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Date:   Mon, 22 Jul 2019 11:11:52 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
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        jglisse@redhat.com, keescook@chromium.org, ldv@altlinux.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
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        namit@vmware.com, peterz@infradead.org,
        syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk,
        wad@chromium.org
Subject: Re: WARNING in __mmdrop
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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:

This stuff looks quite similar to the hmm_mirror use model and other
places in the kernel. I'm still hoping we can share this code a bit more.

There is another bug, this sequence here:

vhost_vring_set_num_addr()
   mmu_notifier_unregister()
   [..]
   mmu_notifier_register()

Which I think is trying to create a lock to protect dev->vqs..

Has the problem that mmu_notifier_unregister() doesn't guarantee that
invalidate_start/end are fully paired.

So after any unregister the code has to clean up any resulting
unbalanced invalidate_count before it can call mmu_notifier_register
again. ie zero the invalidate_count.

It also seems really weird that vhost_map_prefetch() can fail, ie due
to __get_user_pages_fast needing to block, but that just silently
(permanently?) disables the optimization?? At least the usage here
would be better done with a seqcount lock and a normal blocking call
to get_user_pages_fast()...

Jason