diff --git a/docs/manual/customize-post-image.txt b/docs/manual/customize-post-image.txt
index 90ea2b9328..5308093d06 100644
--- a/docs/manual/customize-post-image.txt
+++ b/docs/manual/customize-post-image.txt
@@ -28,7 +28,8 @@ different sets of arguments to each script.
 
 Again just like for the post-build scripts, the scripts have access to
 the environment variables +BR2_CONFIG+, +HOST_DIR+, +STAGING_DIR+,
-+TARGET_DIR+, +BUILD_DIR+, +BINARIES_DIR+ and +BASE_DIR+.
++TARGET_DIR+, +BUILD_DIR+, +BINARIES_DIR+, +CONFIG_DIR+ and
++BASE_DIR+.
 
 The post-image scripts will be executed as the user that executes
 Buildroot, which should normally _not_ be the root user. Therefore, any
diff --git a/docs/manual/customize-rootfs.txt b/docs/manual/customize-rootfs.txt
index e1dbadda5b..e46306fd2a 100644
--- a/docs/manual/customize-rootfs.txt
+++ b/docs/manual/customize-rootfs.txt
@@ -60,6 +60,9 @@ The post-build scripts are run with the main Buildroot tree as current
 In addition, you may also use these environment variables:
 
   - +BR2_CONFIG+: the path to the Buildroot .config file
+  - +CONFIG_DIR+: the directory containing the .config file, and
+    therefore the top-level Buildroot Makefile to use (which is
+    correct for both in-tree and out-of-tree builds)
   - +HOST_DIR+, +STAGING_DIR+, +TARGET_DIR+: see
     xref:generic-package-reference[]
   - +BUILD_DIR+: the directory where packages are extracted and built
diff --git a/package/Makefile.in b/package/Makefile.in
index 51f5cbce4f..f4028bc67c 100644
--- a/package/Makefile.in
+++ b/package/Makefile.in
@@ -318,6 +318,7 @@ EXTRA_ENV = \
 	PATH=$(BR_PATH) \
 	BR2_DL_DIR=$(BR2_DL_DIR) \
 	BUILD_DIR=$(BUILD_DIR) \
+	CONFIG_DIR=$(CONFIG_DIR) \
 	O=$(CANONICAL_O)
 
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