wolfBoot/docs/secure_app_handoff.md

Secure Application Handoff

WOLFBOOT_SECURE_APP tells wolfBoot that the image it boots is a signed Secure-state runtime (for example a TrustZone-M Secure application such as wolfTrust) rather than a Non-secure application. wolfBoot authenticates and measures the image, publishes a measured-boot record at a fixed secure-RAM address, and then branches to the image's reset vector without leaving Secure state.

This is the wolfBoot analog of MCUboot's shared-data area consumed by TF-M: the first stage measures what it launched and hands the measurement to the runtime.

Default TrustZone boot vs. the secure-app handoff

	Default TZ boot	WOLFBOOT_SECURE_APP
Target state	Non-secure	stays Secure
Stack / entry	msp_ns, bxns to the NS vector	msp, branch to the Secure reset vector
Record	none	measured-boot record written before the jump

The Secure jump lives in src/boot_arm.c under WOLFBOOT_SECURE_APP: it turns the wolfBoot MPU off, sets MSP, re-enables interrupts, and branches to the image entry, leaving the runtime to install its own memory map from its Reset_Handler.

The measured-boot record

include/wolfboot/secure_handoff.h defines the record and a single builder, wolfBoot_secure_handoff_build(). wolfBoot writes it from wolfBoot_prepare_secure_handoff() in src/update_flash.c immediately before do_boot(), and panics if the record cannot be built.

Field	Type	Meaning
magic	uint32_t	0x5742484F, written last as the valid flag
magic_inverse	uint32_t	~magic, torn-write guard
version	uint16_t	record layout version (1)
size	uint16_t	sizeof(record) (56)
lifecycle	uint32_t	PSA lifecycle from hal_attestation_get_lifecycle()
image_version	uint32_t	version of the booted image
hash_algorithm	uint16_t	1 = SHA-256
measurement_size	uint16_t	digest length (32)
measurement[32]	uint8_t	SHA-256 of the booted image

The record lives at WOLFBOOT_SECURE_HANDOFF_ADDRESS, a secure-RAM address the port reserves and the config supplies. The builder writes magic_inverse and then magic last, fenced with dmb/dsb, so a consumer that observes magic sees a complete record.

A consumer validates magic and magic_inverse, checks version/size, uses the fields, and then clears the record.

Enabling it

WOLFBOOT_SECURE_APP=1
WOLFBOOT_SECURE_HANDOFF_ADDRESS=0x30020000   # secure-RAM address of the record

The handoff requires authenticated boot: SIGN=NONE/WOLFBOOT_NO_SIGN and WOLFBOOT_SKIP_BOOT_VERIFY are rejected at configure time, and the record currently requires HASH=SHA256. Setting WOLFBOOT_SECURE_APP without WOLFBOOT_SECURE_HANDOFF_ADDRESS is a compile-time error.

Porting the handoff to a new target

The handoff mechanism (record, measurement, Secure jump, build knob) is target-independent. A new port becomes a clean secure-runtime loader by supplying three things:

1. Config only — WOLFBOOT_SECURE_APP=1, WOLFBOOT_SECURE_HANDOFF_ADDRESS, and the usual TZEN/partition layout. No code.
2. hal_attestation_get_lifecycle() (optional) — returns the chip's PSA lifecycle. A weak default in hal/hal.c reports "unknown", so a port links and hands off without it; implement it for a real lifecycle value.
3. TrustZone setup that keeps the secure app Secure across the jump — the port's own SAU / security-controller code must keep the secure app's flash and RAM Secure and must not unsecure peripherals when WOLFBOOT_SECURE_APP is set. Every SoC's security fabric differs (GTZC, AHBSC, TRDC, …), so this is the one piece that is inherently per-port; it is the TrustZone init a port already writes, guarded on WOLFBOOT_SECURE_APP.

STM32H5 reference

config/examples/stm32h5-tz-wolftrust.config is the reference consumer. On the STM32H5 the record lives at 0x30020000, hal_attestation_get_lifecycle() derives the PSA lifecycle from the flash product state and the debug authentication status (hal/stm32h5_lifecycle.h), and hal/stm32_tz.c keeps the whole SRAM1 window and the secure app flash Secure across the handoff.

Tests

• tools/unit-tests/unit-secure-handoff.c — record layout, the builder, and the STM32H5 lifecycle mapping.
• tools/unit-tests/test-secure-handoff-config.sh — the configure-time guards (rejects SIGN=NONE and WOLFBOOT_SKIP_BOOT_VERIFY) and the STM32H5 stack floors.