 8020919a ("mac80211: Properly handle SKB with radiotap
only"), buffers whose length is too short cause a WARN_ON(1) to be
executed. This change exposed a fault in rtlwifi drivers, which is fixed
by regarding packets with skb->len <= FCS_LEN as though they are in error
and dropping them. The test is now annotated as likely.

Cc: Stable <stable@vger.kernel.org> # v5.0+
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/realtek/rtlwifi/pci.c

@@ -822,7 +822,7 @@ static void _rtl_pci_rx_interrupt(struct ieee80211_hw *hw)
 		hdr = rtl_get_hdr(skb);
 		fc = rtl_get_fc(skb);
 
-		if (!stats.crc && !stats.hwerror) {
+		if (!stats.crc && !stats.hwerror && (skb->len > FCS_LEN)) {
 			memcpy(IEEE80211_SKB_RXCB(skb), &rx_status,
 			       sizeof(rx_status));
 
@@ -859,6 +859,7 @@ static void _rtl_pci_rx_interrupt(struct ieee80211_hw *hw)
 				_rtl_pci_rx_to_mac80211(hw, skb, rx_status);
 			}
 		} else {
+			/* drop packets with errors or those too short */
 			dev_kfree_skb_any(skb);
 		}
 new_trx_end: