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

[Stefan Hajnoczi <stefanha@redhat.com>]

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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.

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?

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