From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH v3 1/1] s390: virtio: let arch accept devices without IOMMU feature References: <1592390637-17441-1-git-send-email-pmorel@linux.ibm.com> <1592390637-17441-2-git-send-email-pmorel@linux.ibm.com> <20200629115952-mutt-send-email-mst@kernel.org> From: Pierre Morel Message-ID: <66f808f2-5dd9-9127-d0e8-6bafbf13fc62@linux.ibm.com> Date: Mon, 29 Jun 2020 18:48:28 +0200 MIME-Version: 1.0 In-Reply-To: <20200629115952-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, pasic@linux.ibm.com, borntraeger@de.ibm.com, frankja@linux.ibm.com, jasowang@redhat.com, cohuck@redhat.com, kvm@vger.kernel.org, linux-s390@vger.kernel.org, virtualization@lists.linux-foundation.org, thomas.lendacky@amd.com, david@gibson.dropbear.id.au, linuxram@us.ibm.com, heiko.carstens@de.ibm.com, gor@linux.ibm.com 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