From: "Michael S. Tsirkin" <mst@redhat.com>
To: Pierre Morel <pmorel@linux.ibm.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 17:18:09 -0400 [thread overview]
Message-ID: <20200629171241-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <66f808f2-5dd9-9127-d0e8-6bafbf13fc62@linux.ibm.com>
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
next prev parent reply other threads:[~2020-06-29 21:18 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-17 10:43 [PATCH v3 0/1] s390: virtio: let arch choose to accept devices without IOMMU feature Pierre Morel
2020-06-17 10:43 ` [PATCH v3 1/1] s390: virtio: let arch " Pierre Morel
2020-06-17 11:22 ` Heiko Carstens
2020-06-17 11:59 ` Pierre Morel
2020-06-17 13:36 ` Tom Lendacky
2020-06-17 14:12 ` Pierre Morel
2020-06-17 22:29 ` Halil Pasic
2020-06-19 9:20 ` Cornelia Huck
2020-06-19 12:02 ` Halil Pasic
2020-06-29 13:15 ` Pierre Morel
2020-06-29 13:14 ` Pierre Morel
2020-06-29 13:44 ` Cornelia Huck
2020-06-29 16:10 ` Pierre Morel
2020-06-29 13:21 ` Pierre Morel
2020-06-29 15:57 ` Michael S. Tsirkin
2020-06-29 16:05 ` Cornelia Huck
2020-07-02 13:03 ` Pierre Morel
2020-07-06 13:37 ` Pierre Morel
2020-07-06 14:33 ` Cornelia Huck
2020-07-06 15:01 ` Pierre Morel
2020-06-29 16:09 ` Pierre Morel
2020-06-29 16:09 ` Michael S. Tsirkin
2020-06-29 16:48 ` Pierre Morel
2020-06-29 21:18 ` Michael S. Tsirkin [this message]
2020-06-30 7:08 ` Cornelia Huck
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