Re: [PATCH v2 1/1] s390: virtio: let arch accept devices without IOMMU feature

Pierre Morel
2020-06-16T10:53:09Z

On 2020-06-16 11:52, Halil Pasic wrote:
> On Mon, 15 Jun 2020 14:39:24 +0200
> Pierre Morel <pmorel@linux.ibm.com> wrote:
>
> I find the subject (commit short) sub optimal. The 'arch' is already
> accepting devices 'without IOMMU feature'. What you are introducing is
> the ability to reject.
>
>> An architecture protecting the guest memory against unauthorized host
>> access may want to enforce VIRTIO I/O device protection through the
>> use of VIRTIO_F_IOMMU_PLATFORM.
>>
>> Let's give a chance to the architecture to accept or not devices
>> without VIRTIO_F_IOMMU_PLATFORM.
>>
>
> I don't particularly like the commit message. In general, I believe
> using access_platform instead of iommu_platform would really benefit us.

IOMMU_PLATFORM is used overall in Linux, and I did not find any
occurrence for ACCESS_PLATFORM.

>
>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
>> ---
>> arch/s390/mm/init.c | 6 ++++++
>> drivers/virtio/virtio.c | 9 +++++++++
>> include/linux/virtio.h | 2 ++
>> 3 files changed, 17 insertions(+)
>>
>> diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
>> index 87b2d024e75a..3f04ad09650f 100644
>> --- a/arch/s390/mm/init.c
>> +++ b/arch/s390/mm/init.c
>> @@ -46,6 +46,7 @@
>> #include <asm/kasan.h>
>> #include <asm/dma-mapping.h>
>> #include <asm/uv.h>
>> +#include <linux/virtio.h>
>
> arch/s390/mm/init.c including virtio.h looks a bit strange to me, but
> if Heiko and Vasily don't mind, neither do I.

Do we have a better place to install the hook?
I though that since it is related to memory management and that, since
force_dma_unencrypted already is there, it would be a good place.

However, kvm-s390 is another candidate.

>
>>
>> pgd_t swapper_pg_dir[PTRS_PER_PGD] __section(.bss..swapper_pg_dir);
>>
>> @@ -162,6 +163,11 @@ bool force_dma_unencrypted(struct device *dev)
>> return is_prot_virt_guest();
>> }
>>
>> +int arch_needs_iommu_platform(struct virtio_device *dev)
>
> Maybe prefixing the name with virtio_ would help provide the
> proper context.

The virtio_dev makes it obvious and from the virtio side it should be
obvious that the arch is responsible for this.

However if nobody has something against I change it.

>
>> +{
>> + return is_prot_virt_guest();
>> +}
>> +
>> /* protected virtualization */
>> static void pv_init(void)
>> {
>> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
>> index a977e32a88f2..30091089bee8 100644
>> --- a/drivers/virtio/virtio.c
>> +++ b/drivers/virtio/virtio.c
>> @@ -167,6 +167,11 @@ void virtio_add_status(struct virtio_device *dev, unsigned int status)
>> }
>> EXPORT_SYMBOL_GPL(virtio_add_status);
>>
>> +int __weak arch_needs_iommu_platform(struct virtio_device *dev)
>> +{
>> + return 0;
>> +}
>> +
>
> Adding some people that could be interested in overriding this as well
> to the cc list.

Thanks,

>
>> int virtio_finalize_features(struct virtio_device *dev)
>> {
>> int ret = dev->config->finalize_features(dev);
>> @@ -179,6 +184,10 @@ int virtio_finalize_features(struct virtio_device *dev)
>> if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1))
>> return 0;
>>
>> + if (arch_needs_iommu_platform(dev) &&
>> + !virtio_has_feature(dev, VIRTIO_F_IOMMU_PLATFORM))
>> + return -EIO;
>> +
>
> Why EIO?

Because I/O can not occur correctly?
I am open to suggestions.

>
> Overall, I think it is a good idea to have something that is going to
> protect us from this scenario.
>

It would clearly be a good thing that trusted hypervizors like QEMU
forbid this scenario however should we let the door open?

Thanks,
Pierre

--
Pierre Morel
IBM Lab Boeblingen