[PATCH v3] hw/s390x: Fix a possible crash with passed-through virtio devices

[Thomas Huth <thuth@redhat.com>]

From: Thomas Huth <thuth@redhat.com>

Consider the following nested setup: An L1 host uses some virtio device
(e.g. virtio-keyboard) for the L2 guest, and this L2 guest passes this
device through to the L3 guest. Since the L3 guest sees a virtio device,
it might send virtio notifications to the QEMU in L2 for that device.
But since the QEMU in L2 defined this device as vfio-ccw, the function
handle_virtio_ccw_notify() cannot handle this and crashes: It calls
virtio_ccw_get_vdev() that casts sch->driver_data into a VirtioCcwDevice,
but since "sch" belongs to a vfio-ccw device, that driver_data rather
points to a CcwDevice instead. So as soon as QEMU tries to use some
VirtioCcwDevice specific data from that device, we've lost.

We must not take virtio notifications for such devices. Thus fix the
issue by adding a check to the handle_virtio_ccw_notify() handler to
refuse all devices that are not our own virtio devices. Like in the
other branches that detect wrong settings, we return -EINVAL from the
function, which will later be placed in GPR2 to inform the guest about
the error.

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 v3: Print the subchannel number to ease debugging

 hw/s390x/s390-hypercall.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/hw/s390x/s390-hypercall.c b/hw/s390x/s390-hypercall.c
index ac1b08b2cd5..508dd97ca0d 100644
--- a/hw/s390x/s390-hypercall.c
+++ b/hw/s390x/s390-hypercall.c
@@ -10,6 +10,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/error-report.h"
 #include "cpu.h"
 #include "hw/s390x/s390-virtio-ccw.h"
 #include "hw/s390x/s390-hypercall.h"
@@ -42,6 +43,19 @@ static int handle_virtio_ccw_notify(uint64_t subch_id, uint64_t data)
     if (!sch || !css_subch_visible(sch)) {
         return -EINVAL;
     }
+    if (sch->id.cu_type != VIRTIO_CCW_CU_TYPE) {
+        /*
+         * This might happen in nested setups: If the L1 host defined the
+         * L2 guest with a virtio device (e.g. virtio-keyboard), and the
+         * L2 guest passes this device through to the L3 guest, the L3 guest
+         * might send virtio notifications to the QEMU in L2 for that device.
+         * But since the QEMU in L2 defined this device as vfio-ccw, it's not
+         * a VirtIODevice that we can handle here!
+         */
+        warn_report_once("Got virtio notification for unsupported device "
+                         "on subchannel %02x.%1x.%04x!", cssid, ssid, schid);
+        return -EINVAL;
+    }
 
     vdev = virtio_ccw_get_vdev(sch);
     if (vq_idx >= VIRTIO_QUEUE_MAX || !virtio_queue_get_num(vdev, vq_idx)) {
-- 
2.51.1