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

From: Pierre Morel <pmorel@linux.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
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
Subject: Re: [PATCH v3 1/1] s390: virtio: let arch accept devices without IOMMU feature
Date: Mon, 29 Jun 2020 18:48:28 +0200	[thread overview]
Message-ID: <66f808f2-5dd9-9127-d0e8-6bafbf13fc62@linux.ibm.com> (raw)
In-Reply-To: <20200629115952-mutt-send-email-mst@kernel.org>

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