netdev - Re: [Qemu-devel] BUG: KASAN: use-after-free in free_old_xmit

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Date:   Tue, 27 Jun 2017 10:13:41 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     jpmenil@...il.com, Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        John Fastabend <john.fastabend@...il.com>,
        qemu-devel Developers <qemu-devel@...gnu.org>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [Qemu-devel] BUG: KASAN: use-after-free in free_old_xmit_skbs



On 2017年06月26日 15:35, Jean-Philippe Menil wrote:
> On 06/26/2017 04:50 AM, Jason Wang wrote:
>>
>>
>> On 2017年06月24日 06:32, Cong Wang wrote:
>>> On Fri, Jun 23, 2017 at 1:43 AM, Jason Wang <jasowang@...hat.com> 
>>> wrote:
>>>>
>>>> On 2017年06月23日 02:53, Michael S. Tsirkin wrote:
>>>>> On Thu, Jun 22, 2017 at 08:15:58AM +0200, jean-philippe menil wrote:
>>>>>> Hi Michael,
>>>>>>
>>>>>> from what i see, the race appear when we hit virtnet_reset in
>>>>>> virtnet_xdp_set.
>>>>>> virtnet_reset
>>>>>>     _remove_vq_common
>>>>>>       virtnet_del_vqs
>>>>>>         virtnet_free_queues
>>>>>>           kfree(vi->sq)
>>>>>> when the xdp program (with two instances of the program to 
>>>>>> trigger it
>>>>>> faster)
>>>>>> is added or removed.
>>>>>>
>>>>>> It's easily repeatable, with 2 cpus and 4 queues on the qemu command
>>>>>> line,
>>>>>> running the xdp_ttl tool from Jesper.
>>>>>>
>>>>>> For now, i'm able to continue my qualification, testing if xdp_qp 
>>>>>> is not
>>>>>> null,
>>>>>> but do not seem to be a sustainable trick.
>>>>>> if (xdp_qp && vi->xdp_queues_pairs != xdp_qp)
>>>>>>
>>>>>> Maybe it will be more clear to you with theses informations.
>>>>>>
>>>>>> Best regards.
>>>>>>
>>>>>> Jean-Philippe
>>>>>
>>>>> I'm pretty clear about the issue here, I was trying to figure out 
>>>>> a fix.
>>>>> Jason, any thoughts?
>>>>>
>>>>>
>>>> Hi Jean:
>>>>
>>>> Does the following fix this issue? (I can't reproduce it locally 
>>>> through
>>>> xdp_ttl)
>>> It is tricky here.
>>>
>>>  From my understanding of the code base, the tx_lock is not sufficient
>>> here, because in virtnet_del_vqs() all vqs are deleted and one vp
>>> maps to one txq.
>>>
>>> I am afraid you have to add a spinlock somewhere to serialized
>>> free_old_xmit_skbs() vs. vring_del_virtqueue(). As you can see
>>> they are in different layers, so it is hard to figure out where to add
>>> it...
>>>
>>> Also, make sure we don't sleep inside the spinlock, I see a
>>> synchronize_net().
>>
>> Looks like I miss something. I thought free_old_xmit_skbs() were 
>> serialized in this case since we disable all tx queues after 
>> netif_tx_unlock_bh()?
>>
>> Jean:
>>
>> I thought this could be easily reproduced by e.g produce some traffic 
>> and in the same time try to attach an xdp program. But looks not. How 
>> do you trigger this? What's your qemu command line for this?
>>
>> Thanks
>
> Hi Jason,
>
> this is how i trigger the bug:
> - on the guest, tcpdump on on the interface
> - on the guest, run iperf against the host
> - on the guest, cat /sys/kernel/debug/tracing/trace_pipe
> - on the guest, run one or two instances of xdp_ttl compiled with 
> DEBUG uncommented, that i start stop, until i trigger the bug.
>
> qemu command line is as follow:
>
> qemu-system-x86_64 -name ubuntu --enable-kvm -machine pc,accel=kvm 
> -smp 2 -drive file=/dev/LocalDisk/ubuntu,if=virtio,format=raw -m 2048 
> -rtc base=localtime,clock=host -usbdevice tablet --balloon virtio 
> -netdev 
> tap,id=ubuntu-0,ifname=ubuntu-0,script=/home/jenfi/WORK/jp/qemu/if-up,downscript=/home/jenfi/WORK/jp/qemu/if-down,vhost=on,queues=4 
> -device 
> virtio-net-pci,netdev=ubuntu-0,mac=de:ad:be:ef:01:03,mq=on,guest_tso4=off,guest_tso6=off,guest_ecn=off,guest_ufo=off,vectors=2 
> -vnc 127.0.0.1:3 -nographic -serial 
> file:/home/jenfi/WORK/jp/qemu/ubuntu.out -monitor 
> unix:/home/jenfi/WORK/jp/qemu/ubuntu.sock,server,nowait
>
> Notice, the smp 2, queues to 4 and vectors to 2.
> Seem that if fogot to mention that in the beginning of this thread, 
> sorry for that.
>
> Best regards.
>
> Jean-Philippe
>

Thanks Jean, I manage to reproduce the issue.

I thought netif_tx_unlock_bh() will do tx lock but looks not, that's why 
previous patch doesn't work.

Could you please this this patch? (At least it can't trigger the warning 
after more than 20 times of xdp start/stop).

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 1f8c15c..a18f859 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1802,6 +1802,7 @@ static void virtnet_freeze_down(struct 
virtio_device *vdev)
         flush_work(&vi->config_work);

         netif_device_detach(vi->dev);
+       netif_tx_disable(vi->dev);
         cancel_delayed_work_sync(&vi->refill);

         if (netif_running(vi->dev)) {