Re: [PATCH v3 1/1] s390: virtio: let arch accept devices without IOMMU feature

Michael S. Tsirkin
2020-06-29T21:18:30Z

On Mon, Jun 29, 2020 at 06:48:28PM +0200, Pierre Morel wrote:
>
>
> On 2020-06-29 18:09, Michael S. Tsirkin wrote:
> > On Wed, Jun 17, 2020 at 12:43:57PM +0200, Pierre Morel wrote:
> > > An architecture protecting the guest memory against unauthorized host
> > > access may want to enforce VIRTIO I/O device protection through the
> > > use of VIRTIO_F_IOMMU_PLATFORM.
> > > Let's give a chance to the architecture to accept or not devices
> > > without VIRTIO_F_IOMMU_PLATFORM.
> >
> > I agree it's a bit misleading. Protection is enforced by memory
> > encryption, you can't trust the hypervisor to report the bit correctly
> > so using that as a securoty measure would be pointless.
> > The real gain here is that broken configs are easier to
> > debug.
> >
> > Here's an attempt at a better description:
> >
> > On some architectures, guest knows that VIRTIO_F_IOMMU_PLATFORM is
> > required for virtio to function: e.g. this is the case on s390 protected
> > virt guests, since otherwise guest passes encrypted guest memory to devices,
> > which the device can't read. Without VIRTIO_F_IOMMU_PLATFORM the
> > result is that affected memory (or even a whole page containing
> > it is corrupted). Detect and fail probe instead - that is easier
> > to debug.
>
> Thanks indeed better aside from the "encrypted guest memory": the mechanism
> used to avoid the access to the guest memory from the host by s390 is not
> encryption but a hardware feature denying the general host access and
> allowing pieces of memory to be shared between guest and host.

s/encrypted/protected/

> As a consequence the data read from memory is not corrupted but not read at
> all and the read error kills the hypervizor with a SIGSEGV.

s/(or even a whole page containing it is corrupted)/can not be
read and the read error kills the hypervizor with a SIGSEGV/

As an aside, we could maybe handle that more gracefully
on the hypervisor side.

>
> >
> > however, now that we have described what it is (hypervisor
> > misconfiguration) I ask a question: can we be sure this will never
> > ever work? E.g. what if some future hypervisor gains ability to
> > access the protected guest memory in some abstractly secure manner?
>
> The goal of the s390 PV feature is to avoid this possibility so I don't
> think so; however, there is a possibility that some hardware VIRTIO device
> gain access to the guest's protected memory, even such device does not exist
> yet.
>
> At the moment such device exists we will need a driver for it, at least to
> enable the feature and apply policies, it is also one of the reasons why a
> hook to the architecture is interesting.

Not neessarily, it could also be fully transparent. See e.g.
recent AMD andvances allowing unmodified guests with SEV.

> > We are blocking this here, and it's hard to predict the future,
> > and a broken hypervisor can always find ways to crash the guest ...
>
> yes, this is also something to fix on the hypervizor side, Halil is working
> on it.
>
> >
> > IMHO it would be safer to just print a warning.
> > What do you think?
>
> Sadly, putting a warning may not help as qemu is killed if it accesses the
> protected memory.
> Also note that the crash occurs not only on start but also on hotplug.
>
> Thanks,
> Pierre

Well that depends on where does the warning go. If it's on a serial port
it might be reported host side before the crash triggers. But
interesting point generally. How about a feature to send a warning code
or string to host then?

> --
> Pierre Morel
> IBM Lab Boeblingen