Re: [PATCH V3] virtio: disable notification hardening by default

Jason Wang
2022-06-27T08:11:36Z

On Mon, Jun 27, 2022 at 3:33 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> 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.

Core IRQ won't help in 1), the changes are limited in the virtio.

> 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.

It's not the fault of hardening but the drivers. The hardening just
expose those bugs.

>
> 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.

I'm not sure I see the difference, disable_irq() stuffs also delay the
interrupt processing until enable_irq().

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

I don't see a deadlock here, maybe you can show more detail on this?

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

And ignore the notification if vq is not kicked? It sounds like the
callback needs to be synchronized with the kick.

>
> 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?

There's no much difference with the existing approach:

1) your proposal implicitly makes callbacks ready in virtqueue_kick()
2) my proposal explicitly makes callbacks ready via virtio_device_ready()

Both require careful auditing of all the existing drivers to make sure
no kick before DRIVER_OK.

>
> 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?

Probably.

>
>
> > 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.

Yes, but consider the case of mlx5_vdpa, PCI stuff was hidden under
the auxiliary bus. And that is the way another vendor will go.

Thanks

>
>
> > >
> > >
> > > > >
> > > > >
> > > > > > ---
> > > > > > 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
> > > > >
> > >
>