Re: [PATCH 02/15] drivers, net, ethernet: convert mtk_eth.dma_refcnt from atomic_t to refcount_t

[Sean Wang]

On Fri, 2017-10-20 at 10:23 +0300, Elena Reshetova wrote:
> 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 <keesc...@chromium.org>
> Reviewed-by: David Windsor <dwind...@gmail.com>
> Reviewed-by: Hans Liljestrand <ishkam...@gmail.com>
> Signed-off-by: Elena Reshetova <elena.reshet...@intel.com>
> ---
>  drivers/net/ethernet/mediatek/mtk_eth_soc.c | 8 +++++---
>  drivers/net/ethernet/mediatek/mtk_eth_soc.h | 4 +++-
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mediatek/mtk_eth_soc.c 
> b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
> index 5e81a72..54adfd9 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);

the existing driver seems to have a missing initial atomic_set for the
eth->dma_refcnt. 

>       }
> -     atomic_inc(&eth->dma_refcnt);
> +     else
> +             refcount_inc(&eth->dma_refcnt);
>  

how about add the initial refcount_set into probe handler, and keep
logic else unchanged ? 

>       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);
> diff --git a/drivers/net/ethernet/mediatek/mtk_eth_soc.h 
> b/drivers/net/ethernet/mediatek/mtk_eth_soc.h
> index 3d3c24a..a3af466 100644
> --- a/drivers/net/ethernet/mediatek/mtk_eth_soc.h
> +++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.h
> @@ -15,6 +15,8 @@
>  #ifndef MTK_ETH_H
>  #define MTK_ETH_H
>  
> +#include <linux/refcount.h>
> +
>  #define MTK_QDMA_PAGE_SIZE   2048
>  #define      MTK_MAX_RX_LENGTH       1536
>  #define MTK_TX_DMA_BUF_LEN   0x3fff
> @@ -632,7 +634,7 @@ struct mtk_eth {
>       struct regmap                   *pctl;
>       u32                             chip_id;
>       bool                            hwlro;
> -     atomic_t                        dma_refcnt;
> +     refcount_t                      dma_refcnt;
>       struct mtk_tx_ring              tx_ring;
>       struct mtk_rx_ring              rx_ring[MTK_MAX_RX_RING_NUM];
>       struct mtk_rx_ring              rx_ring_qdma;