rdma: use resp.len after validation in qemu_rdma_registration_stop

resp.len is given from remote host. So should be validated before use.
Otherwise memcpy can access beyond the buffer.

Cc: Michael R. Hines <mrhines@us.ibm.com>
Reviewed-by: NOrit Wasserman <owasserm@redhat.com>
Reviewed-by: NMichael R. Hines <mrhines@us.ibm.com>
Signed-off-by: NIsaku Yamahata <yamahata@private.email.ne.jp>
Signed-off-by: NMichael R. Hines <mrhines@us.ibm.com>
Message-id: 1376078746-24948-2-git-send-email-mrhines@linux.vnet.ibm.com
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

migration-rdma.c

@@ -3045,10 +3045,6 @@ static int qemu_rdma_registration_stop(QEMUFile *f, void *opaque,
             return ret;
         }
 
-        qemu_rdma_move_header(rdma, reg_result_idx, &resp);
-        memcpy(rdma->block,
-            rdma->wr_data[reg_result_idx].control_curr, resp.len);
-
         nb_remote_blocks = resp.len / sizeof(RDMARemoteBlock);
 
         /*
@@ -3070,6 +3066,9 @@ static int qemu_rdma_registration_stop(QEMUFile *f, void *opaque,
             return -EINVAL;
         }
 
+        qemu_rdma_move_header(rdma, reg_result_idx, &resp);
+        memcpy(rdma->block,
+            rdma->wr_data[reg_result_idx].control_curr, resp.len);
         for (i = 0; i < nb_remote_blocks; i++) {
             network_to_remote_block(&rdma->block[i]);