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SubjectRe: [PATCH v8 26/29] vfio-pci: Register an iommu fault handler
On Thu, 6 Jun 2019 19:54:05 +0100
Jean-Philippe Brucker <jean-philippe.brucker@arm.com> wrote:

> On 05/06/2019 23:45, Jacob Pan wrote:
> > On Tue, 4 Jun 2019 18:11:08 +0200
> > Auger Eric <eric.auger@redhat.com> wrote:
> >
> >> Hi Alex,
> >>
> >> On 6/4/19 12:31 AM, Alex Williamson wrote:
> >>> On Sun, 26 May 2019 18:10:01 +0200
> >>> Eric Auger <eric.auger@redhat.com> wrote:
> >>>
> >>>> This patch registers a fault handler which records faults in
> >>>> a circular buffer and then signals an eventfd. This buffer is
> >>>> exposed within the fault region.
> >>>>
> >>>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> >>>>
> >>>> ---
> >>>>
> >>>> v3 -> v4:
> >>>> - move iommu_unregister_device_fault_handler to vfio_pci_release
> >>>> ---
> >>>> drivers/vfio/pci/vfio_pci.c | 49
> >>>> +++++++++++++++++++++++++++++ drivers/vfio/pci/vfio_pci_private.h
> >>>> | 1 + 2 files changed, 50 insertions(+)
> >>>>
> >>>> diff --git a/drivers/vfio/pci/vfio_pci.c
> >>>> b/drivers/vfio/pci/vfio_pci.c index f75f61127277..520999994ba8
> >>>> 100644 --- a/drivers/vfio/pci/vfio_pci.c
> >>>> +++ b/drivers/vfio/pci/vfio_pci.c
> >>>> @@ -30,6 +30,7 @@
> >>>> #include <linux/vfio.h>
> >>>> #include <linux/vgaarb.h>
> >>>> #include <linux/nospec.h>
> >>>> +#include <linux/circ_buf.h>
> >>>>
> >>>> #include "vfio_pci_private.h"
> >>>>
> >>>> @@ -296,6 +297,46 @@ static const struct vfio_pci_regops
> >>>> vfio_pci_fault_prod_regops = { .add_capability =
> >>>> vfio_pci_fault_prod_add_capability, };
> >>>>
> >>>> +int vfio_pci_iommu_dev_fault_handler(struct iommu_fault_event
> >>>> *evt, void *data) +{
> >>>> + struct vfio_pci_device *vdev = (struct vfio_pci_device
> >>>> *) data;
> >>>> + struct vfio_region_fault_prod *prod_region =
> >>>> + (struct vfio_region_fault_prod
> >>>> *)vdev->fault_pages;
> >>>> + struct vfio_region_fault_cons *cons_region =
> >>>> + (struct vfio_region_fault_cons
> >>>> *)(vdev->fault_pages + 2 * PAGE_SIZE);
> >>>> + struct iommu_fault *new =
> >>>> + (struct iommu_fault *)(vdev->fault_pages +
> >>>> prod_region->offset +
> >>>> + prod_region->prod *
> >>>> prod_region->entry_size);
> >>>> + int prod, cons, size;
> >>>> +
> >>>> + mutex_lock(&vdev->fault_queue_lock);
> >>>> +
> >>>> + if (!vdev->fault_abi)
> >>>> + goto unlock;
> >>>> +
> >>>> + prod = prod_region->prod;
> >>>> + cons = cons_region->cons;
> >>>> + size = prod_region->nb_entries;
> >>>> +
> >>>> + if (CIRC_SPACE(prod, cons, size) < 1)
> >>>> + goto unlock;
> >>>> +
> >>>> + *new = evt->fault;
> >>>> + prod = (prod + 1) % size;
> >>>> + prod_region->prod = prod;
> >>>> + mutex_unlock(&vdev->fault_queue_lock);
> >>>> +
> >>>> + mutex_lock(&vdev->igate);
> >>>> + if (vdev->dma_fault_trigger)
> >>>> + eventfd_signal(vdev->dma_fault_trigger, 1);
> >>>> + mutex_unlock(&vdev->igate);
> >>>> + return 0;
> >>>> +
> >>>> +unlock:
> >>>> + mutex_unlock(&vdev->fault_queue_lock);
> >>>> + return -EINVAL;
> >>>> +}
> >>>> +
> >>>> static int vfio_pci_init_fault_region(struct vfio_pci_device
> >>>> *vdev) {
> >>>> struct vfio_region_fault_prod *header;
> >>>> @@ -328,6 +369,13 @@ static int vfio_pci_init_fault_region(struct
> >>>> vfio_pci_device *vdev) header = (struct vfio_region_fault_prod
> >>>> *)vdev->fault_pages; header->version = -1;
> >>>> header->offset = PAGE_SIZE;
> >>>> +
> >>>> + ret =
> >>>> iommu_register_device_fault_handler(&vdev->pdev->dev,
> >>>> +
> >>>> vfio_pci_iommu_dev_fault_handler,
> >>>> + vdev);
> >>>> + if (ret)
> >>>> + goto out;
> >>>> +
> >>>> return 0;
> >>>> out:
> >>>> kfree(vdev->fault_pages);
> >>>> @@ -570,6 +618,7 @@ static void vfio_pci_release(void
> >>>> *device_data) if (!(--vdev->refcnt)) {
> >>>> vfio_spapr_pci_eeh_release(vdev->pdev);
> >>>> vfio_pci_disable(vdev);
> >>>> +
> >>>> iommu_unregister_device_fault_handler(&vdev->pdev->dev);
> >>>
> >>>
> >>> But this can fail if there are pending faults which leaves a
> >>> device reference and then the system is broken :(
> >> This series only features unrecoverable errors and for those the
> >> unregistration cannot fail. Now unrecoverable errors were added I
> >> admit this is confusing. We need to sort this out or clean the
> >> dependencies.
> > As Alex pointed out in 4/29, we can make
> > iommu_unregister_device_fault_handler() never fail and clean up all
> > the pending faults in the host IOMMU belong to that device. But the
> > problem is that if a fault, such as PRQ, has already been injected
> > into the guest, the page response may come back after handler is
> > unregistered and registered again.
>
> I'm trying to figure out if that would be harmful in any way. I guess
> it can be a bit nasty if we handle the page response right after
> having injected a new page request that uses the same PRGI. In any
> other case we discard the page response, but here we forward it to
> the endpoint and:
>
> * If the response status is success, endpoint retries the
> translation. The guest probably hasn't had time to handle the new
> page request and translation will fail, which may lead the endpoint
> to give up (two unsuccessful translation requests). Or send a new
> request
>
Good point, there shouldn't be any harm if the page response is a
"fake" success. In fact it could happen in the normal operation when
PRQs to two devices share the same non-leaf translation structure. The
worst case is just a retry. I am not aware of the retry limit, is it in
the PCIe spec? I cannot find it.

I think we should just document it, similar to having a spurious
interrupt. The PRQ trace event should capture that as well.

> * otherwise the endpoint won't retry the access, and could also
> disable PRI if the status is failure.
>
That would be true regardless this race condition with handler
registration. So should be fine.

> > We need a way to reject such page response belong
> > to the previous life of the handler. Perhaps a sync call to the
> > guest with your fault queue eventfd? I am not sure.
>
> We could simply expect the device driver not to send any page response
> after unregistering the fault handler. Is there any reason VFIO would
> need to unregister and re-register the fault handler on a live guest?
>
There is no reason for VFIO to unregister and register again, I was
just thinking from security perspective. Someone could write a VFIO app
do this attack. But I agree the damage is within the device, may get
PRI disabled as a result.

So it seems we agree on the following:
- iommu_unregister_device_fault_handler() will never fail
- iommu driver cleans up all pending faults when handler is unregistered
- assume device driver or guest not sending more page response _after_
handler is unregistered.
- system will tolerate rare spurious response

Sounds right?

> Thanks,
> Jean

[Jacob Pan]

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