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

[Max Gurtovoy]

On 8/24/2021 4:38 PM, Yongji Xie wrote:
> On Tue, Aug 24, 2021 at 9:30 PM Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote:
> > On 8/24/2021 3:52 PM, Yongji Xie wrote:
> > > 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.
> > So don't say it's related to confidential computing, and fix it in the
> > VDUSE kernel driver in the hypervisor.
> What I mean is in confidential computing cases. An untrusted device
> might corrupt the protected guest memory, it should be avoided.

This patch has nothing to do with confidential computing by definition 
(virtio feature negotiation are not "in-use" data).

It's device configuration space.

MST, I prefer adding quirks for vDPA devices in VDUSE driver and not 
adding workarounds to virtio driver.

I guess this patch can stay but future patches like this shouldn't be 
merged without a very good reason.

> Thanks,
> Yongji