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Date:   Mon, 9 Mar 2020 08:45:13 -0600
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 Mon, 9 Mar 2020 11:36:46 +0800
Jason Wang <jasowang@redhat.com> wrote:

> On 2020/3/7 =E4=B8=8A=E5=8D=8812:24, Alex Williamson wrote:
> > On Fri, 6 Mar 2020 11:35:21 +0800
> > Jason Wang <jasowang@redhat.com> wrote:
> > =20
> >> On 2020/3/6 =E4=B8=8A=E5=8D=881:14, Alex Williamson wrote: =20
> >>> On Tue, 25 Feb 2020 14:09:07 +0800
> >>> Jason Wang <jasowang@redhat.com> wrote:
> >>>    =20
> >>>> On 2020/2/25 =E4=B8=8A=E5=8D=8810:33, Tian, Kevin wrote: =20
> >>>>>> 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 mechanis=
m of
> >>>>>> setting the vf_token.  This seems like a more user friendly API th=
an
> >>>>>> 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 whet=
her
> >>>>>> we absolutely need this new ioctl.  Currently I'm keeping it becau=
se 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 d=
rive
> >>>>>> PCIe SR-IOV PFs from userspace with VFIO.  There's an immediate ne=
ed
> >>>>>> 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 den=
ial
> >>>>>> 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 st=
ate
> >>>>>> otherwise private to the VF or VF driver. =20
> >>>>> Just a loud thinking. While the motivation of VF token sounds reaso=
nable
> >>>>> to me, I'm curious why the same concern is not raised in other usag=
es.
> >>>>> For example, there is no such design in virtio framework, where the
> >>>>> virtio device could also be restarted, putting in separate process =
(vhost-user),
> >>>>> and even in separate VM (virtio-vhost-user), etc. =20
> >>>> AFAIK, the restart could only be triggered by either VM or qemu. But
> >>>> yes, the datapath could be offloaded.
> >>>>
> >>>> But I'm not sure introducing another dedicated mechanism is better t=
han
> >>>> using the exist generic POSIX mechanism to make sure the connection
> >>>> (AF_UINX) is secure.
> >>>>
> >>>>    =20
> >>>>>     Of course the para-
> >>>>> virtualized attribute of virtio implies some degree of trust, but a=
s you
> >>>>> mentioned many SR-IOV implementations support VF->PF communication
> >>>>> which also implies some level of trust. It's perfectly fine if VFIO=
 just 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
> >>>> I'm not quite sure e.g allowing userspace PF driver with kernel VF
> >>>> driver would not break the assumption of kernel security model. At l=
east
> >>>> we should forbid a unprivileged PF driver running in userspace. =20
> >>> 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 participati=
on
> >>> around a PF that is not considered trusted by the kernel. =20
> >>
> >> I may miss something. But what happens if:
> >>
> >> - PF driver is running by unprivileged user
> >> - PF is programmed to send translated DMA request
> >> - Then unprivileged user can mangle the kernel data =20
> > ATS is a security risk regardless of SR-IOV, how does this change it?
> > Thanks, =20
>=20
>=20
> My understanding is the ATS only happen for some bugous devices. Some=20
> hardware has on-chip IOMMU, this probably means unprivileged userspace=20
> PF driver can control the on-chip IOMMU in this case.

Again, how does this relate to SR-IOV?  A PF is currently assignable
regardless of the support in this series.  Thanks,

Alex