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

Jason Wang
2020-03-09T03:37:07Z

On 2020/3/7 上午12:24, Alex Williamson wrote:
> On Fri, 6 Mar 2020 11:35:21 +0800
> Jason Wang <jasowang@redhat.com> wrote:
>
>> On 2020/3/6 上午1:14, Alex Williamson wrote:
>>> On Tue, 25 Feb 2020 14:09:07 +0800
>>> Jason Wang <jasowang@redhat.com> wrote:
>>>
>>>> On 2020/2/25 上午10:33, 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. 😊
>>>>>
>>>>> +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
>>>