efi_loader: add some description about UEFI secure boot

A small text in docs/uefi/uefi.rst was added to explain how we can
configure and utilise UEFI secure boot feature on U-Boot.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

doc/uefi/uefi.rst

@@ -97,6 +97,83 @@ Below you find the output of an example session starting GRUB::
 
 See doc/uImage.FIT/howto.txt for an introduction to FIT images.
 
+Configuring UEFI secure boot
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+UEFI specification[1] defines a secure way of executing UEFI images
+by verifying a signature (or message digest) of image with certificates.
+This feature on U-Boot is enabled with::
+
+    CONFIG_UEFI_SECURE_BOOT=y
+
+To make the boot sequence safe, you need to establish a chain of trust;
+In UEFI secure boot, you can make it with the UEFI variables, "PK"
+(Platform Key), "KEK" (Key Exchange Keys), "db" (white list database)
+and "dbx" (black list database).
+
+There are many online documents that describe what UEFI secure boot is
+and how it works. Please consult some of them for details.
+
+Here is a simple example that you can follow for your initial attempt
+(Please note that the actual steps would absolutely depend on your system
+and environment.):
+
+1. Install utility commands on your host
+    * openssl
+    * efitools
+    * sbsigntool
+
+2. Create signing keys and key database files on your host
+    for PK::
+
+        $ openssl req -x509 -sha256 -newkey rsa:2048 -subj /CN=TEST_PK/ \
+                -keyout PK.key -out PK.crt -nodes -days 365
+        $ cert-to-efi-sig-list -g 11111111-2222-3333-4444-123456789abc \
+                PK.crt PK.esl;
+        $ sign-efi-sig-list -c PK.crt -k PK.key PK PK.esl PK.auth
+
+    for KEK::
+
+        $ openssl req -x509 -sha256 -newkey rsa:2048 -subj /CN=TEST_KEK/ \
+                -keyout KEK.key -out KEK.crt -nodes -days 365
+        $ cert-to-efi-sig-list -g 11111111-2222-3333-4444-123456789abc \
+                KEK.crt KEK.esl
+        $ sign-efi-sig-list -c PK.crt -k PK.key KEK KEK.esl KEK.auth
+
+    for db::
+
+        $ openssl req -x509 -sha256 -newkey rsa:2048 -subj /CN=TEST_db/ \
+                -keyout db.key -out db.crt -nodes -days 365
+        $ cert-to-efi-sig-list -g 11111111-2222-3333-4444-123456789abc \
+                db.crt db.esl
+        $ sign-efi-sig-list -c KEK.crt -k KEK.key db db.esl db.auth
+
+    Copy \*.auth to media, say mmc, that is accessible from U-Boot.
+
+3. Sign an image with one key in "db" on your host::
+
+    $ sbsign --key db.key --cert db.crt helloworld.efi
+
+4. Install keys on your board::
+
+    ==> fatload mmc 0:1 <tmpaddr> PK.auth
+    ==> setenv -e -nv -bs -rt -at -i <tmpaddr>,$filesize PK
+    ==> fatload mmc 0:1 <tmpaddr> KEK.auth
+    ==> setenv -e -nv -bs -rt -at -i <tmpaddr>,$filesize KEK
+    ==> fatload mmc 0:1 <tmpaddr> db.auth
+    ==> setenv -e -nv -bs -rt -at -i <tmpaddr>,$filesize db
+
+5. Set up boot parameters on your board::
+
+    ==> efidebug boot add 1 HELLO mmc 0:1 /helloworld.efi.signed ""
+
+Then your board runs that image from Boot manager (See below).
+You can also try this sequence by running Pytest, test_efi_secboot,
+on sandbox::
+
+    $ cd <U-Boot source directory>
+    $ pytest.py test/py/tests/test_efi_secboot/test_signed.py --bd sandbox
+
 Executing the boot manager
 ~~~~~~~~~~~~~~~~~~~~~~~~~~