Re: [PATCH net] lantiq: net: fix duplicated skb in rx descriptor ring

[Hauke Mehrtens]

On 6/6/21 10:05 PM, Aleksander Jan Bajkowski wrote:
> The previous commit didn't fix the bug properly. By mistake, it replaces
> the pointer of the next skb in the descriptor ring instead of the current
> one. As a result, the two descriptors are assigned the same SKB. The error
> is seen during the iperf test when skb_put tries to insert a second packet
> and exceeds the available buffer.
>
> Fixes: c7718ee96dbc ("net: lantiq: fix memory corruption in RX ring ")
>
> Signed-off-by: Aleksander Jan Bajkowski <olek2@xxxxx>
> ---
>   drivers/net/ethernet/lantiq_xrx200.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/ethernet/lantiq_xrx200.c b/drivers/net/ethernet/lantiq_xrx200.c
> index 36dc3e5f6218..e710f83b3700 100644
> --- a/drivers/net/ethernet/lantiq_xrx200.c
> +++ b/drivers/net/ethernet/lantiq_xrx200.c
> @@ -193,17 +193,18 @@ static int xrx200_hw_receive(struct xrx200_chan *ch)
>   	int ret;
>   
>   	ret = xrx200_alloc_skb(ch);
> -
> -	ch->dma.desc++;
> -	ch->dma.desc %= LTQ_DESC_NUM;
> -
>   	if (ret) {
>   		ch->skb[ch->dma.desc] = skb;
>   		net_dev->stats.rx_dropped++;
> +		ch->dma.desc++;
> +		ch->dma.desc %= LTQ_DESC_NUM;

I would prefer if you handle this problem in the xrx200_alloc_skb() 
function.

You could also provide the skb to xrx200_alloc_skb() and use it in case 
a problem occurs, you would loose the current received packet, but the 
DMA ring stays valid.

An other solution would be to not set LTQ_DMA_OWN, then the DMA hardware 
will not use this memory, but we have to handle it somewhere else.

I am also not sure if the wmb() is needed, the desc_base was allocated 
with dma_alloc_coherent(), I assume that the changes are directly 
written to the memory and not reordered.

>   		netdev_err(net_dev, "failed to allocate new rx buffer\n");
>   		return ret;
>   	}
>   
> +	ch->dma.desc++;
> +	ch->dma.desc %= LTQ_DESC_NUM;
> +
>   	skb_put(skb, len);
>   	skb->protocol = eth_type_trans(skb, net_dev);
>   	netif_receive_skb(skb);

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