[ptxdist] [PATCH v1 2/2] u-boot: Add option to load signed kernel FIT images

[Alexander Dahl via ptxdist <ptxdist@pengutronix.de>]

Requires U-Boot v2023.04 or later, certain options set in board config,
and KERNEL_FIT_SIGNED enabled.  Algorithm "sha256,rsa4096" is hardcoded
to the same value as harcoded in ptxd_make_image_fit_its() which is used
by the kernel-fit package.

With those premises met, U-Boot Verified Boot is possible.  Tested with
U-Boot v2024.04 on Microchip SAM9X60-Curiosity in custom layer on top of
DistroKit-2024.06.0.

    Hit any key to stop autoboot:  0
    8787737 bytes read in 391 ms (21.4 MiB/s)
    ## Loading kernel from FIT Image at 22000000 ...
       Using 'conf-microchip,sam9x60-curiosity' configuration
       Verifying Hash Integrity ... sha256,rsa4096:image-kernel-fit+ OK
       Trying 'kernel' kernel subimage
         Description:  kernel
         Type:         Kernel Image (no loading done)
         Compression:  uncompressed
         Data Start:   0x220000b4
         Data Size:    8757936 Bytes = 8.4 MiB
         Hash algo:    sha256
         Hash value:   8c109fc75263329402c7ae63e3d47c18aabf67307ebc0023e88921227abc7cf3
       Verifying Hash Integrity ... sha256+ OK
    ## Loading fdt from FIT Image at 22000000 ...
       Using 'conf-microchip,sam9x60-curiosity' configuration
       Verifying Hash Integrity ... sha256,rsa4096:image-kernel-fit+ OK
       Trying 'fdt-microchip,sam9x60-curiosity' fdt subimage
         Description:  unavailable
         Type:         Flat Device Tree
         Compression:  uncompressed
         Data Start:   0x2285a45c
         Data Size:    27903 Bytes = 27.2 KiB
         Architecture: ARM
         Hash algo:    sha256
         Hash value:   db2e8047b404f92cc7ca4f7b918be1191aab3ce33d574f046c0f1da959d3cbb2
       Verifying Hash Integrity ... sha256+ OK
       Booting using the fdt blob at 0x2285a45c
    Working FDT set to 2285a45c
       XIP Kernel Image (no loading done) to 220000b4
       Loading Device Tree to 27b31000, end 27b3acfe ... OK
    Working FDT set to 27b31000

    Starting kernel ...

    [    0.000000] Booting Linux on physical CPU 0x0
    [    0.000000] Linux version 6.6.0-tt (ptxdist@ptxdist) (arm-v5te-linux-gnueabi-gcc (OSELAS.Toolchain-2023.07.1 13-20231118) 13.2.1 20231118, GNU ld (GNU Binutils) 2.40) #2 PREEMPT 2024-06-07T12:29:46+00:00

Link: https://docs.u-boot.org/en/latest/usage/fit/verified-boot.html
Signed-off-by: Alexander Dahl <ada@thorsis.com>
---
 platforms/u-boot.in | 30 +++++++++++++++++++++++++++---
 rules/u-boot.make   | 13 +++++++++++++
 2 files changed, 40 insertions(+), 3 deletions(-)

diff --git a/platforms/u-boot.in b/platforms/u-boot.in
index 5ce7387e4..d263135ca 100644
--- a/platforms/u-boot.in
+++ b/platforms/u-boot.in
@@ -7,6 +7,7 @@ config U_BOOT_DEPENDENCIES
 menuconfig U_BOOT
 	select U_BOOT_DEPENDENCIES
 	select BOOTLOADER
+	select CODE_SIGNING		if U_BOOT_VERIFY_SIGNATURE
 	select HOST_SYSTEM_PYTHON3
 	select HOST_OPENSSL		if U_BOOT_NEEDS_HOST_OPENSSL
 	select HOST_GNUTLS		if U_BOOT_NEEDS_HOST_GNUTLS
@@ -18,11 +19,11 @@ if U_BOOT
 
 config U_BOOT_VERSION
 	string
-	default "2019.01"
+	default "2023.04"
 	prompt "U-Boot version"
 	help
-	  Enter the U-Boot version you want to build. Usually something like
-	  "2019.01".
+	  Enter the U-Boot version you want to build.
+	  Usually something like "2019.01".
 
 config U_BOOT_MD5
 	string
@@ -193,6 +194,29 @@ config U_BOOT_BOOT_SCRIPT_ROOTFS_PATH
 
 endif
 
+config U_BOOT_VERIFY_SIGNATURE
+	bool
+	prompt "Verify FIT image signature"
+	select U_BOOT_NEEDS_HOST_OPENSSL
+	imply KERNEL_FIT_SIGNED
+	help
+	  For U-Boot Verified Boot [1] a public key has to be put into
+	  the control dtb built with U-Boot.
+	  The loaded image (most probably a Linux Kernel image) must be
+	  signed with the matching private key.
+	  Uses the PTXdist Code Signing Infrastructure.
+
+	  Note: You need U-Boot 2023.04 or later.
+
+	  Note: You must enable CONFIG_TOOLS_FIT_SIGNATURE in U-Boot
+		Kconfig for the necessary host mkimage features, and
+		enable CONFIG_FIT_SIGNATURE, CONFIG_RSA, and
+		CONFIG_ECDSA for runtime verification.
+		Ensure you have _disabled_ CONFIG_LEGACY_IMAGE_FORMAT to
+		not accidentally allow running unsigned images.
+
+	  [1] https://docs.u-boot.org/en/latest/usage/fit/verified-boot.html
+
 comment "target install"
 
 config U_BOOT_INSTALL_U_BOOT_BIN
diff --git a/rules/u-boot.make b/rules/u-boot.make
index 204475b78..d3ea65cf2 100644
--- a/rules/u-boot.make
+++ b/rules/u-boot.make
@@ -120,6 +120,19 @@ endif
 $(STATEDIR)/u-boot.compile:
 	@$(call targetinfo)
 	@$(call world/compile, U_BOOT)
+ifdef PTXCONF_U_BOOT_VERIFY_SIGNATURE
+	@mv $(U_BOOT_BUILD_DIR)/u-boot.dtb $(U_BOOT_BUILD_DIR)/u-boot-pubkey.dtb
+	@$(CODE_SIGNING_ENV) $(U_BOOT_BUILD_DIR)/tools/mkimage \
+		-f auto-conf -d /dev/null -r \
+		-g image-kernel-fit -N pkcs11 -o "sha256,rsa4096" \
+		-k "$(shell cs_get_uri image-kernel-fit)" \
+		-K "$(U_BOOT_BUILD_DIR)/u-boot-pubkey.dtb" \
+		$(U_BOOT_BUILD_DIR)/unused.itb
+#	# retrigger building dts/dt.dtb to use EXT_DTB, might stay as is otherwise
+	@rm $(U_BOOT_BUILD_DIR)/dts/dt.dtb
+	@$(call compile, U_BOOT, $(U_BOOT_MAKE_OPT) EXT_DTB=$(U_BOOT_BUILD_DIR)/u-boot-pubkey.dtb)
+endif
+
 ifdef PTXCONF_U_BOOT_BOOT_SCRIPT
 	@$(U_BOOT_BUILD_DIR)/tools/mkimage -T script -C none \
 		-d $(U_BOOT_BOOT_SCRIPT_TXT) \
-- 
2.39.5