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Subject: Re: WARNING in __mmdrop
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <b4696f2e-678a-bdb2-4b7c-fb4ce040ec2a@redhat.com>
Date:   Tue, 23 Jul 2019 13:47:04 +0800
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On 2019/7/23 下午1:01, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 12:01:40PM +0800, Jason Wang wrote:
>> On 2019/7/22 下午4:08, Michael S. Tsirkin wrote:
>>> On Mon, Jul 22, 2019 at 01:24:24PM +0800, Jason Wang wrote:
>>>> 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.
>>> synchronize_rcu has very high latency on busy systems.
>>> It is not something that should be used on a syscall path.
>>> KVM had to switch to SRCU to keep it sane.
>>> Otherwise one guest can trivially slow down another one.
>>
>> I think you mean the synchronize_rcu_expedited()? Rethink of the code, the
>> synchronize_rcu() in ioctl() could be removed, since it was serialized with
>> memory accessor.
>
> Really let's just use kfree_rcu. It's way cleaner: fire and forget.


Looks not, you need rate limit the fire as you've figured out? And in 
fact, the synchronization is not even needed, does it help if I leave a 
comment to explain?


>
>> Btw, for kvm ioctl it still uses synchronize_rcu() in kvm_vcpu_ioctl(),
>> (just a little bit more hard to trigger):
>
> AFAIK these never run in response to guest events.
> So they can take very long and guests still won't crash.


What if guest manages to escape to qemu?

Thanks


>
>
>>      case KVM_RUN: {
>> ...
>>          if (unlikely(oldpid != task_pid(current))) {
>>              /* The thread running this VCPU changed. */
>>              struct pid *newpid;
>>
>>              r = kvm_arch_vcpu_run_pid_change(vcpu);
>>              if (r)
>>                  break;
>>
>>              newpid = get_task_pid(current, PIDTYPE_PID);
>>              rcu_assign_pointer(vcpu->pid, newpid);
>>              if (oldpid)
>>                  synchronize_rcu();
>>              put_pid(oldpid);
>>          }
>> ...
>>          break;
>>
>>
>>>>> 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
>>> The root cause of the crash is relevant, but we still need
>>> to fix issues 1-4.
>>>
>>> More issues (my patch tries to fix them too):
>>>
>>> 5. page not dirtied when mappings are torn down outside
>>>      of invalidate callback
>>
>> Yes.
>>
>>
>>> 6. potential cross-VM DOS by one guest keeping system busy
>>>      and increasing synchronize_rcu latency to the point where
>>>      another guest stars timing out and crashes
>>>
>>>
>>>
>> This will be addressed after I remove the synchronize_rcu() from ioctl path.
>>
>> Thanks