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

[Reshetova, Elena]

> 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 <keescook@xxxxxxxxxxxx>
> > Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
> > Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
> > Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> > ---
> >  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 ?

Sure, I guess you mean mtk_probe() function? I can move the refcount_set to be there
and remove this change. 

Should I resend the modified patch to you (maybe then two of the ethernet patches)?

Best Regards,
Elena.
> 
> >  	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;
>