On Mon, 29 Jun 2020 17:18:09 -0400 "Michael S. Tsirkin" <m...@redhat.com> wrote:
> 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. s/guest/the guest/ (x2) > > > > 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/ s/hypervizor/hypervisor/ > > > 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. I guess it depends on the architecture's protection mechanism and threat model whether this makes sense. > > > > > 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. Failing to start a guest is not that bad IMHO, but crashing a guest that is running perfectly fine is. I vote for just failing the probe if preconditions are not met. > > > > 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? I would generally expect a guest warning to stay on the guest side -- especially as the host admin and the guest admin may be different persons. So having a general way to send an alert to from a guest to the host is not uninteresting, although we need to be careful to avoid the guest being able to DOS the host.