Re: [PATCH v2 0/7] vfio/pci: SR-IOV support

[Jason Wang]

On 2020/3/7 äå12:24, Alex Williamson wrote:
> On Fri, 6 Mar 2020 11:35:21 +0800
> Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> > On 2020/3/6 äå1:14, Alex Williamson wrote:
> > > On Tue, 25 Feb 2020 14:09:07 +0800
> > > Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > >   
> > > > On 2020/2/25 äå10:33, 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):
> > > > > Thanks for doing this!
> > > > >      
> > > > > > 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
> > > > > Can you provide a link to the DPDK discussion?
> > > > >      
> > > > > > 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.
> > > > > 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 (vhost-user),
> > > > > and even in separate VM (virtio-vhost-user), etc.
> > > > 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 than
> > > > using the exist generic POSIX mechanism to make sure the connection
> > > > (AF_UINX) is secure.
> > > >
> > > >   
> > > > >     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 just tries
> > > > > to do better than other sub-systems, but knowing how other people
> > > > > tackle the similar problem may make the whole picture clearer. ð
> > > > >
> > > > > +Jason.
> > > > 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 least
> > > > 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.
> >
> > 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
> ATS is a security risk regardless of SR-IOV, how does this change it?
> Thanks,


My understanding is the ATS only happen for some bugous devices. Some 
hardware has on-chip IOMMU, this probably means unprivileged userspace 
PF driver can control the on-chip IOMMU in this case.

Thanks


> Alex
>
> > > 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,
> >
> > Right, but I'm not sure it can be done only via virtio or need support
> > from transport (e.g PCI).
> >
> > Thanks
> >
> >
> > > Alex
> > >