Re: WARNING in __mmdrop

[Jason Wang]

On 2019/7/21 äå8:18, 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@xxxxxxxxxx>
> > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > 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@xxxxxxxxxx>


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

Thanks