linux-kernel - Re: [PATCH] s390: protvirt: virtio: Refuse device without IOMMU

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Date:   Fri, 12 Jun 2020 11:21:08 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Jason Wang <jasowang@...hat.com>, linux-kernel@...r.kernel.org
Cc:     pasic@...ux.ibm.com, borntraeger@...ibm.com, frankja@...ux.ibm.com,
        mst@...hat.com, cohuck@...hat.com, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] s390: protvirt: virtio: Refuse device without IOMMU



On 2020-06-11 05:10, Jason Wang wrote:
> 
> On 2020/6/10 下午9:11, Pierre Morel wrote:
>> Protected Virtualisation protects the memory of the guest and
>> do not allow a the host to access all of its memory.
>>
>> Let's refuse a VIRTIO device which does not use IOMMU
>> protected access.
>>
>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>> ---
>>   drivers/s390/virtio/virtio_ccw.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/drivers/s390/virtio/virtio_ccw.c 
>> b/drivers/s390/virtio/virtio_ccw.c
>> index 5730572b52cd..06ffbc96587a 100644
>> --- a/drivers/s390/virtio/virtio_ccw.c
>> +++ b/drivers/s390/virtio/virtio_ccw.c
>> @@ -986,6 +986,11 @@ static void virtio_ccw_set_status(struct 
>> virtio_device *vdev, u8 status)
>>       if (!ccw)
>>           return;
>> +    /* Protected Virtualisation guest needs IOMMU */
>> +    if (is_prot_virt_guest() &&
>> +        !__virtio_test_bit(vdev, VIRTIO_F_IOMMU_PLATFORM))
>> +            status &= ~VIRTIO_CONFIG_S_FEATURES_OK;
>> +
>>       /* Write the status to the host. */
>>       vcdev->dma_area->status = status;
>>       ccw->cmd_code = CCW_CMD_WRITE_STATUS;
> 
> 
> I wonder whether we need move it to virtio core instead of ccw.
> 
> I think the other memory protection technologies may suffer from this as 
> well.
> 
> Thanks
> 


What would you think of the following, also taking into account Connie's 
comment on where the test should be done:

- declare a weak function in virtio.c code, returning that memory 
protection is not in use.

- overwrite the function in the arch code

- call this function inside core virtio_finalize_features() and if 
required fail if the device don't have VIRTIO_F_IOMMU_PLATFORM.

Alternative could be to test a global variable that the architecture 
would overwrite if needed but I find the weak function solution more 
flexible.

With a function, we also have the possibility to provide the device as 
argument and take actions depending it, this may answer Halil's concern.

Regards,
Pierre

-- 
Pierre Morel
IBM Lab Boeblingen