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Date:   Thu, 5 Mar 2020 10:14:06 -0700
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     "Tian, Kevin" <kevin.tian@intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
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        "Richardson, Bruce" <bruce.richardson@intel.com>,
        "cohuck@redhat.com" <cohuck@redhat.com>
Subject: Re: [PATCH v2 0/7] vfio/pci: SR-IOV support
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On Tue, 25 Feb 2020 14:09:07 +0800
Jason Wang <jasowang@redhat.com> wrote:

> On 2020/2/25 =E4=B8=8A=E5=8D=8810:33, Tian, Kevin wrote:
> >> From: Alex Williamson
> >> Sent: Thursday, February 20, 2020 2:54 AM
> >>
> >> Changes since v1 are primarily to patch 3/7 where the commit log is
> >> rewritten, along with option parsing and failure logging based on
> >> upstream discussions.  The primary user visible difference is that
> >> option parsing is now much more strict.  If a vf_token option is
> >> provided that cannot be used, we generate an error.  As a result of
> >> this, opening a PF with a vf_token option will serve as a mechanism of
> >> setting the vf_token.  This seems like a more user friendly API than
> >> the alternative of sometimes requiring the option (VFs in use) and
> >> sometimes rejecting it, and upholds our desire that the option is
> >> always either used or rejected.
> >>
> >> This also means that the VFIO_DEVICE_FEATURE ioctl is not the only
> >> means of setting the VF token, which might call into question whether
> >> we absolutely need this new ioctl.  Currently I'm keeping it because I
> >> can imagine use cases, for example if a hypervisor were to support
> >> SR-IOV, the PF device might be opened without consideration for a VF
> >> token and we'd require the hypservisor to close and re-open the PF in
> >> order to set a known VF token, which is impractical.
> >>
> >> Series overview (same as provided with v1): =20
> > Thanks for doing this!
> > =20
> >> The synopsis of this series is that we have an ongoing desire to drive
> >> PCIe SR-IOV PFs from userspace with VFIO.  There's an immediate need
> >> for this with DPDK drivers and potentially interesting future use =20
> > Can you provide a link to the DPDK discussion?
> > =20
> >> cases in virtualization.  We've been reluctant to add this support
> >> previously due to the dependency and trust relationship between the
> >> VF device and PF driver.  Minimally the PF driver can induce a denial
> >> of service to the VF, but depending on the specific implementation,
> >> the PF driver might also be responsible for moving data between VFs
> >> or have direct access to the state of the VF, including data or state
> >> otherwise private to the VF or VF driver. =20
> > Just a loud thinking. While the motivation of VF token sounds reasonable
> > to me, I'm curious why the same concern is not raised in other usages.
> > For example, there is no such design in virtio framework, where the
> > virtio device could also be restarted, putting in separate process (vho=
st-user),
> > and even in separate VM (virtio-vhost-user), etc. =20
>=20
>=20
> AFAIK, the restart could only be triggered by either VM or qemu. But=20
> yes, the datapath could be offloaded.
>=20
> But I'm not sure introducing another dedicated mechanism is better than=20
> using the exist generic POSIX mechanism to make sure the connection=20
> (AF_UINX) is secure.
>=20
>=20
> >   Of course the para-
> > virtualized attribute of virtio implies some degree of trust, but as you
> > mentioned many SR-IOV implementations support VF->PF communication
> > which also implies some level of trust. It's perfectly fine if VFIO jus=
t tries
> > to do better than other sub-systems, but knowing how other people
> > tackle the similar problem may make the whole picture clearer. =F0=9F=
=98=8A
> >
> > +Jason. =20
>=20
>=20
> I'm not quite sure e.g allowing userspace PF driver with kernel VF=20
> driver would not break the assumption of kernel security model. At least=
=20
> we should forbid a unprivileged PF driver running in userspace.

It might be useful to have your opinion on this series, because that's
exactly what we're trying to do here.  Various environments, DPDK
specifically, want a userspace PF driver.  This series takes steps to
mitigate the risk of having such a driver, such as requiring this VF
token interface to extend the VFIO interface and validate participation
around a PF that is not considered trusted by the kernel.  We also set
a driver_override to try to make sure no host kernel driver can
automatically bind to a VF of a user owned PF, only vfio-pci, but we
don't prevent the admin from creating configurations where the VFs are
used by other host kernel drivers.

I think the question Kevin is inquiring about is whether virtio devices
are susceptible to the type of collaborative, shared key environment
we're creating here.  For example, can a VM or qemu have access to
reset a virtio device in a way that could affect other devices, ex. FLR
on a PF that could interfere with VF operation.  Thanks,

Alex