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Subject: Re: WARNING in __mmdrop
To:     "Michael S. Tsirkin" <mst@redhat.com>,
        syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>
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From:   Jason Wang <jasowang@redhat.com>
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Date:   Mon, 22 Jul 2019 13:24:24 +0800
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On 2019/7/21 下午8:18, Michael S. Tsirkin wrote:
> On Sun, Jul 21, 2019 at 06:02:52AM -0400, Michael S. Tsirkin wrote:
>> On Sat, Jul 20, 2019 at 03:08:00AM -0700, syzbot wrote:
>>> syzbot has bisected this bug to:
>>>
>>> commit 7f466032dc9e5a61217f22ea34b2df932786bbfc
>>> Author: Jason Wang<jasowang@redhat.com>
>>> Date:   Fri May 24 08:12:18 2019 +0000
>>>
>>>      vhost: access vq metadata through kernel virtual address
>>>
>>> bisection log:https://syzkaller.appspot.com/x/bisect.txt?x=149a8a20600000
>>> start commit:   6d21a41b Add linux-next specific files for 20190718
>>> git tree:       linux-next
>>> final crash:https://syzkaller.appspot.com/x/report.txt?x=169a8a20600000
>>> console output:https://syzkaller.appspot.com/x/log.txt?x=129a8a20600000
>>> kernel config:https://syzkaller.appspot.com/x/.config?x=3430a151e1452331
>>> dashboard link:https://syzkaller.appspot.com/bug?extid=e58112d71f77113ddb7b
>>> syz repro:https://syzkaller.appspot.com/x/repro.syz?x=10139e68600000
>>>
>>> Reported-by:syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com
>>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual
>>> address")
>>>
>>> For information about bisection process see:https://goo.gl/tpsmEJ#bisection
>> OK I poked at this for a bit, I see several things that
>> we need to fix, though I'm not yet sure it's the reason for
>> the failures:
>>
>>
>> 1. mmu_notifier_register shouldn't be called from vhost_vring_set_num_addr
>>     That's just a bad hack, in particular I don't think device
>>     mutex is taken and so poking at two VQs will corrupt
>>     memory.
>>     So what to do? How about a per vq notifier?
>>     Of course we also have synchronize_rcu
>>     in the notifier which is slow and is now going to be called twice.
>>     I think call_rcu would be more appropriate here.
>>     We then need rcu_barrier on module unload.
>>     OTOH if we make pages linear with map then we are good
>>     with kfree_rcu which is even nicer.
>>
>> 2. Doesn't map leak after vhost_map_unprefetch?
>>     And why does it poke at contents of the map?
>>     No one should use it right?
>>
>> 3. notifier unregister happens last in vhost_dev_cleanup,
>>     but register happens first. This looks wrong to me.
>>
>> 4. OK so we use the invalidate count to try and detect that
>>     some invalidate is in progress.
>>     I am not 100% sure why do we care.
>>     Assuming we do, uaddr can change between start and end
>>     and then the counter can get negative, or generally
>>     out of sync.
>>
>> So what to do about all this?
>> I am inclined to say let's just drop the uaddr optimization
>> for now. E.g. kvm invalidates unconditionally.
>> 3 should be fixed independently.
> Above implements this but is only build-tested.
> Jason, pls take a look. If you like the approach feel
> free to take it from here.
>
> One thing the below does not have is any kind of rate-limiting.
> Given it's so easy to restart I'm thinking it makes sense
> to add a generic infrastructure for this.
> Can be a separate patch I guess.


I don't get why must use kfree_rcu() instead of synchronize_rcu() here.


>
> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>


Let me try to figure out the root cause then decide whether or not to go 
for this way.

Thanks