Re: WARNING in __mmdrop

[Jason Wang <jasowang@redhat.com>]

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.

Btw, for kvm ioctl it still uses synchronize_rcu() in kvm_vcpu_ioctl(), 
(just a little bit more hard to trigger):


     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