netdev - Re: [PATCH 1/1] xen-netfront: do not use ~0U as error return value for xennet_fill

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Date:   Mon, 30 Sep 2019 09:56:12 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>,
        xen-devel@...ts.xenproject.org, netdev@...r.kernel.org
Cc:     davem@...emloft.net, sstabellini@...nel.org,
        boris.ostrovsky@...cle.com, joe.jin@...cle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] xen-netfront: do not use ~0U as error return value
 for xennet_fill_frags()

On 30.09.19 09:44, Dongli Zhang wrote:
> xennet_fill_frags() uses ~0U as return value when the sk_buff is not able
> to cache extra fragments. This is incorrect because the return type of
> xennet_fill_frags() is RING_IDX and 0xffffffff is an expected value for
> ring buffer index.
> 
> In the situation when the rsp_cons is approaching 0xffffffff, the return
> value of xennet_fill_frags() may become 0xffffffff which xennet_poll() (the
> caller) would regard as error. As a result, queue->rx.rsp_cons is set
> incorrectly because it is updated only when there is error. If there is no
> error, xennet_poll() would be responsible to update queue->rx.rsp_cons.
> Finally, queue->rx.rsp_cons would point to the rx ring buffer entries whose
> queue->rx_skbs[i] and queue->grant_rx_ref[i] are already cleared to NULL.
> This leads to NULL pointer access in the next iteration to process rx ring
> buffer entries.
> 
> The symptom is similar to the one fixed in
> commit 00b368502d18 ("xen-netfront: do not assume sk_buff_head list is
> empty in error handling").
> 
> This patch uses an extra argument to help return if there is error in
> xennet_fill_frags().

Hmm, I wonder if it wouldn't be better to have the ring index returned
via the new pointer and let return xennet_fill_frags() 0 in case of
success and -ENOENT otherwise.

This would avoid having to introduce the local errno variable.


Juergen

> 
> Fixes: ad4f15dc2c70 ("xen/netfront: don't bug in case of too many frags")
> Signed-off-by: Dongli Zhang <dongli.zhang@...cle.com>
> ---
>   drivers/net/xen-netfront.c | 12 +++++++++---
>   1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index e14ec75..c2a1e09 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -889,11 +889,14 @@ static int xennet_set_skb_gso(struct sk_buff *skb,
>   
>   static RING_IDX xennet_fill_frags(struct netfront_queue *queue,
>   				  struct sk_buff *skb,
> -				  struct sk_buff_head *list)
> +				  struct sk_buff_head *list,
> +				  int *errno)
>   {
>   	RING_IDX cons = queue->rx.rsp_cons;
>   	struct sk_buff *nskb;
>   
> +	*errno = 0;
> +
>   	while ((nskb = __skb_dequeue(list))) {
>   		struct xen_netif_rx_response *rx =
>   			RING_GET_RESPONSE(&queue->rx, ++cons);
> @@ -908,6 +911,7 @@ static RING_IDX xennet_fill_frags(struct netfront_queue *queue,
>   		if (unlikely(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS)) {
>   			queue->rx.rsp_cons = ++cons + skb_queue_len(list);
>   			kfree_skb(nskb);
> +			*errno = -ENOENT;
>   			return ~0U;
>   		}
>   
> @@ -1009,6 +1013,8 @@ static int xennet_poll(struct napi_struct *napi, int budget)
>   	i = queue->rx.rsp_cons;
>   	work_done = 0;
>   	while ((i != rp) && (work_done < budget)) {
> +		int errno;
> +
>   		memcpy(rx, RING_GET_RESPONSE(&queue->rx, i), sizeof(*rx));
>   		memset(extras, 0, sizeof(rinfo.extras));
>   
> @@ -1045,8 +1051,8 @@ static int xennet_poll(struct napi_struct *napi, int budget)
>   		skb->data_len = rx->status;
>   		skb->len += rx->status;
>   
> -		i = xennet_fill_frags(queue, skb, &tmpq);
> -		if (unlikely(i == ~0U))
> +		i = xennet_fill_frags(queue, skb, &tmpq, &errno);
> +		if (unlikely(errno == -ENOENT))
>   			goto err;
>   
>   		if (rx->flags & XEN_NETRXF_csum_blank)
>