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

[Max Gurtovoy]

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 ?

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