Re: [PATCH v2 04/14] net: axienet: Fix DMA descriptor cleanup path

[Andre Przywara <andre.przywara@arm.com>]

On Mon,  9 Mar 2020 18:18:41 +0000
Andre Przywara <andre.przywara@arm.com> wrote:

Hi Radhey,

you looked at this patch before, it was [PATCH 03/14] back then.
You ended up saying "Looks fine then.", but I didn't dare to convert this into a "Reviewed-by:" tag.
Just a hint that I didn't change anything, that might simplify the review.

Cheers,
Andre

> When axienet_dma_bd_init() bails out during the initialisation process,
> it might do so with parts of the structure already allocated and
> initialised, while other parts have not been touched yet. Before
> returning in this case, we call axienet_dma_bd_release(), which does not
> take care of this corner case.
> This is most obvious by the first loop happily dereferencing
> lp->rx_bd_v, which we actually check to be non NULL *afterwards*.
> 
> Make sure we only unmap or free already allocated structures, by:
> - directly returning with -ENOMEM if nothing has been allocated at all
> - checking for lp->rx_bd_v to be non-NULL *before* using it
> - only unmapping allocated DMA RX regions
> 
> This avoids NULL pointer dereferences when initialisation fails.
> 
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> ---
>  .../net/ethernet/xilinx/xilinx_axienet_main.c | 43 ++++++++++++-------
>  1 file changed, 28 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> index 64f73533cabe..9903205d57ec 100644
> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> @@ -160,24 +160,37 @@ static void axienet_dma_bd_release(struct net_device *ndev)
>  	int i;
>  	struct axienet_local *lp = netdev_priv(ndev);
>  
> +	/* If we end up here, tx_bd_v must have been DMA allocated. */
> +	dma_free_coherent(ndev->dev.parent,
> +			  sizeof(*lp->tx_bd_v) * lp->tx_bd_num,
> +			  lp->tx_bd_v,
> +			  lp->tx_bd_p);
> +
> +	if (!lp->rx_bd_v)
> +		return;
> +
>  	for (i = 0; i < lp->rx_bd_num; i++) {
> -		dma_unmap_single(ndev->dev.parent, lp->rx_bd_v[i].phys,
> -				 lp->max_frm_size, DMA_FROM_DEVICE);
> +		/* A NULL skb means this descriptor has not been initialised
> +		 * at all.
> +		 */
> +		if (!lp->rx_bd_v[i].skb)
> +			break;
> +
>  		dev_kfree_skb(lp->rx_bd_v[i].skb);
> -	}
>  
> -	if (lp->rx_bd_v) {
> -		dma_free_coherent(ndev->dev.parent,
> -				  sizeof(*lp->rx_bd_v) * lp->rx_bd_num,
> -				  lp->rx_bd_v,
> -				  lp->rx_bd_p);
> -	}
> -	if (lp->tx_bd_v) {
> -		dma_free_coherent(ndev->dev.parent,
> -				  sizeof(*lp->tx_bd_v) * lp->tx_bd_num,
> -				  lp->tx_bd_v,
> -				  lp->tx_bd_p);
> +		/* For each descriptor, we programmed cntrl with the (non-zero)
> +		 * descriptor size, after it had been successfully allocated.
> +		 * So a non-zero value in there means we need to unmap it.
> +		 */
> +		if (lp->rx_bd_v[i].cntrl)
> +			dma_unmap_single(ndev->dev.parent, lp->rx_bd_v[i].phys,
> +					 lp->max_frm_size, DMA_FROM_DEVICE);
>  	}
> +
> +	dma_free_coherent(ndev->dev.parent,
> +			  sizeof(*lp->rx_bd_v) * lp->rx_bd_num,
> +			  lp->rx_bd_v,
> +			  lp->rx_bd_p);
>  }
>  
>  /**
> @@ -207,7 +220,7 @@ static int axienet_dma_bd_init(struct net_device *ndev)
>  					 sizeof(*lp->tx_bd_v) * lp->tx_bd_num,
>  					 &lp->tx_bd_p, GFP_KERNEL);
>  	if (!lp->tx_bd_v)
> -		goto out;
> +		return -ENOMEM;
>  
>  	lp->rx_bd_v = dma_alloc_coherent(ndev->dev.parent,
>  					 sizeof(*lp->rx_bd_v) * lp->rx_bd_num,