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Subject: Re: [PATCH v5] virtio-blk: Add validation for block size in config
 space
To:     Yongji Xie <xieyongji@bytedance.com>
CC:     "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        virtualization <virtualization@lists.linux-foundation.org>,
        <linux-block@vger.kernel.org>,
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From:   Max Gurtovoy <mgurtovoy@nvidia.com>
Message-ID: <6d6154d7-7947-68be-4e1e-4c1d0a94b2bc@nvidia.com>
Date:   Mon, 23 Aug 2021 12:38:19 +0300
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On 8/23/2021 12:27 PM, Yongji Xie wrote:
> On Mon, Aug 23, 2021 at 5:04 PM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>>
>> On 8/23/2021 11:35 AM, Yongji Xie wrote:
>>> On Mon, Aug 23, 2021 at 4:07 PM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>>>> On 8/23/2021 7:31 AM, Yongji Xie wrote:
>>>>> On Mon, Aug 23, 2021 at 7:17 AM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>>>>>> On 8/9/2021 1:16 PM, Xie Yongji wrote:
>>>>>>> An untrusted device might presents an invalid block size
>>>>>>> in configuration space. This tries to add validation for it
>>>>>>> in the validate callback and clear the VIRTIO_BLK_F_BLK_SIZE
>>>>>>> feature bit if the value is out of the supported range.
>>>>>> This is not clear to me. What is untrusted device ? is it a buggy device ?
>>>>>>
>>>>> A buggy device, the devices in an encrypted VM, or a userspace device
>>>>> created by VDUSE [1].
>>>>>
>>>>> [1] https://lore.kernel.org/kvm/20210818120642.165-1-xieyongji@bytedance.com/
>>>> if it's a userspace device, why don't you fix its control path code
>>>> instead of adding workarounds in the kernel driver ?
>>>>
>>> VDUSE kernel module would not touch (be aware of) the device specific
>>> configuration space. It should be more reasonable to fix it in the
>>> device driver. There is also some existing interface (.validate()) for
>>> doing that.
>> who is emulating the device configuration space ?
>>
> A userspace daemon will initialize the device configuration space and
> pass the contents to the VDUSE kernel module. The VDUSE kernel module
> will handle the access of the config space from the virtio device
> driver, but it doesn't need to know the contents (although we can know
> that).

So you add a workaround in the guest kernel drivers instead of checking 
these quirks in the hypervisor ?

VDUSE kernel should enforce the security for the devices it 
emulates/presents to the VM.

>
>>> 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 ?

Do you suggest we do these workarounds in all device drivers in the kernel ?


>
> Thanks,
> Yongji