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

[Jason Wang]

On 2020/6/12 äå7:38, Pierre Morel wrote:
> On 2020-06-12 11:21, Pierre Morel wrote:
> > On 2020-06-11 05:10, Jason Wang wrote:
> > > On 2020/6/10 äå9:11, 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@xxxxxxxxxxxxx>
> > > > ---
> > > > Â drivers/s390/virtio/virtio_ccw.c | 5 +++++
> > > > Â 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)
> > > > ÂÂÂÂÂ if (!ccw)
> > > > ÂÂÂÂÂÂÂÂÂ return;
> > > > +ÂÂÂ /* Protected Virtualisation guest needs IOMMU */
> > > > +ÂÂÂ if (is_prot_virt_guest() &&
> > > > +ÂÂÂÂÂÂÂ !__virtio_test_bit(vdev, VIRTIO_F_IOMMU_PLATFORM))
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ status &= ~VIRTIO_CONFIG_S_FEATURES_OK;
> > > > +
> > > > ÂÂÂÂÂ /* Write the status to the host. */
> > > > ÂÂÂÂÂ vcdev->dma_area->status = status;
> > > > ÂÂÂÂÂ 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.


I think this is fine.


> > 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
>
> hum, in between I found another way which seems to me much better:
>
> We already have the force_dma_unencrypted() function available which 
> AFAIU is what we want for encrypted memory protection and is already 
> used by power and x86 SEV/SME in a way that seems AFAIU compatible 
> with our problem.
>
> Even DMA and IOMMU are different things, I think they should be used 
> together in our case.
>
> What do you think?
>
> The patch would then be something like:
>
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index a977e32a88f2..53476d5bbe35 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -4,6 +4,7 @@
> Â#include <linux/virtio_config.h>
> Â#include <linux/module.h>
> Â#include <linux/idr.h>
> +#include <linux/dma-direct.h>
> Â#include <uapi/linux/virtio_ids.h>
>
> Â/* Unique numbering for virtio devices. */
> @@ -179,6 +180,10 @@ int virtio_finalize_features(struct virtio_device 
> *dev)
> ÂÂÂÂÂÂÂ if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1))
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return 0;
>
> +ÂÂÂÂÂÂ if (force_dma_unencrypted(&dev->dev) &&
> +ÂÂÂÂÂÂÂÂÂÂ !virtio_has_feature(dev, VIRTIO_F_IOMMU_PLATFORM))
> +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return -EIO;
> +
> ÂÂÂÂÂÂÂ virtio_add_status(dev, VIRTIO_CONFIG_S_FEATURES_OK);
> ÂÂÂÂÂÂÂ status = dev->config->get_status(dev);
> ÂÂÂÂÂÂÂ if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {


I think this can work but need to listen from Michael.

Thanks


> Regards,
> Pierre