Re: [Xen-devel] [PATCH 1/1] xen-netfront: do not use ~0U as error return value for xennet_fill_frags()

[Jürgen Groß <jgross@xxxxxxxx>]

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@xxxxxxxxxx>
> ---
>   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)

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