Re: CONFIG_XEN_VIRTIO{_FORCE_GRANT} interferes with nested virt

[Juergen Gross <jgross@xxxxxxxx>]

On 05.10.22 17:35, Marek Marczykowski-Górecki wrote:
> On Wed, Oct 05, 2022 at 05:04:29PM +0200, Juergen Gross wrote:
> > On 05.10.22 15:51, Marek Marczykowski-Górecki wrote:
> > > On Wed, Oct 05, 2022 at 03:34:56PM +0200, Juergen Gross wrote:
> > > > On 05.10.22 15:25, Marek Marczykowski-Górecki wrote:
> > > > > On Wed, Oct 05, 2022 at 02:57:01PM +0200, Juergen Gross wrote:
> > > > > > On 05.10.22 14:41, Marek Marczykowski-Górecki wrote:
> > > > > > > Hi,
> > > > > > >
> > > > > > > When booting Xen with Linux dom0 nested under KVM,
> > > > > > > CONFIG_XEN_VIRTIO_FORCE_GRANT=y makes it unable to use virtio devices
> > > > > > > provided by L0 hypervisor (KVM with qemu). With PV dom0, grants are
> > > > > > > required for virtio even if just CONFIG_XEN_VIRTIO is enabled.
> > > > > > >
> > > > > > > This is probably uncommon corner case, but one that has bitten me in my
> > > > > > > CI setup... I think Xen should set smarter
> > > > > > > virtio_require_restricted_mem_acc(), that enforces it only for devices
> > > > > > > really provided by another Xen VM (not by the "outer host"), but I'm not
> > > > > > > sure how that could be done. Any ideas?
> > > > > > It should be possible to add a boot parameter for that purpose. Using it
> > > > > > would open a security hole, though (basically like all PCI passthrough to
> > > > > > PV guests).
> > > > > What about excluding just dom0? At least currently, there is no way for
> > > > > dom0 to see virtio devices provided by another Xen domU.
> > > > Even not via hotplug?
> > > That's why I said "currently", IIUC hotplug of virtio devices under Xen
> > > doesn't work yet, no?
> > > With hotplug working, it would need to be a proper detection where the
> > > backend lives, and probably with some extra considerations re Xen on
> > > Xen (based on below, pv-shim could use grants).
> > As stated before, this isn't a problem specific to virtio devices. The same
> > applies to Xen PV devices.
> Why is that an issue for Xen PV devices? They always use grants, so no need
> for exception. But more relevant here, there is no protocol for L0
> hypervisor (that would need to be Xen) to provide PV device to nested L1
> guest (besides pv-shim case, which is already handled), so L1 guest
> cannot confuse PV device provided by L0 and L1.
That's the point. Today using virtio the way you are using it is possible
only because virtio devices don't have the security features of Xen PV
devices. With adding grant support for virtio devices this has changed.

BTW, you can have the old virtio behavior back by not enabling
CONFIG_XEN_VIRTIO.

> > > For me specifically, a command line option would work (because I don't
> > > use Xen-based virtio devices when nested under KVM, or anywhere at all,
> > > at least not yet), but I can see future cases where you have virtio
> > > devices from both L0 and L1 in the same guest, and then it wouldn't be
> > > that simple.
> > Lets think of a general solution covering all PV devices (Xen and virtio).
> In fact, I wonder what's the security benefit of
> CONFIG_XEN_VIRTIO_FORCE_GRANT. If the backend lives in dom0 (or
> stubdomain), it can access whole guest memory anyway, whether frontend
> likes it or not. But if the backend is elsewhere (or guest is protected
> with AMD SEV-SNP, XSM or similar), then the backend won't be able to access
> memory outside of what frontend shares explicitly. So, in the non-dom0 case,
> backend trying to provide non-grant-based virtio device will simply not
> function (because of inability to access guest's memory), instead of
> gaining unintended access. Am I missing some implicit memory sharing
> here?
You are missing the possibility to have a deprivileged user land virtio
backend.

And BTW, SEV won't disable guest memory access, it will just make it
impossible to interprete memory contents from outside. A malicious
backend can still easily crash a SEV guest by clobbering its memory.


Juergen

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