--- a/sys/dev/apple/smc/smc_mmio.c
+++ b/sys/dev/apple/smc/smc_mmio.c
@@ -74,6 +74,7 @@
 
 	rlen = bus_read_1(sc->sc_iomem, ASMC_MMIO_DATA_LEN);
 	if (rlen > len) rlen = len;
+	bzero(buf, len);	/* never leak stale stack tail to sysctl */
 	for (i = 0; i < rlen; i++)
 		buf[i] = bus_read_1(sc->sc_iomem, ASMC_MMIO_DATA + i);