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

[Pierre Morel]

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.
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.


> 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.

> 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

--
Pierre Morel
IBM Lab Boeblingen