netdev - Re: [PATCH V2] amd-xgbe: set skb to NULL after freeing it

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Date:   Wed, 28 Nov 2018 14:23:48 +0000
From:   "Lendacky, Thomas" <Thomas.Lendacky@....com>
To:     Pan Bian <bianpan2016@....com>,
        "David S. Miller" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] amd-xgbe: set skb to NULL after freeing it

On 11/28/2018 03:06 AM, Pan Bian wrote:
> The buffer skb is freed via dev_kfree_skb in a loop. After freeing skb, 
> the value of packet_count is updated via packet_count++. If packet_count
> happens to equal the upper bound budget, the loop will be broken and skb 
> may be assigned to rdata->state.skb. Resulting that rdata->state.skb may

Except that rdata->state.skb is only assigned if we have received
something and it is (!last || context_next).  If error is ever set,
then we read descriptors as long as (!last || context_next).  Also,
packet->errors can only be set for the last packet which implies no
following context descriptors.  So rdata->state.skb can never be set
with the "bad" skb value.

The only way I can see this occurring is if a context descriptor is
indicated (context_next) along with packet->errors. Are you actually
seeing an error/crash where this is occurring?  If so, then the fix
should be to goto read_again if context_next is set because we want
to remove the context descriptor in this case and not save the state.

Thanks,
Tom

> point to a freed memory chunk. To fix this, the patch sets skb to NULL 
> after dev_kfree_skb(skb).
> 
> Signed-off-by: Pan Bian <bianpan2016@....com>
> ---
> V2: correct the commit log
> ---
>  drivers/net/ethernet/amd/xgbe/xgbe-drv.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/net/ethernet/amd/xgbe/xgbe-drv.c b/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
> index 0cc911f..ac6b82d 100644
> --- a/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
> +++ b/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
> @@ -2754,6 +2754,7 @@ static int xgbe_rx_poll(struct xgbe_channel *channel, int budget)
>  				netif_err(pdata, rx_err, netdev,
>  					  "error in received packet\n");
>  			dev_kfree_skb(skb);
> +			skb = NULL;
>  			goto next_packet;
>  		}
>  
> @@ -2806,6 +2807,7 @@ static int xgbe_rx_poll(struct xgbe_channel *channel, int budget)
>  			netif_err(pdata, rx_err, netdev,
>  				  "packet length exceeds configured MTU\n");
>  			dev_kfree_skb(skb);
> +			skb = NULL;
>  			goto next_packet;
>  		}
>  
>