[PATCH v6 8/8] doc: uefi: add anti-rollback documentation

[Masahisa Kojima]

This commit describe the procedure to configure lowest supported
version in the device tree for anti-rollback protection.

Signed-off-by: Masahisa Kojima <masahisa.kojima at linaro.org>
---
Newly created in v6

 doc/develop/uefi/uefi.rst | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/doc/develop/uefi/uefi.rst b/doc/develop/uefi/uefi.rst
index efab0fc7b1..f1f13bb993 100644
--- a/doc/develop/uefi/uefi.rst
+++ b/doc/develop/uefi/uefi.rst
@@ -539,6 +539,38 @@ since FMP protocol handles multiple image indexes.
       [--fit | --raw | --guid <guid-string] \
       <image_blob> <capsule_file_name>
 
+Anti-rollback Protection
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+The anti-rollback protection is implemented differently from firmware versioning.
+U-Boot implements the file-based EFI variable storage, it can be tampered
+and not the right place to store the lowest supported version.
+U-Boot uses device tree to store the lowest supported version, it is secured
+as long as dtb is authenticated together with U-Boot image by the authenticated
+capsule update, and the former stage boot loader verifies the image containing the dtb
+when the system boots.
+
+1. Insert the lowest supported version into a device tree
+
+.. code-block:: console
+
+    $ dtc -@ -I dts -O dtb -o version.dtbo version.dts
+    $ fdtoverlay -i orig.dtb -o new.dtb -v version.dtbo
+
+where version.dts looks like::
+
+    /dts-v1/;
+    /plugin/;
+    &{/} {
+            firmware-version {
+                    image1 {
+                            image-type-id = "09D7CF52-0720-4710-91D1-08469B7FE9C8";
+                            image-index = <1>;
+                            lowest-supported-version = <3>;
+                    };
+            };
+    };
+
 Executing the boot manager
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-- 
2.17.1