Re: [PATCH V6 9/9] virtio: use WARN_ON() to warning illegal status value

Jason Wang
2022-05-30T03:48:28Z

On Fri, May 27, 2022 at 6:50 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> At a minimum, I don't see why it's part of the series. Host can always
> crash the guest if it wants to ...

Probably not with some recent technology. In those cases, a fault will
be generated if the hypervisor tries to access the memory that is
private to the guest.

> The point of BUG_ON is device or driver is already corrupted so we
> should not try to drive it. If you still want this in pls come up with
> a better commit log explaining the why.

A question here, should we always use BUG_ON for the buggy/malicious hypervisor?

The interrupt hardening logic in this series tries to make guest
survive, so did this patch.

>
> On Fri, May 27, 2022 at 02:01:20PM +0800, Jason Wang wrote:
> > We used to use BUG_ON() in virtio_device_ready() to detect illegal
>
> not really, BUG_ON just crashes the kernel. we detect by checking
> status.

We need a kind of notification otherwise there's no way for the user
to know about this expected value.

>
> > status value, this seems sub-optimal since the value is under the
> > control of the device. Switch to use WARN_ON() instead.
>
> some people use crash on warn so ...

Yes, but the policy is under the control of the user.

>
> >
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: "Paul E. McKenney" <paulmck@kernel.org>
> > Cc: Marc Zyngier <maz@kernel.org>
> > Cc: Halil Pasic <pasic@linux.ibm.com>
> > Cc: Cornelia Huck <cohuck@redhat.com>
> > Cc: Vineeth Vijayan <vneethv@linux.ibm.com>
> > Cc: Peter Oberparleiter <oberpar@linux.ibm.com>
> > Cc: linux-s390@vger.kernel.org
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
>
> > ---
> > include/linux/virtio_config.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> > index d4edfd7d91bb..9a36051ceb76 100644
> > --- a/include/linux/virtio_config.h
> > +++ b/include/linux/virtio_config.h
> > @@ -255,7 +255,7 @@ void virtio_device_ready(struct virtio_device *dev)
> > {
> > unsigned status = dev->config->get_status(dev);
> >
> > - BUG_ON(status & VIRTIO_CONFIG_S_DRIVER_OK);
> > + WARN_ON(status & VIRTIO_CONFIG_S_DRIVER_OK);
> >
>
> we lose debuggability as guest will try to continue.
> if we are doing this let us print a helpful message and dump a lot of
> state right here.

I'm ok with dropping this patch from the series. And revisit it in the future.

Thanks

>
> > /*
> > * The virtio_synchronize_cbs() makes sure vring_interrupt()
> > --
> > 2.25.1
>