Re: [PATCH 7/9] virtio-pci: harden INTX interrupts

[Jason Wang]

On Mon, Sep 13, 2021 at 2:41 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Mon, Sep 13, 2021 at 02:36:54PM +0800, Jason Wang wrote:
> > On Mon, Sep 13, 2021 at 2:33 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > >
> > > On Mon, Sep 13, 2021 at 01:53:51PM +0800, Jason Wang wrote:
> > > > This patch tries to make sure the virtio interrupt handler for INTX
> > > > won't be called after a reset and before virtio_device_ready(). We
> > > > can't use IRQF_NO_AUTOEN since we're using shared interrupt
> > > > (IRQF_SHARED). So this patch tracks the INTX enabling status in a new
> > > > intx_soft_enabled variable and toggle it during in
> > > > vp_disable/enable_vectors(). The INTX interrupt handler will check
> > > > intx_soft_enabled before processing the actual interrupt.
> > > >
> > > > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > >
> > >
> > > Not all that excited about all the memory barriers for something
> > > that should be an extremely rare event (for most kernels -
> > > literally once per boot). Can't we do better?
> >
> > I'm not sure, but do we need to care about the slow path (INTX)?
>
> Otherwise we won't try to support this, right?

Sorry, what I meant is "do we need to care about the performance of
the slow path".

>
> > (Or do you have a better approach?)
> >
> > Thanks
>
> Don't know really, maybe rcu or whatever?

I am sure it's worth it to bother since it's the slow path.

> But let's try to be much more specific - is there anything
> specific we are trying to protect against here?

The unexpected calling of the vring or config interrupt handler. (The
same as MSI-X, e.g the untrusted device can send irq at any time).

Thanks

>
>
>
> > >
> > > > ---
> > > >  drivers/virtio/virtio_pci_common.c | 18 ++++++++++++++++--
> > > >  drivers/virtio/virtio_pci_common.h |  1 +
> > > >  2 files changed, 17 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> > > > index 0b9523e6dd39..835197151dc1 100644
> > > > --- a/drivers/virtio/virtio_pci_common.c
> > > > +++ b/drivers/virtio/virtio_pci_common.c
> > > > @@ -30,8 +30,12 @@ void vp_disable_vectors(struct virtio_device *vdev)
> > > >       struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> > > >       int i;
> > > >
> > > > -     if (vp_dev->intx_enabled)
> > > > +     if (vp_dev->intx_enabled) {
> > > > +             vp_dev->intx_soft_enabled = false;
> > > > +             /* ensure the vp_interrupt see this intx_soft_enabled value */
> > > > +             smp_wmb();
> > > >               synchronize_irq(vp_dev->pci_dev->irq);
> > > > +     }
> > > >
> > > >       for (i = 0; i < vp_dev->msix_vectors; ++i)
> > > >               disable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> > > > @@ -43,8 +47,12 @@ 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)
> > > > +     if (vp_dev->intx_enabled) {
> > > > +             vp_dev->intx_soft_enabled = true;
> > > > +             /* ensure the vp_interrupt see this intx_soft_enabled value */
> > > > +             smp_wmb();
> > > >               return;
> > > > +     }
> > > >
> > > >       for (i = 0; i < vp_dev->msix_vectors; ++i)
> > > >               enable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> > > > @@ -97,6 +105,12 @@ static irqreturn_t vp_interrupt(int irq, void *opaque)
> > > >       struct virtio_pci_device *vp_dev = opaque;
> > > >       u8 isr;
> > > >
> > > > +     if (!vp_dev->intx_soft_enabled)
> > > > +             return IRQ_NONE;
> > > > +
> > > > +     /* read intx_soft_enabled before read others */
> > > > +     smp_rmb();
> > > > +
> > > >       /* reading the ISR has the effect of also clearing it so it's very
> > > >        * important to save off the value. */
> > > >       isr = ioread8(vp_dev->isr);
> > > > diff --git a/drivers/virtio/virtio_pci_common.h b/drivers/virtio/virtio_pci_common.h
> > > > index a235ce9ff6a5..3c06e0f92ee4 100644
> > > > --- a/drivers/virtio/virtio_pci_common.h
> > > > +++ b/drivers/virtio/virtio_pci_common.h
> > > > @@ -64,6 +64,7 @@ struct virtio_pci_device {
> > > >       /* MSI-X support */
> > > >       int msix_enabled;
> > > >       int intx_enabled;
> > > > +     bool intx_soft_enabled;
> > > >       cpumask_var_t *msix_affinity_masks;
> > > >       /* Name strings for interrupts. This size should be enough,
> > > >        * and I'm too lazy to allocate each name separately. */
> > > > --
> > > > 2.25.1
> > >
>