[PATCH] s390: protvirt: virtio: Refuse device without IOMMU

Pierre Morel
2020-06-12T09:21:08Z

On 2020-06-11 05:10, Jason Wang wrote:
>
> On 2020/6/10 ??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 at linux.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