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

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Message-ID: <CACGkMEvoFapNoqsqq59iH+z_qx1swecjnbbPs7=nN4bn6XdbtA@mail.gmail.com>
Date:   Mon, 30 May 2022 11:48:08 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Halil Pasic <pasic@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        eperezma <eperezma@...hat.com>, Cindy Lu <lulu@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Vineeth Vijayan <vneethv@...ux.ibm.com>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH V6 9/9] virtio: use WARN_ON() to warning illegal status value

On Fri, May 27, 2022 at 6:50 PM Michael S. Tsirkin <mst@...hat.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@...utronix.de>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: "Paul E. McKenney" <paulmck@...nel.org>
> > Cc: Marc Zyngier <maz@...nel.org>
> > Cc: Halil Pasic <pasic@...ux.ibm.com>
> > Cc: Cornelia Huck <cohuck@...hat.com>
> > Cc: Vineeth Vijayan <vneethv@...ux.ibm.com>
> > Cc: Peter Oberparleiter <oberpar@...ux.ibm.com>
> > Cc: linux-s390@...r.kernel.org
> > Signed-off-by: Jason Wang <jasowang@...hat.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
>