drivers, net, ethernet: convert mtk_eth.dma_refcnt from atomic_t to refcount_t

atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable mtk_eth.dma_refcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 3 deletions

diff --git a/drivers/net/ethernet/mediatek/mtk_eth_soc.c b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
index 5e81a7263654..54adfd967858 100644
--- a/drivers/net/ethernet/mediatek/mtk_eth_soc.c
+++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
@@ -1817,7 +1817,7 @@ static int mtk_open(struct net_device *dev)
 	struct mtk_eth *eth = mac->hw;
 
 	/* we run 2 netdevs on the same dma ring so we only bring it up once */
-	if (!atomic_read(&eth->dma_refcnt)) {
+	if (!refcount_read(&eth->dma_refcnt)) {
 		int err = mtk_start_dma(eth);
 
 		if (err)
@@ -1827,8 +1827,10 @@ static int mtk_open(struct net_device *dev)
 		napi_enable(&eth->rx_napi);
 		mtk_tx_irq_enable(eth, MTK_TX_DONE_INT);
 		mtk_rx_irq_enable(eth, MTK_RX_DONE_INT);
+		refcount_set(&eth->dma_refcnt, 1);
 	}
-	atomic_inc(&eth->dma_refcnt);
+	else
+		refcount_inc(&eth->dma_refcnt);
 
 	phy_start(dev->phydev);
 	netif_start_queue(dev);
@@ -1868,7 +1870,7 @@ static int mtk_stop(struct net_device *dev)
 	phy_stop(dev->phydev);
 
 	/* only shutdown DMA if this is the last user */
-	if (!atomic_dec_and_test(&eth->dma_refcnt))
+	if (!refcount_dec_and_test(&eth->dma_refcnt))
 		return 0;
 
 	mtk_tx_irq_disable(eth, MTK_TX_DONE_INT);