Re: [PATCH v5 04/14] driver core: platform: Add driver dma ownership management

[Jason Gunthorpe]

On Mon, Feb 14, 2022 at 02:37:15PM +0100, Greg Kroah-Hartman wrote:
> On Mon, Feb 14, 2022 at 09:18:53AM -0400, Jason Gunthorpe wrote:
> > On Mon, Feb 14, 2022 at 10:59:50AM +0100, Greg Kroah-Hartman wrote:
> > 
> > > > +	if (ret && !drv->no_kernel_api_dma)
> > > > +		iommu_device_unuse_dma_api(dev);
> > > 
> > > So you are now going to call this for every platform driver _unless_
> > > they set this flag?
> > 
> > Yes, it is necessary because VFIO supports platform devices as well
> > and needs to ensure security. Conflicting kernel driver attachements
> > must be blocked, just like for PCI.
> 
> A platform device shouldn't be using VFIO, but ugh, oh well, that ship
> has sailed :(

I don't know why you say that, but yes, this is was set long ago.

> And stop it with the "security" mess, do not give people a false sense
> of it here please.

I'm confused by what you mean. This is all about what we tend to refer
to as DMA security - meaning a device's DMA can be controled by a
hostile environment and not impact the integrity of the kernel.

Jason