Re: [RFC] /dev/ioasid uAPI proposal

Jason Gunthorpe
2021-06-09T14:45:37Z

On Wed, Jun 09, 2021 at 08:31:34AM -0600, Alex Williamson wrote:

> If we go back to the wbinvd ioctl mechanism, if I call that ioctl with
> an ioasidfd that contains no devices, then I shouldn't be able to
> generate a wbinvd on the processor, right? If I add a device,
> especially in a configuration that can generate non-coherent DMA, now
> that ioctl should work. If I then remove all devices from that ioasid,
> what then is the difference from the initial state. Should the ioctl
> now work because it worked once in the past?

The ioctl is fine, but telling KVM to enable WBINVD is very similar to
open and then reconfiguring the ioasid_fd is very similar to
chmod. From a security perspective revoke is not strictly required,
IMHO.

> access. This is no different than starting a shell via sudo (ie. an
> ongoing reference) or having the previous authentication time out, or
> in our case be notified it has expired.

Those are all authentication gates as well, yes sudo has a timer, but
once the timer expires it doesn't forcibly revoke & close all the
existing sudo sessions. It just means you can't create new ones
without authenticating.

> > > That's already more or less meaningless for both KVM and VFIO, since they
> > > are tied to an mm.
> >
> > vfio isn't supposed to be tied to a mm.
>
> vfio does accounting against an mm, why shouldn't it be tied to an mm?

It looks like vfio type 1 is doing it properly, each ranch of of user
VA is stuffed into a struct vfio_dma and that contains a struct task
(which can be a mm_struct these days) that refers to the owning mm.

Looks like a single fd can hold multiple vfio_dma's and I don't see an
enforcment that current is locked to any specific process.

When the accounting is done it is done via the mm obtained through the
vfio_dma struct, not a global FD wide mm.

This appears all fine for something using pin_user_pages(). We don't
expect FDs to become locked to a single process on the first call to
pin_user_pages() that is un-unixy.

kvm is special in this regard.

Jason