Re: [PATCH v7 2/2] s390: virtio: PV needs VIRTIO I/O device protection

[Michael S. Tsirkin]

On Wed, Jul 15, 2020 at 10:31:09AM +0200, Pierre Morel wrote:
> If protected virtualization is active on s390, the virtio queues are
> not accessible to the host, unless VIRTIO_F_IOMMU_PLATFORM has been
> negotiated. Use the new arch_validate_virtio_features() interface to
> fail probe if that's not the case, preventing a host error on access
> attempt.
> 
> Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
> Reviewed-by: Cornelia Huck <cohuck@xxxxxxxxxx>
> Acked-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
> Acked-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> ---
>  arch/s390/mm/init.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
> index 6dc7c3b60ef6..d39af6554d4f 100644
> --- a/arch/s390/mm/init.c
> +++ b/arch/s390/mm/init.c
> @@ -45,6 +45,7 @@
>  #include <asm/kasan.h>
>  #include <asm/dma-mapping.h>
>  #include <asm/uv.h>
> +#include <linux/virtio_config.h>
>  
>  pgd_t swapper_pg_dir[PTRS_PER_PGD] __section(.bss..swapper_pg_dir);
>  
> @@ -161,6 +162,33 @@ bool force_dma_unencrypted(struct device *dev)
>  	return is_prot_virt_guest();
>  }
>  
> +/*
> + * arch_validate_virtio_features
> + * @dev: the VIRTIO device being added
> + *
> + * Return an error if required features are missing on a guest running
> + * with protected virtualization.
> + */
> +int arch_validate_virtio_features(struct virtio_device *dev)
> +{
> +	if (!is_prot_virt_guest())
> +		return 0;
> +
> +	if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1)) {
> +		dev_warn(&dev->dev,
> +			 "legacy virtio not supported with protected virtualization\n");
> +		return -ENODEV;
> +	}
> +
> +	if (!virtio_has_feature(dev, VIRTIO_F_IOMMU_PLATFORM)) {
> +		dev_warn(&dev->dev,
> +			 "support for limited memory access required for protected virtualization\n");
> +		return -ENODEV;
> +	}
> +
> +	return 0;
> +}
> +
>  /* protected virtualization */
>  static void pv_init(void)
>  {

What bothers me here is that arch code depends on virtio now.
It works even with a modular virtio when functions are inline,
but it seems fragile: e.g. it breaks virtio as an out of tree module,
since layout of struct virtio_device can change.

I'm not sure what to do with this yet, will try to think about it
over the weekend. Thanks!


> -- 
> 2.25.1