Re: [PATCH] virtio: skip guest index check on device load

[Michael S. Tsirkin]

On Tue, Oct 27, 2020 at 12:53:29PM +0000, Felipe Franciosi wrote:
> 
> 
> > On Oct 27, 2020, at 12:25 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > 
> > On Tue, Oct 27, 2020 at 11:30:49AM +0000, Stefan Hajnoczi wrote:
> >> On Mon, Oct 26, 2020 at 03:13:32PM +0000, Felipe Franciosi wrote:
> >>> QEMU must be careful when loading device state off migration streams to
> >>> prevent a malicious source from exploiting the emulator. Overdoing these
> >>> checks has the side effect of allowing a guest to "pin itself" in cloud
> >>> environments by messing with state which is entirely in its control.
> >>> 
> >>> Similarly to what f3081539 achieved in usb_device_post_load(), this
> >>> commit removes such a check from virtio_load(). Worth noting, the result
> >>> of a load without this check is the same as if a guest enables a VQ with
> >>> invalid indexes to begin with. That is, the virtual device is set in a
> >>> broken state (by the datapath handler) and must be reset.
> >>> 
> >>> Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
> >>> ---
> >>> hw/virtio/virtio.c | 12 ------------
> >>> 1 file changed, 12 deletions(-)
> >>> 
> >>> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> >>> index 6f8f865aff..0561bdb857 100644
> >>> --- a/hw/virtio/virtio.c
> >>> +++ b/hw/virtio/virtio.c
> >>> @@ -3136,8 +3136,6 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, 
> >>> int version_id)
> >>>     RCU_READ_LOCK_GUARD();
> >>>     for (i = 0; i < num; i++) {
> >>>         if (vdev->vq[i].vring.desc) {
> >>> -            uint16_t nheads;
> >>> -
> >>>             /*
> >>>              * VIRTIO-1 devices migrate desc, used, and avail ring 
> >>> addresses so
> >>>              * only the region cache needs to be set up.  Legacy devices 
> >>> need
> >>> @@ -3157,16 +3155,6 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, 
> >>> int version_id)
> >>>                 continue;
> >>>             }
> >>> 
> >>> -            nheads = vring_avail_idx(&vdev->vq[i]) - 
> >>> vdev->vq[i].last_avail_idx;
> >>> -            /* Check it isn't doing strange things with descriptor 
> >>> numbers. */
> >>> -            if (nheads > vdev->vq[i].vring.num) {
> >>> -                error_report("VQ %d size 0x%x Guest index 0x%x "
> >>> -                             "inconsistent with Host index 0x%x: delta 
> >>> 0x%x",
> >>> -                             i, vdev->vq[i].vring.num,
> >>> -                             vring_avail_idx(&vdev->vq[i]),
> >>> -                             vdev->vq[i].last_avail_idx, nheads);
> >>> -                return -1;
> >>> -            }
> >> 
> >> Michael, the commit that introduced this check seems to have been for
> >> debugging rather than to prevent a QEMU crash, so this removing the
> >> check may be safe:
> >> 
> >>  commit 258dc7c96bb4b7ca71d5bee811e73933310e168c
> >>  Author: Michael S. Tsirkin <mst@redhat.com>
> >>  Date:   Sun Oct 17 20:23:48 2010 +0200
> >> 
> >>      virtio: sanity-check available index
> >> 
> >>      Checking available index upon load instead of
> >>      only when vm is running makes is easier to
> >>      debug failures.
> > 
> > Agreed. Given this, let's keep the message around, just with
> > LOG_GUEST_ERROR ?
> 
> I thought about it. Happy to send a v2 which keeps the check and logs
> without throwing an error.
> 
> Separately, I'm thinking of hooking up QEMU with VRING_ERR so datapath
> handlers can notify QEMU that something went broken and NEEDS_RESET
> can be flipped on the status register, possibly along a configuration
> interrupt. I can see libvhost-user supports that, but are there any
> reasons QEMU doesn't do this already?

Mostly because guest support isn't there. That in turn isn't easy,
lots of synchronization is needed within guests.


> > 
> >> Felipe: Did you audit the code to make sure the invalid avail_idx value
> >> and the fields it is propagated to (e.g. shadow_avail_idx) are always
> >> used in a safe way?
> > 
> 
> I did it briefly. I also wrote a mock userspace driver that creates
> this condition in a very controlled way (so I can step half-way
> through setting up VQs and trigger a migration, for example). But you
> know how manual tests are... I may have missed something.
> Your expert eyes are most welcome. :)
> 
> F.