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

[Yongji Xie]

On Mon, Aug 23, 2021 at 5:38 PM Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote:
>
>
> On 8/23/2021 12:27 PM, Yongji Xie wrote:
> > On Mon, Aug 23, 2021 at 5:04 PM Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote:
> >>
> >> On 8/23/2021 11:35 AM, Yongji Xie wrote:
> >>> On Mon, Aug 23, 2021 at 4:07 PM Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote:
> >>>> On 8/23/2021 7:31 AM, Yongji Xie wrote:
> >>>>> On Mon, Aug 23, 2021 at 7:17 AM Max Gurtovoy <mgurtovoy@xxxxxxxxxx> 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@xxxxxxxxxxxxx/
> >>>> 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 ?
>

I didn't see any problem adding this validation in the device driver.

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

I agree that the VDUSE kernel should enforce the security for the
emulated devices. But I still think the virtio device driver should
handle this case since nobody can make sure the device can always set
the correct value. Adding this validation would be helpful.

> >>> 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.

> 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?

Thanks,
Yongji