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
