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

[Max Gurtovoy]

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.

> That's why we need to validate any input from them.
>
> Thanks
>
> > > 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.