fix(arm): add extra hash config to validate ROTPK

The default mbedTLS configuration enables hash algorithms based on
the HASH_ALG or MBOOT_EL_HASH_ALG selected. However, the Arm ROTPK
is always embedded as a SHA256 hash in BL1 and BL2. In the future,
we may need to adjust this to use the HASH_ALG algorithm for
embedding the ROTPK hash.

As a temporary workaround, a separate mbedTLS configuration has
been created for Arm platforms to explicitly set SHA256 defines,
rather than relying on the default configuration. This adjustment
is reflected in the mbedTLS configuration file for the TC platform
as well as in the PSA Crypto configuration file.

Change-Id: Ib3128ce7b0fb5c0858624ecbc998d456968beddf
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>

plat/arm/board/fvp_r/platform.mk

@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2021, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2021-2024, Arm Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -25,7 +25,7 @@ FVP_R_INTERCONNECT_SOURCES	:= 	drivers/arm/cci/cci.c
 include plat/arm/board/common/board_common.mk
 include plat/arm/common/arm_common.mk
 
-PLAT_INCLUDES		:=	-Iplat/arm/board/fvp_r/include
+PLAT_INCLUDES		+=	-Iplat/arm/board/fvp_r/include
 
 FVP_R_BL_COMMON_SOURCES	:=	plat/arm/board/fvp_r/fvp_r_common.c		\
 				plat/arm/board/fvp_r/fvp_r_context_mgmt.c	\

plat/arm/board/tc/plat_tc_mbedtls_config.h

@@ -21,6 +21,20 @@
 #endif
 #endif /* TF_MBEDTLS_HEAP_SIZE */
 
+/**
+ * On Arm TC platforms, the ROTPK is always hashed using the SHA-256
+ * algorithm.
+ * TODO: Update to hash the ROTPK with the selected HASH_ALG to avoid
+ * the need for explicitly enabling the SHA-256 configuration in mbedTLS.
+ */
+#define MBEDTLS_SHA256_C
+
+/*
+ * Use an implementation of SHA-256 with a smaller memory footprint
+ * but reduced speed.
+ */
+#define MBEDTLS_SHA256_SMALLER
+
 #define MBEDTLS_PSA_CRYPTO_C
 #define MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG
 #define MBEDTLS_ECP_C

plat/arm/common/arm_common.mk

@@ -119,10 +119,6 @@ ifeq (${ARM_LINUX_KERNEL_AS_BL33},1)
   endif
 endif
 
-# Use an implementation of SHA-256 with a smaller memory footprint but reduced
-# speed.
-$(eval $(call add_define,MBEDTLS_SHA256_SMALLER))
-
 # Add the build options to pack Trusted OS Extra1 and Trusted OS Extra2 images
 # in the FIP if the platform requires.
 ifneq ($(BL32_EXTRA1),)
@@ -364,6 +360,17 @@ ifeq (${DRTM_SUPPORT},1)
 BL31_SOURCES            +=	plat/arm/common/arm_err.c
 endif
 
+ifneq ($(filter 1,${MEASURED_BOOT} ${TRUSTED_BOARD_BOOT} ${DRTM_SUPPORT}),)
+    PLAT_INCLUDES		+=	-Iplat/arm/common	\
+					-Iinclude/drivers/auth/mbedtls
+    # Specify mbed TLS configuration file
+    ifeq (${PSA_CRYPTO},1)
+      MBEDTLS_CONFIG_FILE	?=	"<plat_arm_psa_mbedtls_config.h>"
+    else
+      MBEDTLS_CONFIG_FILE	?=	"<plat_arm_mbedtls_config.h>"
+    endif
+endif
+
 ifneq (${TRUSTED_BOARD_BOOT},0)
 
     # Include common TBB sources

plat/arm/common/plat_arm_mbedtls_config.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2024, Arm Ltd. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef PLAT_ARM_MBEDTLS_CONFIG_H
+#define PLAT_ARM_MBEDTLS_CONFIG_H
+
+#include <mbedtls_config-3.h>
+
+/**
+ * On Arm platforms, the ROTPK is always hashed using the SHA-256
+ * algorithm.
+ * TODO: Update to hash the ROTPK with the selected HASH_ALG to avoid
+ * the need for explicitly enabling the SHA-256 configuration in mbedTLS.
+ */
+#define MBEDTLS_SHA256_C
+
+/*
+ * Use an implementation of SHA-256 with a smaller memory footprint
+ * but reduced speed.
+ */
+#define MBEDTLS_SHA256_SMALLER
+
+#endif /* PLAT_ARM_MBEDTLS_CONFIG_H */

plat/arm/common/plat_arm_psa_mbedtls_config.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2024, Arm Ltd. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef PLAT_ARM_PSA_MBEDTLS_CONFIG_H
+#define PLAT_ARM_PSA_MBEDTLS_CONFIG_H
+
+#include "plat_arm_mbedtls_config.h"
+
+#define MBEDTLS_PSA_CRYPTO_C
+#define MBEDTLS_PSA_ASSUME_EXCLUSIVE_BUFFERS
+
+/*
+ * Using PSA crypto API requires an RNG right now. If we don't define the macro
+ * below then we get build errors.
+ *
+ * This is a functionality gap in mbedTLS. The technical limitation is that
+ * psa_crypto_init() is all-or-nothing, and fixing that would require separate
+ * initialization of the keystore, the RNG, etc.
+ *
+ * By defining MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG, we pretend using an external
+ * RNG. As a result, the PSA crypto init code does nothing when it comes to
+ * initializing the RNG, as we are supposed to take care of that ourselves.
+ */
+#define MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG
+
+#endif /* PLAT_ARM_PSA_MBEDTLS_CONFIG_H */