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Date:   Fri, 26 Apr 2019 00:04:02 +0800
From:   YueHaibing <yuehaibing@...wei.com>
To:     Jason Wang <jasowang@...hat.com>,
        Cong Wang <xiyou.wangcong@...il.com>
CC:     netdev <netdev@...r.kernel.org>
Subject: Re: BUG: KASAN: use-after-free Read in tun_net_xmit

On 2019/4/24 20:25, YueHaibing wrote:
> On 2019/4/24 17:11, Jason Wang wrote:
>>
>> On 2019/4/24 上午12:41, Cong Wang wrote:
>>> On Mon, Apr 22, 2019 at 11:42 PM Jason Wang <jasowang@...hat.com> wrote:
>>>>
>>>> On 2019/4/23 下午2:00, Cong Wang wrote:
>>>>> On Mon, Apr 22, 2019 at 2:41 AM Jason Wang <jasowang@...hat.com> wrote:
>>>>>> On 2019/4/22 上午11:57, YueHaibing wrote:
>>>>>>> We get a KASAN report as below, but don't have any reproducer.
>>>>>>>
>>>>>>> Any comments are appreciated.
>>>>>>>
>>>>>>> ==================================================================
>>>>>>> BUG: KASAN: use-after-free in tun_net_xmit+0x1670/0x1750 drivers/net/tun.c:1104
>>>>>>> Read of size 8 at addr ffff88836cc26a70 by task swapper/3/0
>>>>>> Which kernel version did you use? The calltrace points out the a use
>>>>>> after free for tun_file structure which should be synchronized through
>>>>>> RCU + RTNL lock.
>>>>> The tfile socket has to be marked with SOCK_RCU_FREE in order
>>>>> to fully respect the RCU grace period.
>>>>>
>>>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>>>>> index e9ca1c088d0b..31c3210288cb 100644
>>>>> --- a/drivers/net/tun.c
>>>>> +++ b/drivers/net/tun.c
>>>>> @@ -3431,6 +3431,7 @@ static int tun_chr_open(struct inode *inode,
>>>>> struct file * file)
>>>>>           file->private_data = tfile;
>>>>>           INIT_LIST_HEAD(&tfile->next);
>>>>>
>>>>> +       sock_set_flag(&tfile->sk, SOCK_RCU_FREE);
>>>>>           sock_set_flag(&tfile->sk, SOCK_ZEROCOPY);
>>>>>
>>>>>           return 0;
>>>>
>>>> We did a synchronize_net() when socket is detached from netdevice in
>>>> __tun_detach() so it looks to me this is unnecessary.
>>> I knew, but it is only called conditionally, that is:
>>>
>>>   695         if (tun && !tfile->detached) {
>>> ...
>>>   710
>>>   711                 synchronize_net();
>>>
>>> And it looks like syzbot just skipped this condition,
>>
>>
>> If tfile is detached, it should have gone for the path of synchronize_net() before. If tfile is never attached, tun_net_xmit() doesn't have the chance to access that. I wonder whether or not we should use WRITE_ONCE() for tun->numqueues-- in this fucntion. If the value was not committed to memory before synchronize_net(), we may race with tun_net_xmit() which check txq against tun->numqueues.
>>
>>
>>> this is why I believe
>>> you still need to respect RCU grace period _unconditionally_ for tfile.
>>
>>
>> This is true if I miss subtle race in the code.
>>
>>
>> Haibing: could you please try the following test?
>>
>> 1) start VM with multiple queue

 I configured 8 queues with virtio driver to start vm

>>
>> 2) using pktgen to inject packets to all queues through tap

 inject packet into tap nic in host

>>
>> 3) using ethtool to change the combined channels in guest in a loop

 repeat do as follow in vm:
 	ethtool -L eth0 combined 4
 	ethtool -L eth0 combined 8

>>
>> 4) kill the guest
>>

This cannot reproduce the issue.


>>
> 
> Ok, I will try this.
> 
>> Thanks
>>
>>
>>> Thanks.
>>
>> .
>>
> 
> 
> .
> 

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