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Subject: Re: [PATCH v2 0/7] vfio/pci: SR-IOV support
To:     Alex Williamson <alex.williamson@redhat.com>
Cc:     "Tian, Kevin" <kevin.tian@intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
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        "thomas@monjalon.net" <thomas@monjalon.net>,
        "bluca@debian.org" <bluca@debian.org>,
        "jerinjacobk@gmail.com" <jerinjacobk@gmail.com>,
        "Richardson, Bruce" <bruce.richardson@intel.com>,
        "cohuck@redhat.com" <cohuck@redhat.com>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <d9ac428f-c764-014c-db5b-3f94d8f3e626@redhat.com>
Date:   Mon, 9 Mar 2020 11:36:46 +0800
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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:
>
>> On 2020/3/6 =E4=B8=8A=E5=8D=881:14, Alex Williamson wrote:
>>> 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:
>>>>>> From: Alex Williamson
>>>>>> Sent: Thursday, February 20, 2020 2:54 AM
>>>>>>
>>>>>> Changes since v1 are primarily to patch 3/7 where the commit log i=
s
>>>>>> 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 o=
f
>>>>>> 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):
>>>>> 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
>>>>> 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 th=
e
>>>>>> 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 VF=
s
>>>>>> or have direct access to the state of the VF, including data or st=
ate
>>>>>> otherwise private to the VF or VF driver.
>>>>> 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.
>>>> 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.
>>>> 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.
>>> 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.
>>
>> 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=20
hardware has on-chip IOMMU, this probably means unprivileged userspace=20
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 ar=
e
>>> used by other host kernel drivers.
>>>
>>> I think the question Kevin is inquiring about is whether virtio devic=
es
>>> 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. F=
LR
>>> 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
>>>  =20