Messages in this thread | | | Subject | Re: [PATCH v3 1/1] s390: virtio: let arch accept devices without IOMMU feature | From | Pierre Morel <> | Date | Mon, 29 Jun 2020 18:48:28 +0200 |
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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
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