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

[Yongji Xie]

On Tue, Aug 24, 2021 at 6:11 PM Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote:
>
>
> On 8/24/2021 5:47 AM, Jason Wang wrote:
> > On Tue, Aug 24, 2021 at 6:31 AM Max Gurtovoy <mgurtovoy@xxxxxxxxxx> 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.

Thanks,
Yongji