linux-kernel - Re: [PATCH v6 4/7] vfio: ap: register IOMMU VFIO notifier

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Date:   Fri, 29 Mar 2019 09:14:48 -0400
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     pmorel@...ux.ibm.com, borntraeger@...ibm.com
Cc:     alex.williamson@...hat.com, cohuck@...hat.com,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, frankja@...ux.ibm.com, pasic@...ux.ibm.com,
        david@...hat.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, freude@...ux.ibm.com, mimu@...ux.ibm.com
Subject: Re: [PATCH v6 4/7] vfio: ap: register IOMMU VFIO notifier

On 3/29/19 5:31 AM, Pierre Morel wrote:
> On 28/03/2019 21:46, Tony Krowiak wrote:
>> On 3/22/19 10:43 AM, Pierre Morel wrote:
>>> To be able to use the VFIO interface to facilitate the
>>> mediated device memory pinning/unpinning we need to register
>>> a notifier for IOMMU.
>>>
>>> While we will start to pin one guest page for the interrupt indicator
>>> byte, this is still ok with ballooning as this page will never be
>>> used by the guest virtio-balloon driver.
>>> So the pinned page will never be freed. And even a broken guest does
>>> so, that would not impact the host as the original page is still
>>> in control by vfio.
>>
>> I apologize, but I do not understand what you are saying in the second
>> sentence of the paragraph above. Why will the pinned page never be freed?
> Because it is in use by the guest's kernel as a notification information 
> byte for the original PQAP AQIC.

Your comment says the pinned page will never be free, doesn't it get
freed when the guest is terminated?

> 
>   I understand that the pinned page is under the control of vfio
>> until it is freed, but have no idea what you mean by "and even a broken
>> guest does so"? A broken guest does what? Can you please reword this so
>> it makes more sense?
> 
> A broken guest could free the page used for the NIB. What is obviously 
> wrong.

Then why not simply say a pinned page is under the control of the
vfio driver, so if a broken (malicious?) guest frees the page, it will
not impact the host or something to that effect.

> 
>>
>>>
>>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>>> Reviewed-by: Cornelia Huck <cohuck@...hat.com>
>>> ---
>>>   drivers/s390/crypto/vfio_ap_ops.c     | 38 
>>> +++++++++++++++++++++++++++++++++++
>>>   drivers/s390/crypto/vfio_ap_private.h |  2 ++
>>>   2 files changed, 40 insertions(+)
>>>
>>> diff --git a/drivers/s390/crypto/vfio_ap_ops.c 
>>> b/drivers/s390/crypto/vfio_ap_ops.c
>>> index bdb36e0..3478499 100644
>>> --- a/drivers/s390/crypto/vfio_ap_ops.c
>>> +++ b/drivers/s390/crypto/vfio_ap_ops.c
>>> @@ -787,6 +787,35 @@ static const struct attribute_group 
>>> *vfio_ap_mdev_attr_groups[] = {
>>>       NULL
>>>   };
>>> +/**
>>> + * vfio_ap_mdev_iommu_notifier: IOMMU notifier callback
>>> + *
>>> + * @nb: The notifier block
>>> + * @action: Action to be taken
>>> + * @data: data associated with the request
>>> + *
>>> + * For an UNMAP request, unpin the guest IOVA (the NIB guest address we
>>> + * pinned before). Other requests are ignored.
>>> + *
>>> + */
>>> +static int vfio_ap_mdev_iommu_notifier(struct notifier_block *nb,
>>> +                       unsigned long action, void *data)
>>> +{
>>> +    struct ap_matrix_mdev *matrix_mdev;
>>> +
>>> +    matrix_mdev = container_of(nb, struct ap_matrix_mdev, 
>>> iommu_notifier);
>>> +
>>
>> I don't understand why we registered this notifier. I may be wrong, but
>> AFAIU, this notifier will be invoked only when the VFIO_IOMMU_UNMAP_DMA
>> ioctl is called from userspace. I did an experiment and inserted some
>> printf's to see if this ever gets called and verified it does not. Maybe
>> you have a good reason of which I'm not aware. Can you enlighten me
>> here?
> 
> The vfio_iommu_type1 pin page requires a notifier.
> 
> Regards,
> Pierre
>