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@xxxxxxxxxxxxx>
>>
>>
>> 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();
...
}
}
---
But strictly speaking, virtnet_set_queues() should not be necessary if
(prog != NULL && old_prog != NULL).
If you prefer this, I can modify it accordingly.
--
Toshiaki Makita
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