[PULL 14/15] virtio: skip guest index check on device load

["Michael S. Tsirkin" <mst@redhat.com>]

From: Felipe Franciosi <felipe@nutanix.com>

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>
Message-Id: <20201028134643.110698-1-felipe@nutanix.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/virtio/virtio.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 6f8f865aff..ceb58fda6c 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -17,6 +17,7 @@
 #include "trace.h"
 #include "exec/address-spaces.h"
 #include "qemu/error-report.h"
+#include "qemu/log.h"
 #include "qemu/main-loop.h"
 #include "qemu/module.h"
 #include "hw/virtio/virtio.h"
@@ -3160,12 +3161,12 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
             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;
+                qemu_log_mask(LOG_GUEST_ERROR,
+                              "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);
             }
             vdev->vq[i].used_idx = vring_used_idx(&vdev->vq[i]);
             vdev->vq[i].shadow_avail_idx = vring_avail_idx(&vdev->vq[i]);
-- 
MST