Re: [PATCH 04/10] vfio: Move storage of allow_unsafe_interrupts to vfio_main.c

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 7 Nov 2022 09:19:43 -0400
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Mon, Oct 31, 2022 at 04:45:26PM -0600, Alex Williamson wrote:
> 
> > > It is one idea, it depends how literal you want to be on "module
> > > parameters are ABI". IMHO it is a weak form of ABI and the need of
> > > this paramter in particular is not that common in modern times, AFAIK.
> > > 
> > > So perhaps we just also expose it through vfio.ko and expect people to
> > > migrate. That would give a window were both options are available.  
> > 
> > That might be best.  Ultimately this is an opt-out of a feature that
> > has security implications, so I'd rather error on the side of requiring
> > the user to re-assert that opt-out.  It seems the potential good in
> > eliminating stale or unnecessary options outweighs any weak claims of
> > preserving an ABI for a module that's no longer in service.  
> 
> Ok, lets do this
> 
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -55,6 +55,11 @@ static struct vfio {
>  bool vfio_allow_unsafe_interrupts;
>  EXPORT_SYMBOL_GPL(vfio_allow_unsafe_interrupts);
>  
> +module_param_named(allow_unsafe_interrupts,
> +                  vfio_allow_unsafe_interrupts, bool, S_IRUGO | S_IWUSR);
> +MODULE_PARM_DESC(allow_unsafe_interrupts,
> +                "Enable VFIO IOMMU support for on platforms without interrupt remapping support.");
> +
>  static DEFINE_XARRAY(vfio_device_set_xa);
>  static const struct file_operations vfio_group_fops;
> 
> > However, I'd question whether vfio is the right place for that new
> > module option.  As proposed, vfio is only passing it through to
> > iommufd, where an error related to lack of the hardware feature is
> > masked behind an -EPERM by the time it gets back to vfio, making any
> > sort of advisory to the user about the module option convoluted.  It
> > seems like iommufd should own the option to opt-out universally, not
> > just through the vfio use case.  Thanks,  
> 
> My thinking is this option shouldn't exist at all in other iommufd
> users. eg I don't see value in VDPA supporting it.

I disagree, the IOMMU interface is responsible for isolating the
device, this option doesn't make any sense to live in vfio-main, which
is the reason it was always a type1 option.  If vdpa doesn't allow full
device access such that it can guarantee that a device cannot generate
a DMA that can spoof MSI, then it sounds like the flag we pass when
attaching a device to iommfd should to reflect this difference in usage.
The driver either requires full isolation, default, or can indicate a
form of restricted DMA programming that prevents interrupt spoofing.
The policy whether to permit unsafe configurations should exist in one
place, iommufd.  Thanks,

Alex