# Re: \[PATCH v3\] virtio-net: Add validation for used length

- From: Jason Wang \<hidden\>
- Date: 2021-05-31 07:51:41 
- List: virtualization
- Page: https://ratatoskr.run/virtualization/2021/05/1396277
- Thread: https://ratatoskr.run/netdev/2021/05/1395149/t.md

```text

在 2021/5/31 下午3:19, Yongji Xie 写道:
> On Mon, May 31, 2021 at 2:49 PM Jason Wang [ref] wrote:
>>
>> 在 2021/5/28 下午8:11, Xie Yongji 写道:
>>> This adds validation for used length (might come
>>> from an untrusted device) to avoid data corruption
>>> or loss.
>>>
>>> Signed-off-by: Xie Yongji <redacted>
>>> ---
>>>    drivers/net/virtio_net.c | 28 +++++++++++++++++++++-------
>>>    1 file changed, 21 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index 073fec4c0df1..01f15b65824c 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -732,6 +732,17 @@ static struct sk_buff *receive_small(struct net_device *dev,
>>>
>>>        rcu_read_lock();
>>>        xdp_prog = rcu_dereference(rq->xdp_prog);
>>> +     if (unlikely(len > GOOD_PACKET_LEN)) {
>>> +             pr_debug("%s: rx error: len %u exceeds max size %d\n",
>>> +                      dev->name, len, GOOD_PACKET_LEN);
>>> +             dev->stats.rx_length_errors++;
>>> +             if (xdp_prog)
>>> +                     goto err_xdp;
>>> +
>>> +             rcu_read_unlock();
>>> +             put_page(page);
>>> +             return NULL;
>>> +     }
>>>        if (xdp_prog) {
>>>                struct virtio_net_hdr_mrg_rxbuf *hdr = buf + header_offset;
>>>                struct xdp_frame *xdpf;
>>> @@ -888,6 +899,16 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>>>
>>>        rcu_read_lock();
>>>        xdp_prog = rcu_dereference(rq->xdp_prog);
>>> +     if (unlikely(len > truesize)) {
>>> +             pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
>>> +                      dev->name, len, (unsigned long)ctx);
>>> +             dev->stats.rx_length_errors++;
>>> +             if (xdp_prog)
>>> +                     goto err_xdp;
>>> +
>>> +             rcu_read_unlock();
>>> +             goto err_skb;
>>> +     }
>>
>> Patch looks correct but I'd rather not bother XDP here. It would be
>> better if we just do the check before rcu_read_lock() and use err_skb
>> directly() to avoid RCU/XDP stuffs.
>>
> If so, we will miss the statistics of xdp_drops. Is it OK?


It should be ok, we still had drops and it was dropped before dealing with XDP.

The motivation is to have simple codes.

Thanks


>
> Thanks,
> Yongji
>

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```