linux-kernel - Re: [PATCH net v2] net: neigh: don't call kfree_skb() under spin_lock

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Date:   Fri, 19 Aug 2022 15:15:19 +0300
From:   Nikolay Aleksandrov <razor@...ckwall.org>
To:     Yang Yingliang <yangyingliang@...wei.com>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Cc:     den@...nvz.org, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org
Subject: Re: [PATCH net v2] net: neigh: don't call kfree_skb() under
 spin_lock_irqsave()

On 19/08/2022 07:47, Yang Yingliang wrote:
> It is not allowed to call kfree_skb() from hardware interrupt
> context or with interrupts being disabled. So add all skb to
> a tmp list, then free them after spin_unlock_irqrestore() at
> once.
> 
> Fixes: 66ba215cb513 ("neigh: fix possible DoS due to net iface start/stop loop")
> Signed-off-by: Yang Yingliang <yangyingliang@...wei.com>
> ---
>  net/core/neighbour.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index 5b669eb80270..d21c7de1ff1a 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -309,14 +309,17 @@ static int neigh_del_timer(struct neighbour *n)
>  
>  static void pneigh_queue_purge(struct sk_buff_head *list, struct net *net)
>  {
> +	struct sk_buff_head tmp;
>  	unsigned long flags;
>  	struct sk_buff *skb;
>  
> +	skb_queue_head_init(&tmp);
>  	spin_lock_irqsave(&list->lock, flags);
>  	skb = skb_peek(list);
>  	while (skb != NULL) {
>  		struct sk_buff *skb_next = skb_peek_next(skb, list);
>  		struct net_device *dev = skb->dev;
> +
>  		if (net == NULL || net_eq(dev_net(dev), net)) {
>  			struct in_device *in_dev;
>  
> @@ -328,11 +331,16 @@ static void :q

(struct sk_buff_head *list, struct net *net)
>  			__skb_unlink(skb, list);
>  
>  			dev_put(dev);
> -			kfree_skb(skb);
> +			dev_kfree_skb_irq(skb);

this is still doing dev_kfree_skb_irq() instead of attaching the skb to tmp, in fact
tmp seems unused so the loop below does nothing

>  		}
>  		skb = skb_next;
>  	}
>  	spin_unlock_irqrestore(&list->lock, flags);
> +
> +	while ((skb = __skb_dequeue(&tmp))) {
> +		dev_put(skb->dev);

Also note that there's already a dev_put() above

> +		kfree_skb(skb);
> +	}
>  }
>  
>  static void neigh_flush_dev(struct neigh_table *tbl, struct net_device *dev,