Re: [PATCH] vhost/vsock: don't check owner in vhost_vsock_stop() while releasing

[Stefano Garzarella]

On Mon, Feb 21, 2022 at 11:33:11PM +0530, Anirudh Rayabharam wrote:
> On Mon, Feb 21, 2022 at 05:44:20PM +0100, Stefano Garzarella wrote:
> > On Mon, Feb 21, 2022 at 09:44:39PM +0530, Anirudh Rayabharam wrote:
> > > On Mon, Feb 21, 2022 at 02:59:30PM +0100, Stefano Garzarella wrote:
> > > > On Mon, Feb 21, 2022 at 12:49 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
> > > > >
> > > > > vhost_vsock_stop() calls vhost_dev_check_owner() to check the device
> > > > > ownership. It expects current->mm to be valid.
> > > > >
> > > > > vhost_vsock_stop() is also called by vhost_vsock_dev_release() when
> > > > > the user has not done close(), so when we are in do_exit(). In this
> > > > > case current->mm is invalid and we're releasing the device, so we
> > > > > should clean it anyway.
> > > > >
> > > > > Let's check the owner only when vhost_vsock_stop() is called
> > > > > by an ioctl.
> > > > >
> > > > > Fixes: 433fc58e6bf2 ("VSOCK: Introduce vhost_vsock.ko")
> > > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > > Reported-by: syzbot+1e3ea63db39f2b4440e0@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> > > > > ---
> > > > >  drivers/vhost/vsock.c | 14 ++++++++------
> > > > >  1 file changed, 8 insertions(+), 6 deletions(-)
> > > >
> > > > Reported-and-tested-by: syzbot+0abd373e2e50d704db87@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > I don't think this patch fixes "INFO: task hung in vhost_work_dev_flush"
> > > even though syzbot says so. I am able to reproduce the issue locally
> > > even with this patch applied.
> >
> > Are you using the sysbot reproducer or another test?
> > In that case, can you share it?
>
> I am using the syzbot reproducer.
>
> > From the stack trace it seemed to me that the worker accesses a zone that
> > has been cleaned (iotlb), so it is invalid and fails.
>
> Would the thread hang in that case? How?

Looking at this log [1] it seems that the process is blocked on the 
wait_for_completion() in vhost_work_dev_flush().

Since we're not setting the backend to NULL to stop the worker, it's 
likely that the worker will keep running, preventing the flush work from 
completing.

[1] https://syzkaller.appspot.com/text?tag=CrashLog&x=153f0852700000