modules: tf-m: support MCUboot signature types other than RSA-3072

With TF-M updated to 2.1.0 it now makes the signature type default
to EC-P256 for the mps2/an521/cpu0/ns board.
So far Zephyr had only supported and assumed that it was RSA-3072.

This brings support for other signature types, and changes the global
default to EC-P256.
The switch from RSA-3072 to EC-P256 reduces the flash usage by ~3.3KB
while having a negligible impact on RAM usage (increase of ~70 bytes)
when compiling the tfm_psa_test sample on mps2/an521/cpu0/ns and
nrf9160dk/nrf9160/ns without explicit optimizations.

The TFM_KEY_FILE_{S,NS} Kconfig options are moved inside an
`if TFM_BL2` as they are only used if MCUboot is included in TF-M.

The TF-M CMake variables MCUBOOT_KEY_{S,NS} are now set so that it's
possible to use signing keys located elsewhere than the default
location.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>

doc/releases/migration-guide-3.7.rst

@@ -96,6 +96,14 @@ MbedTLS
 MCUboot
 =======
 
+Trusted Firmware-M
+==================
+
+* The default MCUboot signature type has been changed from RSA-3072 to EC-P256.
+  This affects builds that have MCUboot enabled in TF-M (:kconfig:option:`CONFIG_TFM_BL2`).
+  If you wish to keep using RSA-3072, you need to set :kconfig:option:`CONFIG_TFM_MCUBOOT_SIGNATURE_TYPE`
+  to `"RSA-3072"`. Otherwise, make sure to have your own signing keys of the signature type in use.
+
 zcbor
 =====
 

doc/releases/release-notes-3.7.rst

@@ -427,6 +427,10 @@ Trusted Firmware-M
 * TF-M was updated to 2.1.0. Release notes can be found at:
   https://tf-m-user-guide.trustedfirmware.org/releases/2.1.0.html
 
+* Support for MCUboot signature types other than RSA-3072 has been added.
+  The type can be chosen with the :kconfig:option:`CONFIG_TFM_MCUBOOT_SIGNATURE_TYPE` Kconfig option.
+  Using EC-P256, the new default, reduces flash usage by several KBs compared to RSA.
+
 zcbor
 *****
 

modules/trusted-firmware-m/CMakeLists.txt

@@ -35,6 +35,25 @@ if (CONFIG_BUILD_WITH_TFM)
     list(APPEND TFM_CMAKE_ARGS -DBL2=TRUE)
     list(APPEND TFM_CMAKE_ARGS -DMCUBOOT_IMAGE_VERSION_S=${CONFIG_TFM_IMAGE_VERSION_S})
     list(APPEND TFM_CMAKE_ARGS -DMCUBOOT_IMAGE_VERSION_NS=${CONFIG_TFM_IMAGE_VERSION_NS})
+    list(APPEND TFM_CMAKE_ARGS -DMCUBOOT_SIGNATURE_TYPE=${CONFIG_TFM_MCUBOOT_SIGNATURE_TYPE})
+
+    # TF-M's config/check_config.cmake requires MCUBOOT_BUILTIN_KEY=OFF for RSA
+    # and MCUBOOT_USE_PSA_CRYPTO for EC-P. The others are dependencies needed
+    # for either the build or the boot to succeed.
+    if (${CONFIG_TFM_MCUBOOT_SIGNATURE_TYPE} MATCHES "^RSA")
+      list(APPEND TFM_CMAKE_ARGS -DMCUBOOT_BUILTIN_KEY=OFF)
+      list(APPEND TFM_CMAKE_ARGS -DMCUBOOT_HW_KEY=ON)
+    elseif (${CONFIG_TFM_MCUBOOT_SIGNATURE_TYPE} MATCHES "^EC-P")
+      list(APPEND TFM_CMAKE_ARGS -DMCUBOOT_USE_PSA_CRYPTO=ON)
+      list(APPEND TFM_CMAKE_ARGS -DMCUBOOT_BUILTIN_KEY=ON)
+      list(APPEND TFM_CMAKE_ARGS -DMCUBOOT_HW_KEY=OFF)
+    endif()
+
+    foreach(SUFFIX IN ITEMS "S" "NS")
+      string(CONFIGURE ${CONFIG_TFM_KEY_FILE_${SUFFIX}} CONFIG_TFM_KEY_FILE_${SUFFIX})
+      list(APPEND TFM_CMAKE_ARGS -DMCUBOOT_KEY_${SUFFIX}=${CONFIG_TFM_KEY_FILE_${SUFFIX}})
+    endforeach()
+
   else()
     list(APPEND TFM_CMAKE_ARGS -DBL2=FALSE)
   endif()

modules/trusted-firmware-m/Kconfig.tfm

@@ -53,27 +53,8 @@ menuconfig BUILD_WITH_TFM
 
 if BUILD_WITH_TFM
 
-config TFM_KEY_FILE_S
-	string "Path to private key used to sign secure firmware images."
-	depends on BUILD_WITH_TFM
-	default "${ZEPHYR_TRUSTED_FIRMWARE_M_MODULE_DIR}/bl2/ext/mcuboot/root-RSA-3072.pem"
-	help
-	  The path and filename for the .pem file containing the private key
-	  that should be used by the BL2 bootloader when signing secure
-	  firmware images.
-
-config TFM_KEY_FILE_NS
-	string "Path to private key used to sign non-secure firmware images."
-	depends on BUILD_WITH_TFM
-	default "${ZEPHYR_TRUSTED_FIRMWARE_M_MODULE_DIR}/bl2/ext/mcuboot/root-RSA-3072_1.pem"
-	help
-	  The path and filename for the .pem file containing the private key
-	  that should be used by the BL2 bootloader when signing non-secure
-	  firmware images.
-
 config TFM_PROFILE
 	string
-	depends on BUILD_WITH_TFM
 	default "profile_small" if TFM_PROFILE_TYPE_SMALL
 	default "profile_medium" if TFM_PROFILE_TYPE_MEDIUM
 	default "profile_medium_arotless" if TFM_PROFILE_TYPE_AROTLESS
@@ -85,7 +66,6 @@ config TFM_PROFILE
 
 choice TFM_PROFILE_TYPE
 	prompt "TF-M build profile"
-	depends on BUILD_WITH_TFM
 	default TFM_PROFILE_TYPE_NOT_SET
 	help
 	  The TF-M build profile selection. Can be empty (not set),
@@ -134,7 +114,6 @@ endchoice
 config TFM_ISOLATION_LEVEL
 	int "Isolation level setting." if (TFM_PROFILE_TYPE_NOT_SET && TFM_IPC)
 	range 1 3
-	depends on BUILD_WITH_TFM
 	default 1 if TFM_PROFILE_TYPE_SMALL || !TFM_IPC
 	default 2 if TFM_PROFILE_TYPE_MEDIUM
 	default 3 if TFM_PROFILE_TYPE_LARGE
@@ -259,6 +238,28 @@ config TFM_CONNECTION_BASED_SERVICE_API
 
 if TFM_BL2
 
+config TFM_MCUBOOT_SIGNATURE_TYPE
+	string "The signature type used to sign the secure and non-secure firmware images."
+	default "EC-P256"
+	help
+	  Available types: RSA-2048, RSA-3072, EC-P256, EC-P384.
+
+config TFM_KEY_FILE_S
+	string "Path to private key used to sign secure firmware images."
+	default "${ZEPHYR_TRUSTED_FIRMWARE_M_MODULE_DIR}/bl2/ext/mcuboot/root-${CONFIG_TFM_MCUBOOT_SIGNATURE_TYPE}.pem"
+	help
+	  The path and filename for the .pem file containing the private key
+	  that should be used by the BL2 bootloader when signing secure
+	  firmware images.
+
+config TFM_KEY_FILE_NS
+	string "Path to private key used to sign non-secure firmware images."
+	default "${ZEPHYR_TRUSTED_FIRMWARE_M_MODULE_DIR}/bl2/ext/mcuboot/root-${CONFIG_TFM_MCUBOOT_SIGNATURE_TYPE}_1.pem"
+	help
+	  The path and filename for the .pem file containing the private key
+	  that should be used by the BL2 bootloader when signing non-secure
+	  firmware images.
+
 config TFM_MCUBOOT_IMAGE_NUMBER
 	int "Granularity of FW updates of TFM and app"
 	range 1 2