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Date:   Mon, 27 Jun 2022 03:33:50 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     Cornelia Huck <cohuck@redhat.com>,
        Halil Pasic <pasic@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        linux-s390@vger.kernel.org,
        virtualization <virtualization@lists.linux-foundation.org>,
        kvm <kvm@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ben Hutchings <ben@decadent.org.uk>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH V3] virtio: disable notification hardening by default
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On Mon, Jun 27, 2022 at 10:50:17AM +0800, Jason Wang wrote:
> On Fri, Jun 24, 2022 at 2:31 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Wed, Jun 22, 2022 at 03:09:31PM +0800, Jason Wang wrote:
> > > On Wed, Jun 22, 2022 at 3:03 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > On Wed, Jun 22, 2022 at 09:29:40AM +0800, Jason Wang wrote:
> > > > > We try to harden virtio device notifications in 8b4ec69d7e09 ("virtio:
> > > > > harden vring IRQ"). It works with the assumption that the driver or
> > > > > core can properly call virtio_device_ready() at the right
> > > > > place. Unfortunately, this seems to be not true and uncover various
> > > > > bugs of the existing drivers, mainly the issue of using
> > > > > virtio_device_ready() incorrectly.
> > > > >
> > > > > So let's having a Kconfig option and disable it by default. It gives
> > > > > us a breath to fix the drivers and then we can consider to enable it
> > > > > by default.
> > > > >
> > > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > >
> > > >
> > > > OK I will queue, but I think the problem is fundamental.
> > >
> > > If I understand correctly, you want some core IRQ work?
> >
> > Yes.
> >
> > > As discussed
> > > before, it doesn't solve all the problems, we still need to do per
> > > driver audit.
> > >
> > > Thanks
> >
> > Maybe, but we don't need to tie things to device_ready then.
> > We can do
> >
> > - disable irqs
> > - device ready
> > - setup everything
> > - enable irqs
> >
> >
> > and this works for most things, the only issue is
> > this deadlocks if "setup everything" waits for interrupts.
> >
> >
> > With the current approach there's really no good time:
> > 1.- setup everything
> > - device ready
> >
> > can cause kicks before device is ready
> >
> > 2.- device ready
> > - setup everything
> >
> > can cause callbacks before setup.
> >
> > So I prefer the 1. and fix the hardening in the core.
> 
> So my question is:
> 
> 1) do similar hardening like config interrupt
> or
> 2) per transport notification work (e.g for PCI core IRQ work)
> 
> 1) seems easier and universal, but we pay little overhead which could
> be eliminated by the config option.

I doubt 1 is easy and I am not even sure core IRQ changes will help.
My concern with adding overhead is that I'm not sure these are not just
wasted CPU cycles.  We spent a bunch of time on irq hardening and so far
we are still at the "all drivers need to be fixed" stage.

The reason config was kind of easy is that config interrupt is rarely
vital for device function so arbitrarily deferring that does not lead to
deadlocks - what you are trying to do with VQ interrupts is
fundamentally different. Things are especially bad if we just drop
an interrupt but deferring can lead to problems too.

Consider as an example
    virtio-net: fix race between ndo_open() and virtio_device_ready()
if you just defer vq interrupts you get deadlocks.


So, thinking about all this, how about a simple per vq flag meaning
"this vq was kicked since reset"?

If driver does not kick then it's not ready to get callbacks, right?

Sounds quite clean, but we need to think through memory ordering
concerns - I guess it's only when we change the value so
	if (!vq->kicked) {
		vq->kicked = true;
		mb();
	}

will do the trick, right?

need to think about the reset path - it already synchronizes callbacks
and already can lose interrupts so we just need to clear vq->kicked
before that, right?


> 2) seems require more work in the IRQ core and it can not work for all
> transports (e.g vDPA would be kind of difficult)
> 
> Thanks

Hmm I don't really get why would it be difficult.
VDPA is mostly PCI isn't it? With PCI both level INT#x and edge MSI
have interrupt masking support.


> >
> >
> > > >
> > > >
> > > > > ---
> > > > > Changes since V2:
> > > > > - Tweak the Kconfig help
> > > > > - Add comment for the read_lock() pairing in virtio_ccw
> > > > > ---
> > > > >  drivers/s390/virtio/virtio_ccw.c |  9 ++++++++-
> > > > >  drivers/virtio/Kconfig           | 13 +++++++++++++
> > > > >  drivers/virtio/virtio.c          |  2 ++
> > > > >  drivers/virtio/virtio_ring.c     | 12 ++++++++++++
> > > > >  include/linux/virtio_config.h    |  2 ++
> > > > >  5 files changed, 37 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c
> > > > > index 97e51c34e6cf..1f6a358f65f0 100644
> > > > > --- a/drivers/s390/virtio/virtio_ccw.c
> > > > > +++ b/drivers/s390/virtio/virtio_ccw.c
> > > > > @@ -1136,8 +1136,13 @@ static void virtio_ccw_int_handler(struct ccw_device *cdev,
> > > > >                       vcdev->err = -EIO;
> > > > >       }
> > > > >       virtio_ccw_check_activity(vcdev, activity);
> > > > > -     /* Interrupts are disabled here */
> > > > > +#ifdef CONFIG_VIRTIO_HARDEN_NOTIFICATION
> > > > > +     /*
> > > > > +      * Paried with virtio_ccw_synchronize_cbs() and interrupts are
> > > > > +      * disabled here.
> > > > > +      */
> > > > >       read_lock(&vcdev->irq_lock);
> > > > > +#endif
> > > > >       for_each_set_bit(i, indicators(vcdev),
> > > > >                        sizeof(*indicators(vcdev)) * BITS_PER_BYTE) {
> > > > >               /* The bit clear must happen before the vring kick. */
> > > > > @@ -1146,7 +1151,9 @@ static void virtio_ccw_int_handler(struct ccw_device *cdev,
> > > > >               vq = virtio_ccw_vq_by_ind(vcdev, i);
> > > > >               vring_interrupt(0, vq);
> > > > >       }
> > > > > +#ifdef CONFIG_VIRTIO_HARDEN_NOTIFICATION
> > > > >       read_unlock(&vcdev->irq_lock);
> > > > > +#endif
> > > > >       if (test_bit(0, indicators2(vcdev))) {
> > > > >               virtio_config_changed(&vcdev->vdev);
> > > > >               clear_bit(0, indicators2(vcdev));
> > > > > diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
> > > > > index b5adf6abd241..c04f370a1e5c 100644
> > > > > --- a/drivers/virtio/Kconfig
> > > > > +++ b/drivers/virtio/Kconfig
> > > > > @@ -35,6 +35,19 @@ menuconfig VIRTIO_MENU
> > > > >
> > > > >  if VIRTIO_MENU
> > > > >
> > > > > +config VIRTIO_HARDEN_NOTIFICATION
> > > > > +        bool "Harden virtio notification"
> > > > > +        help
> > > > > +          Enable this to harden the device notifications and suppress
> > > > > +          those that happen at a time where notifications are illegal.
> > > > > +
> > > > > +          Experimental: Note that several drivers still have bugs that
> > > > > +          may cause crashes or hangs when correct handling of
> > > > > +          notifications is enforced; depending on the subset of
> > > > > +          drivers and devices you use, this may or may not work.
> > > > > +
> > > > > +          If unsure, say N.
> > > > > +
> > > > >  config VIRTIO_PCI
> > > > >       tristate "PCI driver for virtio devices"
> > > > >       depends on PCI
> > > > > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > > > > index ef04a96942bf..21dc08d2f32d 100644
> > > > > --- a/drivers/virtio/virtio.c
> > > > > +++ b/drivers/virtio/virtio.c
> > > > > @@ -220,6 +220,7 @@ static int virtio_features_ok(struct virtio_device *dev)
> > > > >   * */
> > > > >  void virtio_reset_device(struct virtio_device *dev)
> > > > >  {
> > > > > +#ifdef CONFIG_VIRTIO_HARDEN_NOTIFICATION
> > > > >       /*
> > > > >        * The below virtio_synchronize_cbs() guarantees that any
> > > > >        * interrupt for this line arriving after
> > > > > @@ -228,6 +229,7 @@ void virtio_reset_device(struct virtio_device *dev)
> > > > >        */
> > > > >       virtio_break_device(dev);
> > > > >       virtio_synchronize_cbs(dev);
> > > > > +#endif
> > > > >
> > > > >       dev->config->reset(dev);
> > > > >  }
> > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > > index 13a7348cedff..d9d3b6e201fb 100644
> > > > > --- a/drivers/virtio/virtio_ring.c
> > > > > +++ b/drivers/virtio/virtio_ring.c
> > > > > @@ -1688,7 +1688,11 @@ static struct virtqueue *vring_create_virtqueue_packed(
> > > > >       vq->we_own_ring = true;
> > > > >       vq->notify = notify;
> > > > >       vq->weak_barriers = weak_barriers;
> > > > > +#ifdef CONFIG_VIRTIO_HARDEN_NOTIFICATION
> > > > >       vq->broken = true;
> > > > > +#else
> > > > > +     vq->broken = false;
> > > > > +#endif
> > > > >       vq->last_used_idx = 0;
> > > > >       vq->event_triggered = false;
> > > > >       vq->num_added = 0;
> > > > > @@ -2135,9 +2139,13 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
> > > > >       }
> > > > >
> > > > >       if (unlikely(vq->broken)) {
> > > > > +#ifdef CONFIG_VIRTIO_HARDEN_NOTIFICATION
> > > > >               dev_warn_once(&vq->vq.vdev->dev,
> > > > >                             "virtio vring IRQ raised before DRIVER_OK");
> > > > >               return IRQ_NONE;
> > > > > +#else
> > > > > +             return IRQ_HANDLED;
> > > > > +#endif
> > > > >       }
> > > > >
> > > > >       /* Just a hint for performance: so it's ok that this can be racy! */
> > > > > @@ -2180,7 +2188,11 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> > > > >       vq->we_own_ring = false;
> > > > >       vq->notify = notify;
> > > > >       vq->weak_barriers = weak_barriers;
> > > > > +#ifdef CONFIG_VIRTIO_HARDEN_NOTIFICATION
> > > > >       vq->broken = true;
> > > > > +#else
> > > > > +     vq->broken = false;
> > > > > +#endif
> > > > >       vq->last_used_idx = 0;
> > > > >       vq->event_triggered = false;
> > > > >       vq->num_added = 0;
> > > > > diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> > > > > index 9a36051ceb76..d15c3cdda2d2 100644
> > > > > --- a/include/linux/virtio_config.h
> > > > > +++ b/include/linux/virtio_config.h
> > > > > @@ -257,6 +257,7 @@ void virtio_device_ready(struct virtio_device *dev)
> > > > >
> > > > >       WARN_ON(status & VIRTIO_CONFIG_S_DRIVER_OK);
> > > > >
> > > > > +#ifdef CONFIG_VIRTIO_HARDEN_NOTIFICATION
> > > > >       /*
> > > > >        * The virtio_synchronize_cbs() makes sure vring_interrupt()
> > > > >        * will see the driver specific setup if it sees vq->broken
> > > > > @@ -264,6 +265,7 @@ void virtio_device_ready(struct virtio_device *dev)
> > > > >        */
> > > > >       virtio_synchronize_cbs(dev);
> > > > >       __virtio_unbreak_device(dev);
> > > > > +#endif
> > > > >       /*
> > > > >        * The transport should ensure the visibility of vq->broken
> > > > >        * before setting DRIVER_OK. See the comments for the transport
> > > > > --
> > > > > 2.25.1
> > > >
> >