Re: [PATCH v8 26/29] vfio-pci: Register an iommu fault handler

[Auger Eric]

Hi Jean, Jacob,

On 6/6/19 10:29 PM, Jacob Pan wrote:
> On Thu, 6 Jun 2019 19:54:05 +0100
> Jean-Philippe Brucker <jean-philippe.brucker@xxxxxxx> wrote:
> 
>> On 05/06/2019 23:45, Jacob Pan wrote:
>>> On Tue, 4 Jun 2019 18:11:08 +0200
>>> Auger Eric <eric.auger@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>>>>
>>>>>> ---
>>>>>>
>>>>>> 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.

At the moment the handler unregistration is done on the vfio-pci release
function() when the last reference is released so I am not sure this can
even be achieved.
> 
> 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?

sounds good for me

Thanks

Eric
> 
>> Thanks,
>> Jean
> 
> [Jacob Pan]
>