linux-kernel - Re: [PATCH net] net: sonic: remove dev_kfree_skb before return NETDEV_TX

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Date:   Wed, 4 Sep 2019 12:19:51 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Mao Wenan <maowenan@...wei.com>, tsbogend@...ha.franken.de,
        davem@...emloft.net
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH net] net: sonic: remove dev_kfree_skb before return
 NETDEV_TX_BUSY



On 9/4/19 11:42 AM, Mao Wenan wrote:
> When dma_map_single is failed to map buffer, skb can't be freed
> before sonic driver return to stack with NETDEV_TX_BUSY, because
> this skb may be requeued to qdisc, it might trigger use-after-free.
> 
> Fixes: d9fb9f384292 ("*sonic/natsemi/ns83829: Move the National Semi-conductor drivers")
> Signed-off-by: Mao Wenan <maowenan@...wei.com>
> ---
>  drivers/net/ethernet/natsemi/sonic.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/natsemi/sonic.c b/drivers/net/ethernet/natsemi/sonic.c
> index d0a01e8f000a..248a8f22a33b 100644
> --- a/drivers/net/ethernet/natsemi/sonic.c
> +++ b/drivers/net/ethernet/natsemi/sonic.c
> @@ -233,7 +233,6 @@ static int sonic_send_packet(struct sk_buff *skb, struct net_device *dev)
>  	laddr = dma_map_single(lp->device, skb->data, length, DMA_TO_DEVICE);
>  	if (!laddr) {
>  		printk(KERN_ERR "%s: failed to map tx DMA buffer.\n", dev->name);
> -		dev_kfree_skb(skb);
>  		return NETDEV_TX_BUSY;
>  	}
>  
> 

That is the wrong way to fix this bug.

What guarantee do we have that the mapping operation will succeed next time we attempt
the transmit (and the dma_map_single() operation) ?

NETDEV_TX_BUSY is very dangerous, this might trigger an infinite loop.

I would rather leave the dev_kfree_skb(skb), and return NETDEV_TX_OK

Also the printk(KERN_ERR ...) should be replaced by pr_err_ratelimited(...)

NETDEV_TX_BUSY really should only be used by drivers that call netif_tx_stop_queue()
at the wrong moment.