Re: [Xen-devel] Question on xen-netfront code to fix a potential ring buffer corruption

[Dongli Zhang <dongli.zhang@xxxxxxxxxx>]

Hi Juergen,

On 8/27/19 2:13 PM, Juergen Gross wrote:
> On 18.08.19 10:31, Dongli Zhang wrote:
>> Hi,
>>
>> Would you please help confirm why the condition at line 908 is ">="?
>>
>> In my opinion, we would only hit "skb_shinfo(skb)->nr_frag == MAX_SKB_FRAGS" 
>> at
>> line 908.
>>
>> 890 static RING_IDX xennet_fill_frags(struct netfront_queue *queue,
>> 891                                   struct sk_buff *skb,
>> 892                                   struct sk_buff_head *list)
>> 893 {
>> 894         RING_IDX cons = queue->rx.rsp_cons;
>> 895         struct sk_buff *nskb;
>> 896
>> 897         while ((nskb = __skb_dequeue(list))) {
>> 898                 struct xen_netif_rx_response *rx =
>> 899                         RING_GET_RESPONSE(&queue->rx, ++cons);
>> 900                 skb_frag_t *nfrag = &skb_shinfo(nskb)->frags[0];
>> 901
>> 902                 if (skb_shinfo(skb)->nr_frags == MAX_SKB_FRAGS) {
>> 903                         unsigned int pull_to = 
>> NETFRONT_SKB_CB(skb)->pull_to;
>> 904
>> 905                         BUG_ON(pull_to < skb_headlen(skb));
>> 906                         __pskb_pull_tail(skb, pull_to - 
>> skb_headlen(skb));
>> 907                 }
>> 908                 if (unlikely(skb_shinfo(skb)->nr_frags >= 
>> MAX_SKB_FRAGS)) {
>> 909                         queue->rx.rsp_cons = ++cons;
>> 910                         kfree_skb(nskb);
>> 911                         return ~0U;
>> 912                 }
>> 913
>> 914                 skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,
>> 915                                 skb_frag_page(nfrag),
>> 916                                 rx->offset, rx->status, PAGE_SIZE);
>> 917
>> 918                 skb_shinfo(nskb)->nr_frags = 0;
>> 919                 kfree_skb(nskb);
>> 920         }
>> 921
>> 922         return cons;
>> 923 }
>>
>>
>> The reason that I ask about this is because I am considering below patch to
>> avoid a potential xen-netfront ring buffer corruption.
>>
>> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
>> index 8d33970..48a2162 100644
>> --- a/drivers/net/xen-netfront.c
>> +++ b/drivers/net/xen-netfront.c
>> @@ -906,7 +906,7 @@ static RING_IDX xennet_fill_frags(struct netfront_queue
>> *queue,
>>                          __pskb_pull_tail(skb, pull_to - skb_headlen(skb));
>>                  }
>>                  if (unlikely(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS)) {
>> -                       queue->rx.rsp_cons = ++cons;
>> +                       queue->rx.rsp_cons = cons + skb_queue_len(list) + 1;
>>                          kfree_skb(nskb);
>>                          return ~0U;
>>                  }
>>
>>
>> If there is skb left in list when we return ~0U, queue->rx.rsp_cons may be 
>> set
>> incorrectly.
> 
> Sa basically you want to consume the responses for all outstanding skbs
> in the list?
> 

I think there would be bug if there is skb left in the list.

This is what is implanted in xennet_poll() when there is error of processing
extra info like below. As at line 1034, if there is error, all outstanding skb
are consumed.

 985 static int xennet_poll(struct napi_struct *napi, int budget)
... ...
1028                 if (extras[XEN_NETIF_EXTRA_TYPE_GSO - 1].type) {
1029                         struct xen_netif_extra_info *gso;
1030                         gso = &extras[XEN_NETIF_EXTRA_TYPE_GSO - 1];
1031
1032                         if (unlikely(xennet_set_skb_gso(skb, gso))) {
1033                                 __skb_queue_head(&tmpq, skb);
1034                                 queue->rx.rsp_cons += skb_queue_len(&tmpq);
1035                                 goto err;
1036                         }
1037                 }

The reason we need to consume all outstanding skb is because
xennet_get_responses() would reset both queue->rx_skbs[i] and
queue->grant_rx_ref[i] to NULL before enqueue all outstanding skb to the list
(e.g., &tmpq), by xennet_get_rx_skb() and xennet_get_rx_ref().

If we do not consume all of them, we would hit NULL in queue->rx_skbs[i] in next
iteration of while loop in xennet_poll().

That is, if we do not consume all outstanding skb, the queue->rx.rsp_cons may
refer to a response whose queue->rx_skbs[i] and queue->grant_rx_ref[i] are
already reset to NULL.

Dongli Zhang

>>
>> While I am trying to make up a case that would hit the corruption, I could 
>> not
>> explain why (unlikely(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS)), but not
>> just "==". Perhaps __pskb_pull_tail() may fail although pull_to is less than
>> skb_headlen(skb).
> 
> I don't think nr_frags can be larger than MAX_SKB_FRAGS. OTOH I don't
> think it hurts to have a safety net here in order to avoid problems.
> 
> Originally this was BUG_ON(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS)
> so that might explain the ">=".
> 
> 
> Juergen
> 
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