--- a/sys/dev/disk/sili/sili.c
+++ b/sys/dev/disk/sili/sili.c
@@ -116,6 +116,23 @@
 int
 sili_port_alloc(struct sili_softc *sc, u_int port)
 {
+	/*
+	 * The port-register window must actually cover this port's
+	 * subregion.  bus_space_subregion() performs no validation of the
+	 * offset against the parent mapping, so a device decoding an
+	 * undersized BAR(2) (see sili_pci_attach()) would otherwise drive
+	 * MMIO reads/writes past the pmap-mapped window and panic the
+	 * kernel (or scribble on an adjacent device mapping).
+	 */
+	if (sc->sc_pregs == NULL ||
+	    rman_get_size(sc->sc_pregs) < SILI_PORT_REGION(port) +
+	    SILI_PORT_SIZE) {
+		device_printf(sc->sc_dev,
+			      "port regs window too small for port %u\n",
+			      port);
+		return (EINVAL);
+	}
+
 	struct sili_port	*ap;
 	struct ata_port		*at;
 	struct sili_prb		*prb;