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

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Date:   Tue, 24 Aug 2021 21:38:40 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Max Gurtovoy <mgurtovoy@...dia.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-block@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] virtio-blk: Add validation for block size in config space

On Tue, Aug 24, 2021 at 9:30 PM Max Gurtovoy <mgurtovoy@...dia.com> wrote:
>
>
> On 8/24/2021 3:52 PM, Yongji Xie wrote:
> > On Tue, Aug 24, 2021 at 6:11 PM Max Gurtovoy <mgurtovoy@...dia.com> wrote:
> >>
> >> On 8/24/2021 5:47 AM, Jason Wang wrote:
> >>> On Tue, Aug 24, 2021 at 6:31 AM Max Gurtovoy <mgurtovoy@...dia.com> 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.

Thanks,
Yongji