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

From: Jason Wang
Date: Mon Sep 13 2021 - 03:08:16 EST

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On Mon, Sep 13, 2021 at 2:50 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Mon, Sep 13, 2021 at 02:37:42AM -0400, Michael S. Tsirkin wrote:
> > On Mon, Sep 13, 2021 at 02:34:01PM +0800, Jason Wang wrote:
> > > On Mon, Sep 13, 2021 at 2:28 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > > >
> > > > On Mon, Sep 13, 2021 at 02:08:02PM +0800, Jason Wang wrote:
> > > > > 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
> > > >
> > > > Hmm but I don't see how do these patches do this.
> > > > They call disable_irq but can't the hypervisor keep
> > > > sending interrupts after disable_irq, too?
> > >
> > > Yes, but since the irq is disabled, the vring or config callback won't
> > > be called in this case.
> > >
> > > Thanks
> >
> > But doen't "irq is disabled" basically mean "we told the hypervisor
> > to disable the irq"? What extractly prevents hypervisor from
> > sending the irq even if guest thinks it disabled it?
>
> More generally, can't we for example blow away the
> indir_desc array that we use to keep the ctx pointers?
> Won't that be enough?

I'm not sure how it is related to the indirect descriptor but an
example is that all the current driver will assume:

1) the interrupt won't be raised before virtio_device_ready()
2) the interrupt won't be raised after reset()

All of these above two are not true if we don't trust the device. If
we want to audit all the drivers it would be not trivial and hard to
be correct. So it looks to me it's better to avoid those in the virtio
core.

>
>
> And looking at all this closely, I suspect it is wise to just completely
> disable hotplug/unplug for encrypted guests. Or maybe it's already
> the case?

I think not, it's more about:

1) what happens if the vq interrupt handler is called between
init_vqs() and virito_device_ready()
2) what happens if vq interrupt handler is called after reset

Instead of trying to answer those hard questions, it's better simply
not to have those conditions.

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
> > > > > >
> > > >
>

Next message: Geert Uytterhoeven: "Build regressions/improvements in v5.15-rc1"
Previous message: Vitaly Kuznetsov: "Re: [PATCH 2/2] KVM: x86: Identify vCPU0 by its vcpu_idx instead of walking vCPUs array"
In reply to: Michael S. Tsirkin: "Re: [PATCH 6/9] virtio_pci: harden MSI-X interrupts"
Next in thread: Thomas Gleixner: "Re: [PATCH 6/9] virtio_pci: harden MSI-X interrupts"
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