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

Jason Wang
2020-03-06T03:35:40Z

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

> 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
>