device: don't allow mutating the device's hardware address length

We repeatedly call nm_device_update_hw_address() to reset the cached
MAC address of the device. However, we don't allow changing the address
length once it is set.

Multiple entities (initial, current and permanent MAC address) are all
checked to have the same address length. Changing the length would be a
very strange thing (and probably indicate a bug somewhere else).

Just don't allow that.

src/devices/nm-device.c

@@ -227,7 +227,10 @@ typedef struct _NMDevicePrivate {
 	char *        iface;   /* may change, could be renamed by user */
 	int           ifindex;
 
-	guint         hw_addr_len;
+	union {
+		const guint8 hw_addr_len; /* read-only */
+		guint8 hw_addr_len_;
+	};
 	guint8 /*HwAddrType*/ hw_addr_type;
 
 	bool          real;
@@ -2516,11 +2519,6 @@ nm_device_unrealize (NMDevice *self, gboolean remove_resources, GError **error)
 		g_clear_pointer (&priv->udi, g_free);
 		_notify (self, PROP_UDI);
 	}
-	if (priv->hw_addr) {
-		priv->hw_addr_len = 0;
-		g_clear_pointer (&priv->hw_addr, g_free);
-		_notify (self, PROP_HW_ADDRESS);
-	}
 	if (priv->physical_port_id) {
 		g_clear_pointer (&priv->physical_port_id, g_free);
 		_notify (self, PROP_PHYSICAL_PORT_ID);
@@ -2529,8 +2527,11 @@ nm_device_unrealize (NMDevice *self, gboolean remove_resources, GError **error)
 	nm_clear_g_source (&priv->stats.timeout_id);
 	_stats_update_counters (self, 0, 0);
 
+	priv->hw_addr_len_ = 0;
+	if (nm_clear_g_free (&priv->hw_addr))
+		_notify (self, PROP_HW_ADDRESS);
 	priv->hw_addr_type = HW_ADDR_TYPE_UNSET;
-	g_clear_pointer (&priv->hw_addr_perm, g_free);
+	if (nm_clear_g_free (&priv->hw_addr_perm))
 		_notify (self, PROP_PERM_HW_ADDRESS);
 	g_clear_pointer (&priv->hw_addr_initial, g_free);
 
@@ -11587,11 +11588,17 @@ const char *
 nm_device_get_hw_address (NMDevice *self)
 {
 	NMDevicePrivate *priv;
+	char buf[NM_UTILS_HWADDR_LEN_MAX];
+	gsize l;
 
 	g_return_val_if_fail (NM_IS_DEVICE (self), NULL);
+
 	priv = NM_DEVICE_GET_PRIVATE (self);
 
-	nm_assert ((!priv->hw_addr) ^ (priv->hw_addr_len > 0));
+	nm_assert (   (!priv->hw_addr && priv->hw_addr_len == 0)
+	           || (   priv->hw_addr
+	               && _nm_utils_hwaddr_aton (priv->hw_addr, buf, sizeof (buf), &l)
+	               && l == priv->hw_addr_len));
 
 	return priv->hw_addr;
 }
@@ -11616,9 +11623,25 @@ nm_device_update_hw_address (NMDevice *self)
 		hwaddrlen = 0;
 
 	if (hwaddrlen) {
-		priv->hw_addr_len = hwaddrlen;
+		if (   priv->hw_addr_len
+		    && priv->hw_addr_len != hwaddrlen) {
+			char s_buf[NM_UTILS_HWADDR_LEN_MAX_STR];
+
+			/* we cannot change the address length of a device once it is set (except
+			 * unrealizing the device).
+			 *
+			 * The reason is that the permanent and initial MAC addresses also must have the
+			 * same address length, so it's unclear what it would mean that the length changes. */
+			_LOGD (LOGD_PLATFORM | LOGD_DEVICE,
+			       "hw-addr: read a MAC address with differing length (%s vs. %s)",
+			       priv->hw_addr,
+			       nm_utils_hwaddr_ntoa_buf (hwaddr, hwaddrlen, TRUE, s_buf, sizeof (s_buf)));
+			return FALSE;
+		}
+
 		if (!priv->hw_addr || !nm_utils_hwaddr_matches (priv->hw_addr, -1, hwaddr, hwaddrlen)) {
 			g_free (priv->hw_addr);
+			priv->hw_addr_len_ = hwaddrlen;
 			priv->hw_addr = nm_utils_hwaddr_ntoa (hwaddr, hwaddrlen);
 
 			_LOGD (LOGD_PLATFORM | LOGD_DEVICE, "hw-addr: hardware address now %s", priv->hw_addr);
@@ -11867,11 +11890,9 @@ _hw_addr_set (NMDevice *self,
 		return TRUE;
 	}
 
-	if (priv->hw_addr_len != addr_len) {
+	if (   priv->hw_addr_len
-		if (priv->hw_addr_len)
+	    && priv->hw_addr_len != addr_len)
 		g_return_val_if_reached (FALSE);
-		priv->hw_addr_len = addr_len;
-	}
 
 	_LOGT (LOGD_DEVICE, "set-hw-addr: setting MAC address to '%s' (%s, %s)...", addr, operation, detail);
 
@@ -12255,7 +12276,7 @@ constructor (GType type,
 			g_return_val_if_reached (object);
 		}
 
-		priv->hw_addr_len = l;
+		priv->hw_addr_len_ = l;
 		priv->hw_addr = nm_utils_hwaddr_ntoa (buf, l);
 		_LOGT (LOGD_DEVICE, "hw-addr: has permanent hw-address '%s'", priv->hw_addr_perm);
 	}