diff --git a/core/config.mk b/core/config.mk
index 0c8a87f..bd905dc 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -1250,7 +1250,11 @@
 TARGET_SYSTEM_PROP := $(wildcard $(TARGET_DEVICE_DIR)/system.prop)
 endif
 
-.KATI_READONLY += TARGET_SYSTEM_PROP
+ifeq ($(TARGET_SYSTEM_EXT_PROP),)
+TARGET_SYSTEM_EXT_PROP := $(wildcard $(TARGET_DEVICE_DIR)/system_ext.prop)
+endif
+
+.KATI_READONLY += TARGET_SYSTEM_PROP TARGET_SYSTEM_EXT_PROP
 
 include $(BUILD_SYSTEM)/sysprop_config.mk
 
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 09ee938..5fca203 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -341,6 +341,7 @@
 $(call add_json_list, OemProperties, $(PRODUCT_OEM_PROPERTIES))
 
 $(call add_json_list, SystemPropFiles, $(TARGET_SYSTEM_PROP))
+$(call add_json_list, SystemExtPropFiles, $(TARGET_SYSTEM_EXT_PROP))
 
 # Do not set ArtTargetIncludeDebugBuild into any value if PRODUCT_ART_TARGET_INCLUDE_DEBUG_BUILD is not set,
 # to have the same behavior from runtime_libart.mk.
diff --git a/core/sysprop.mk b/core/sysprop.mk
index 7dd756a..6d65e19 100644
--- a/core/sysprop.mk
+++ b/core/sysprop.mk
@@ -395,25 +395,10 @@
 # -----------------------------------------------------------------
 # system_ext/etc/build.prop
 #
-_prop_files_ := $(if $(TARGET_SYSTEM_EXT_PROP),\
-    $(TARGET_SYSTEM_EXT_PROP),\
-    $(wildcard $(TARGET_DEVICE_DIR)/system_ext.prop))
-
-# Order matters here. When there are duplicates, the last one wins.
-# TODO(b/117892318): don't allow duplicates so that the ordering doesn't matter
-_prop_vars_ := PRODUCT_SYSTEM_EXT_PROPERTIES
+# system_ext/build.prop is built by Soong. See system-build.prop module in
+# build/soong/Android.bp.
 
 INSTALLED_SYSTEM_EXT_BUILD_PROP_TARGET := $(TARGET_OUT_SYSTEM_EXT)/etc/build.prop
-$(eval $(call build-properties,\
-    system_ext,\
-    $(INSTALLED_SYSTEM_EXT_BUILD_PROP_TARGET),\
-    $(_prop_files_),\
-    $(_prop_vars_),\
-    $(empty),\
-    $(empty),\
-    $(empty)))
-
-$(eval $(call declare-1p-target,$(INSTALLED_SYSTEM_EXT_BUILD_PROP_TARGET)))
 
 # ----------------------------------------------------------------
 # ramdisk/boot/etc/build.prop