diff --git a/Android.mk b/Android.mk
index b667fd206..26ec09862 100644
--- a/Android.mk
+++ b/Android.mk
@@ -52,8 +52,18 @@ PLAT_PUBLIC_POLICY := $(LOCAL_PATH)/public
 PLAT_PRIVATE_POLICY := $(LOCAL_PATH)/private
 PLAT_VENDOR_POLICY := $(LOCAL_PATH)/vendor
 REQD_MASK_POLICY := $(LOCAL_PATH)/reqd_mask
-SYSTEM_EXT_PUBLIC_POLICY := $(BOARD_PLAT_PUBLIC_SEPOLICY_DIR)
-SYSTEM_EXT_PRIVATE_POLICY := $(BOARD_PLAT_PRIVATE_SEPOLICY_DIR)
+
+SYSTEM_EXT_PUBLIC_POLICY := $(SYSTEM_EXT_PUBLIC_SEPOLICY_DIRS)
+ifneq (,$(BOARD_PLAT_PUBLIC_SEPOLICY_DIR))
+  # TODO: Disallow BOARD_PLAT_*
+  SYSTEM_EXT_PUBLIC_POLICY += $(BOARD_PLAT_PUBLIC_SEPOLICY_DIR)
+endif
+SYSTEM_EXT_PRIVATE_POLICY := $(SYSTEM_EXT_PRIVATE_SEPOLICY_DIRS)
+ifneq (,$(BOARD_PLAT_PRIVATE_SEPOLICY_DIR))
+  # TODO: Disallow BOARD_PLAT_*
+  SYSTEM_EXT_PRIVATE_POLICY += $(BOARD_PLAT_PRIVATE_SEPOLICY_DIR)
+endif
+
 PRODUCT_PUBLIC_POLICY := $(PRODUCT_PUBLIC_SEPOLICY_DIRS)
 PRODUCT_PRIVATE_POLICY := $(PRODUCT_PRIVATE_SEPOLICY_DIRS)
 
diff --git a/README b/README
index 43d9bbca9..f14ac67e6 100644
--- a/README
+++ b/README
@@ -34,6 +34,17 @@ From the Tuna device BoardConfig.mk, device/samsung/tuna/BoardConfig.mk
 
 BOARD_VENDOR_SEPOLICY_DIRS += device/samsung/tuna/sepolicy
 
+Alongside vendor sepolicy dirs, OEMs can also amend the public and private
+policy of the product and system_ext partitions:
+
+SYSTEM_EXT_PUBLIC_SEPOLICY_DIRS += device/acme/roadrunner-sepolicy/systemext/public
+SYSTEM_EXT_PRIVATE_SEPOLICY_DIRS += device/acme/roadrunner-sepolicy/systemext/private
+PRODUCT_PUBLIC_SEPOLICY_DIRS += device/acme/roadrunner-sepolicy/product/public
+PRODUCT_PRIVATE_SEPOLICY_DIRS += device/acme/roadrunner-sepolicy/product/private
+
+The old BOARD_PLAT_PUBLIC_SEPOLICY_DIR and BOARD_PLAT_PRIVATE_SEPOLICY_DIR
+variables have been deprecated in favour of SYSTEM_EXT_*.
+
 Additionally, OEMs can specify BOARD_SEPOLICY_M4DEFS to pass arbitrary m4
 definitions during the build. A definition consists of a string in the form
 of macro-name=value. Spaces must NOT be present. This is useful for building modular