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Date:   Mon, 22 Jul 2019 04:08:44 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
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Subject: Re: WARNING in __mmdrop
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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.

> 
> > 
> > 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

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


-- 
MST