virtio-scsi: fix buffer overrun on invalid state load

CVE-2013-4542

hw/scsi/scsi-bus.c invokes load_request.

 virtio_scsi_load_request does:
    qemu_get_buffer(f, (unsigned char *)&req->elem, sizeof(req->elem));

this probably can make elem invalid, for example,
make in_num or out_num huge, then:

    virtio_scsi_parse_req(s, vs->cmd_vqs[n], req);

will do:

    if (req->elem.out_num > 1) {
        qemu_sgl_init_external(req, &req->elem.out_sg[1],
                               &req->elem.out_addr[1],
                               req->elem.out_num - 1);
    } else {
        qemu_sgl_init_external(req, &req->elem.in_sg[1],
                               &req->elem.in_addr[1],
                               req->elem.in_num - 1);
    }

and this will access out of array bounds.

Note: this adds security checks within assert calls since
SCSIBusInfo's load_request cannot fail.
For now simply disable builds with NDEBUG - there seems
to be little value in supporting these.

Cc: Andreas Färber <afaerber@suse.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
(cherry picked from commit 3c3ce98)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/scsi/virtio-scsi.c

@@ -147,6 +147,15 @@ static void *virtio_scsi_load_request(QEMUFile *f, SCSIRequest *sreq)
     qemu_get_be32s(f, &n);
     assert(n < vs->conf.num_queues);
     qemu_get_buffer(f, (unsigned char *)&req->elem, sizeof(req->elem));
+    /* TODO: add a way for SCSIBusInfo's load_request to fail,
+     * and fail migration instead of asserting here.
+     * When we do, we might be able to re-enable NDEBUG below.
+     */
+#ifdef NDEBUG
+#error building with NDEBUG is not supported
+#endif
+    assert(req->elem.in_num <= ARRAY_SIZE(req->elem.in_sg));
+    assert(req->elem.out_num <= ARRAY_SIZE(req->elem.out_sg));
     virtio_scsi_parse_req(s, vs->cmd_vqs[n], req);
 
     scsi_req_ref(sreq);