net: ll_temac: Fix bug causing buffer descriptor overrun

mainline inclusion
from mainline-v5.2-rc1
commit 2c9938e7
category: bugfix
bugzilla: NA
CVE: CVE-2021-38207

---------------------------

As we are actually using a BD for both the skb and each frag contained in
it, the oldest TX BD would be overwritten when there was exactly one BD
less than needed.
Signed-off-by: NEsben Haabendal <esben@geanix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NYue Haibing <yuehaibing@huawei.com>
Reviewed-by: NXiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

drivers/net/ethernet/xilinx/ll_temac_main.c

@@ -689,7 +689,7 @@ temac_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 	start_p = lp->tx_bd_p + sizeof(*lp->tx_bd_v) * lp->tx_bd_tail;
 	cur_p = &lp->tx_bd_v[lp->tx_bd_tail];
 
-	if (temac_check_tx_bd_space(lp, num_frag)) {
+	if (temac_check_tx_bd_space(lp, num_frag + 1)) {
 		if (!netif_queue_stopped(ndev))
 			netif_stop_queue(ndev);
 		return NETDEV_TX_BUSY;