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 <fel...@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 <m...@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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