linux-kernel - Re: [PATCH v5] virtio-blk: Add validation for block size in config space

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Date:   Tue, 24 Aug 2021 01:31:10 +0300
From:   Max Gurtovoy <mgurtovoy@...dia.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     Yongji Xie <xieyongji@...edance.com>,
        Jason Wang <jasowang@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] virtio-blk: Add validation for block size in config
 space


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.

>
> Yongji - I think the commit log should be much more explicit that
> this is hardening. Otherwise people get confused and think this
> needs a CVE or a backport for security.
>