Re: [PATCH net 5/7] virtio_net: Don't process redirected XDP frames when XDP is disabled

[Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>]

On 2019/01/18 12:52, Jason Wang wrote:
> On 2019/1/18 上午9:56, Toshiaki Makita wrote:
>> On 2019/01/17 22:05, Jason Wang wrote:
>>> On 2019/1/17 下午8:53, Jason Wang wrote:
>>>> On 2019/1/17 下午7:20, Toshiaki Makita wrote:
>>>>> Commit 8dcc5b0ab0ec ("virtio_net: fix ndo_xdp_xmit crash towards
>>>>> dev not
>>>>> ready for XDP") tried to avoid access to unexpected sq while XDP is
>>>>> disabled, but was not complete.
>>>>>
>>>>> There was a small window which causes out of bounds sq access in
>>>>> virtnet_xdp_xmit() while disabling XDP.
>>>>>
>>>>> An example case of
>>>>>    - curr_queue_pairs = 6 (2 for SKB and 4 for XDP)
>>>>>    - online_cpu_num = xdp_queue_paris = 4
>>>>> when XDP is enabled:
>>>>>
>>>>> CPU 0                         CPU 1
>>>>> (Disabling XDP)               (Processing redirected XDP frames)
>>>>>
>>>>>                                 virtnet_xdp_xmit()
>>>>> virtnet_xdp_set()
>>>>>    _virtnet_set_queues()
>>>>>     set curr_queue_pairs (2)
>>>>>                                  check if rq->xdp_prog is not NULL
>>>>>                                  virtnet_xdp_sq(vi)
>>>>>                                   qp = curr_queue_pairs -
>>>>>                                        xdp_queue_pairs +
>>>>>                                        smp_processor_id()
>>>>>                                      = 2 - 4 + 1 = -1
>>>>>                                   sq = &vi->sq[qp] // out of bounds
>>>>> access
>>>>>     set xdp_queue_pairs (0)
>>>>>     rq->xdp_prog = NULL
>>>>>
>>>>> Basically we should not change curr_queue_pairs and xdp_queue_pairs
>>>>> while someone can read the values. Thus, when disabling XDP, assign
>>>>> NULL
>>>>> to rq->xdp_prog first, and wait for RCU grace period, then change
>>>>> xxx_queue_pairs.
>>>>> Note that we need to keep the current order when enabling XDP though.
>>>>>
>>>>> Fixes: 186b3c998c50 ("virtio-net: support XDP_REDIRECT")
>>>>> Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
>>>>
>>>> I wonder whether or not we could simply do:
>>>>
>>>>
>>>> if (prog) {
>>>
>>> Should be !prog
>>>
>>>
>>>>      rcu_assign_pointer()
>>>>
>>>>      synchronize_net()
>>>>
>>>> }
>>>>
>>>> set queues
>>>>
>>>> if (!prog) {
>>>
>>> Should be prog.
>> Either would work.
>>
>> With your suggestion the code will look like:
>>
>> ---
>> if (!prog) {
>>     for (...) {
>>         rcu_assign_pointer();
>>         ...
>>     }
>>     synchronize_net();
>> }
>>
>> virtnet_set_queues();
>> netif_set_real_num_rx_queues();
>> vi->xdp_queue_pairs = xdp_qp;
>>
>> if (prog) {
>>     for (...) {
>>         rcu_assign_pointer();
>>         ...
>>     }
>> }
> 
> 
> Yes, I think this makes code more easier to be understand.
> 
> 
>> ---
>>
>> But strictly speaking, virtnet_set_queues() should not be necessary if
>> (prog != NULL && old_prog != NULL).
> 
> 
> Yes, but it was another possible 'issue'.
> 
> 
>> If you prefer this, I can modify it accordingly.
> 
> 
> I prefer to do this change.

OK, will do in v2.

-- 
Toshiaki Makita