Re: [PATCH 6/9] virtio_pci: harden MSI-X interrupts

[Jason Wang]

On Mon, Sep 13, 2021 at 2:04 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Mon, Sep 13, 2021 at 01:53:50PM +0800, Jason Wang wrote:
> > We used to synchronize pending MSI-X irq handlers via
> > synchronize_irq(), this may not work for the untrusted device which
> > may keep sending interrupts after reset which may lead unexpected
> > results. Similarly, we should not enable MSI-X interrupt until the
> > device is ready. So this patch fixes those two issues by:
> >
> > 1) switching to use disable_irq() to prevent the virtio interrupt
> >    handlers to be called after the device is reset.
> > 2) using IRQF_NO_AUTOEN and enable the MSI-X irq during .ready()
> >
> > This can make sure the virtio interrupt handler won't be called before
> > virtio_device_ready() and after reset.
> >
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
>
> I don't get the threat model here. Isn't disabling irqs done by the
> hypervisor anyway? Is there a reason to trust disable_irq but not
> device reset?

My understanding is that e.g in the case of SEV/TDX we don't trust the
hypervisor. So the hypervisor can keep sending interrupts even if the
device is reset. The guest can only trust its own software interrupt
management logic to avoid call virtio callback in this case.

Thanks

>
> Cc a bunch more people ...
>
>
> > ---
> >  drivers/virtio/virtio_pci_common.c | 27 +++++++++++++++++++++------
> >  drivers/virtio/virtio_pci_common.h |  6 ++++--
> >  drivers/virtio/virtio_pci_legacy.c |  5 +++--
> >  drivers/virtio/virtio_pci_modern.c |  6 ++++--
> >  4 files changed, 32 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> > index b35bb2d57f62..0b9523e6dd39 100644
> > --- a/drivers/virtio/virtio_pci_common.c
> > +++ b/drivers/virtio/virtio_pci_common.c
> > @@ -24,8 +24,8 @@ MODULE_PARM_DESC(force_legacy,
> >                "Force legacy mode for transitional virtio 1 devices");
> >  #endif
> >
> > -/* wait for pending irq handlers */
> > -void vp_synchronize_vectors(struct virtio_device *vdev)
> > +/* disable irq handlers */
> > +void vp_disable_vectors(struct virtio_device *vdev)
> >  {
> >       struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> >       int i;
> > @@ -34,7 +34,20 @@ void vp_synchronize_vectors(struct virtio_device *vdev)
> >               synchronize_irq(vp_dev->pci_dev->irq);
> >
> >       for (i = 0; i < vp_dev->msix_vectors; ++i)
> > -             synchronize_irq(pci_irq_vector(vp_dev->pci_dev, i));
> > +             disable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> > +}
> > +
> > +/* enable irq handlers */
> > +void vp_enable_vectors(struct virtio_device *vdev)
> > +{
> > +     struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> > +     int i;
> > +
> > +     if (vp_dev->intx_enabled)
> > +             return;
> > +
> > +     for (i = 0; i < vp_dev->msix_vectors; ++i)
> > +             enable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> >  }
> >
> >  /* the notify function used when creating a virt queue */
> > @@ -141,7 +154,8 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
> >       snprintf(vp_dev->msix_names[v], sizeof *vp_dev->msix_names,
> >                "%s-config", name);
> >       err = request_irq(pci_irq_vector(vp_dev->pci_dev, v),
> > -                       vp_config_changed, 0, vp_dev->msix_names[v],
> > +                       vp_config_changed, IRQF_NO_AUTOEN,
> > +                       vp_dev->msix_names[v],
> >                         vp_dev);
> >       if (err)
> >               goto error;
> > @@ -160,7 +174,8 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
> >               snprintf(vp_dev->msix_names[v], sizeof *vp_dev->msix_names,
> >                        "%s-virtqueues", name);
> >               err = request_irq(pci_irq_vector(vp_dev->pci_dev, v),
> > -                               vp_vring_interrupt, 0, vp_dev->msix_names[v],
> > +                               vp_vring_interrupt, IRQF_NO_AUTOEN,
> > +                               vp_dev->msix_names[v],
> >                                 vp_dev);
> >               if (err)
> >                       goto error;
> > @@ -337,7 +352,7 @@ static int vp_find_vqs_msix(struct virtio_device *vdev, unsigned nvqs,
> >                        "%s-%s",
> >                        dev_name(&vp_dev->vdev.dev), names[i]);
> >               err = request_irq(pci_irq_vector(vp_dev->pci_dev, msix_vec),
> > -                               vring_interrupt, 0,
> > +                               vring_interrupt, IRQF_NO_AUTOEN,
> >                                 vp_dev->msix_names[msix_vec],
> >                                 vqs[i]);
> >               if (err)
> > diff --git a/drivers/virtio/virtio_pci_common.h b/drivers/virtio/virtio_pci_common.h
> > index beec047a8f8d..a235ce9ff6a5 100644
> > --- a/drivers/virtio/virtio_pci_common.h
> > +++ b/drivers/virtio/virtio_pci_common.h
> > @@ -102,8 +102,10 @@ static struct virtio_pci_device *to_vp_device(struct virtio_device *vdev)
> >       return container_of(vdev, struct virtio_pci_device, vdev);
> >  }
> >
> > -/* wait for pending irq handlers */
> > -void vp_synchronize_vectors(struct virtio_device *vdev);
> > +/* disable irq handlers */
> > +void vp_disable_vectors(struct virtio_device *vdev);
> > +/* enable irq handlers */
> > +void vp_enable_vectors(struct virtio_device *vdev);
> >  /* the notify function used when creating a virt queue */
> >  bool vp_notify(struct virtqueue *vq);
> >  /* the config->del_vqs() implementation */
> > diff --git a/drivers/virtio/virtio_pci_legacy.c b/drivers/virtio/virtio_pci_legacy.c
> > index d62e9835aeec..bdf6bc667ab5 100644
> > --- a/drivers/virtio/virtio_pci_legacy.c
> > +++ b/drivers/virtio/virtio_pci_legacy.c
> > @@ -97,8 +97,8 @@ static void vp_reset(struct virtio_device *vdev)
> >       /* Flush out the status write, and flush in device writes,
> >        * including MSi-X interrupts, if any. */
> >       ioread8(vp_dev->ioaddr + VIRTIO_PCI_STATUS);
> > -     /* Flush pending VQ/configuration callbacks. */
> > -     vp_synchronize_vectors(vdev);
> > +     /* Disable VQ/configuration callbacks. */
> > +     vp_disable_vectors(vdev);
> >  }
> >
> >  static u16 vp_config_vector(struct virtio_pci_device *vp_dev, u16 vector)
> > @@ -194,6 +194,7 @@ static void del_vq(struct virtio_pci_vq_info *info)
> >  }
> >
> >  static const struct virtio_config_ops virtio_pci_config_ops = {
> > +     .ready          = vp_enable_vectors,
> >       .get            = vp_get,
> >       .set            = vp_set,
> >       .get_status     = vp_get_status,
> > diff --git a/drivers/virtio/virtio_pci_modern.c b/drivers/virtio/virtio_pci_modern.c
> > index 30654d3a0b41..acf0f6b6381d 100644
> > --- a/drivers/virtio/virtio_pci_modern.c
> > +++ b/drivers/virtio/virtio_pci_modern.c
> > @@ -172,8 +172,8 @@ static void vp_reset(struct virtio_device *vdev)
> >        */
> >       while (vp_modern_get_status(mdev))
> >               msleep(1);
> > -     /* Flush pending VQ/configuration callbacks. */
> > -     vp_synchronize_vectors(vdev);
> > +     /* Disable VQ/configuration callbacks. */
> > +     vp_disable_vectors(vdev);
> >  }
> >
> >  static u16 vp_config_vector(struct virtio_pci_device *vp_dev, u16 vector)
> > @@ -380,6 +380,7 @@ static bool vp_get_shm_region(struct virtio_device *vdev,
> >  }
> >
> >  static const struct virtio_config_ops virtio_pci_config_nodev_ops = {
> > +     .ready          = vp_enable_vectors,
> >       .get            = NULL,
> >       .set            = NULL,
> >       .generation     = vp_generation,
> > @@ -397,6 +398,7 @@ static const struct virtio_config_ops virtio_pci_config_nodev_ops = {
> >  };
> >
> >  static const struct virtio_config_ops virtio_pci_config_ops = {
> > +     .ready          = vp_enable_vectors,
> >       .get            = vp_get,
> >       .set            = vp_set,
> >       .generation     = vp_generation,
> > --
> > 2.25.1
>