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Subject: Re: [PATCH v5] virtio-blk: Add validation for block size in config
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To:     Yongji Xie <xieyongji@bytedance.com>
CC:     Jason Wang <jasowang@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Stefan Hajnoczi <stefanha@redhat.com>,
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From:   Max Gurtovoy <mgurtovoy@nvidia.com>
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Date:   Tue, 24 Aug 2021 16:48:18 +0300
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On 8/24/2021 4:38 PM, Yongji Xie wrote:
> On Tue, Aug 24, 2021 at 9:30 PM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>>
>> On 8/24/2021 3:52 PM, Yongji Xie wrote:
>>> On Tue, Aug 24, 2021 at 6:11 PM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>>>> On 8/24/2021 5:47 AM, Jason Wang wrote:
>>>>> On Tue, Aug 24, 2021 at 6:31 AM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>>>>>> On 8/23/2021 3:13 PM, Michael S. Tsirkin wrote:
>>>>>>> On Mon, Aug 23, 2021 at 01:45:31PM +0300, Max Gurtovoy wrote:
>>>>>>>> It helpful if there is a justification for this.
>>>>>>>>
>>>>>>>> In this case, no such HW device exist and the only device that can cause
>>>>>>>> this trouble today is user space VDUSE device that must be validated by the
>>>>>>>> emulation VDUSE kernel driver.
>>>>>>>>
>>>>>>>> Otherwise, will can create 1000 commit like this in the virtio level (for
>>>>>>>> example for each feature for each virtio device).
>>>>>>> Yea, it's a lot of work but I don't think it's avoidable.
>>>>>>>
>>>>>>>>>>>>> And regardless of userspace device, we still need to fix it for other cases.
>>>>>>>>>>>> which cases ? Do you know that there is a buggy HW we need to workaround ?
>>>>>>>>>>>>
>>>>>>>>>>> No, there isn't now. But this could be a potential attack surface if
>>>>>>>>>>> the host doesn't trust the device.
>>>>>>>>>> If the host doesn't trust a device, why it continues using it ?
>>>>>>>>>>
>>>>>>>>> IIUC this is the case for the encrypted VMs.
>>>>>>>> what do you mean encrypted VM ?
>>>>>>>>
>>>>>>>> And how this small patch causes a VM to be 100% encryption supported ?
>>>>>>>>
>>>>>>>>>> Do you suggest we do these workarounds in all device drivers in the kernel ?
>>>>>>>>>>
>>>>>>>>> Isn't it the driver's job to validate some unreasonable configuration?
>>>>>>>> The check should be in different layer.
>>>>>>>>
>>>>>>>> Virtio blk driver should not cover on some strange VDUSE stuff.
>>>>>>> Yes I'm not convinced VDUSE is a valid use-case. I think that for
>>>>>>> security and robustness it should validate data it gets from userspace
>>>>>>> right there after reading it.
>>>>>>> But I think this is useful for the virtio hardening thing.
>>>>>>> https://lwn.net/Articles/865216/
>>>>>> I don't see how this change is assisting confidential computing.
>>>>>>
>>>>>> Confidential computingtalks about encrypting guest memory from the host,
>>>>>> and not adding some quirks to devices.
>>>>> In the case of confidential computing, the hypervisor and hard device
>>>>> is not in the trust zone. It means the guest doesn't trust the cloud
>>>>> vendor.
>>>> Confidential computing protects data during processing ("in-use" data).
>>>>
>>>> Nothing to do with virtio feature negotiation.
>>>>
>>> But if a misbehaving device can corrupt the guest memory, I think it
>>> should be avoided.
>> So don't say it's related to confidential computing, and fix it in the
>> VDUSE kernel driver in the hypervisor.
>>
> What I mean is in confidential computing cases. An untrusted device
> might corrupt the protected guest memory, it should be avoided.

This patch has nothing to do with confidential computing by definition 
(virtio feature negotiation are not "in-use" data).

It's device configuration space.

MST, I prefer adding quirks for vDPA devices in VDUSE driver and not 
adding workarounds to virtio driver.

I guess this patch can stay but future patches like this shouldn't be 
merged without a very good reason.

>
> Thanks,
> Yongji