[media] dvb-core: prevent some corruption the legacy ioctl

Quite a few of the ->diseqc_send_master_cmd() implementations don't
check cmd->msg_len so it can lead to memory corruption.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/dvb-core/dvb_frontend.c

@@ -2384,7 +2384,13 @@ static int dvb_frontend_ioctl_legacy(struct file *file,
 
 	case FE_DISEQC_SEND_MASTER_CMD:
 		if (fe->ops.diseqc_send_master_cmd) {
-			err = fe->ops.diseqc_send_master_cmd(fe, (struct dvb_diseqc_master_cmd*) parg);
+			struct dvb_diseqc_master_cmd *cmd = parg;
+
+			if (cmd->msg_len > sizeof(cmd->msg)) {
+				err = -EINVAL;
+				break;
+			}
+			err = fe->ops.diseqc_send_master_cmd(fe, cmd);
 			fepriv->state = FESTATE_DISEQC;
 			fepriv->status = 0;
 		}