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

[Alex Williamson]

On Mon, 9 Mar 2020 11:36:46 +0800
Jason Wang <jasowang@xxxxxxxxxx> wrote:

> 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.

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

Alex