net: stmmac: WARN if tx_skbuff entries are reused before cleared

The current code assumes that a tx_skbuff entry has been cleared
by stmmac_tx_clean() before stmmac_xmit()/stmmac_tso_xmit()
assigns a new skb to that entry. However, since we never check
the current value before overwriting it, it is theoretically
possible that a non-NULL value is overwritten.

Add WARN_ONs to verify that each entry in tx_skbuff is NULL
before it is assigned a new value.
Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -2794,6 +2794,7 @@ static void stmmac_tso_allocator(struct stmmac_priv *priv, unsigned int des,
 
 	while (tmp_len > 0) {
 		tx_q->cur_tx = STMMAC_GET_ENTRY(tx_q->cur_tx, DMA_TX_SIZE);
+		WARN_ON(tx_q->tx_skbuff[tx_q->cur_tx]);
 		desc = tx_q->dma_tx + tx_q->cur_tx;
 
 		desc->des0 = cpu_to_le32(des + (total_len - tmp_len));
@@ -2878,6 +2879,7 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
 		priv->hw->desc->set_mss(mss_desc, mss);
 		tx_q->mss = mss;
 		tx_q->cur_tx = STMMAC_GET_ENTRY(tx_q->cur_tx, DMA_TX_SIZE);
+		WARN_ON(tx_q->tx_skbuff[tx_q->cur_tx]);
 	}
 
 	if (netif_msg_tx_queued(priv)) {
@@ -2888,6 +2890,7 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	first_entry = tx_q->cur_tx;
+	WARN_ON(tx_q->tx_skbuff[first_entry]);
 
 	desc = tx_q->dma_tx + first_entry;
 	first = desc;
@@ -3062,6 +3065,7 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	entry = tx_q->cur_tx;
 	first_entry = entry;
+	WARN_ON(tx_q->tx_skbuff[first_entry]);
 
 	csum_insertion = (skb->ip_summed == CHECKSUM_PARTIAL);
 
@@ -3090,6 +3094,7 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		bool last_segment = (i == (nfrags - 1));
 
 		entry = STMMAC_GET_ENTRY(entry, DMA_TX_SIZE);
+		WARN_ON(tx_q->tx_skbuff[entry]);
 
 		if (likely(priv->extend_desc))
 			desc = (struct dma_desc *)(tx_q->dma_etx + entry);