cortexm_common: add SRAM based PRNG seeder

Makefile.dep

@@ -616,6 +616,12 @@ ifneq (,$(filter conn_can,$(USEMODULE)))
   USEMODULE += xtimer
 endif
 
+ifneq (,$(filter puf_sram,$(USEMODULE)))
+  USEMODULE += hashes
+  USEMODULE += random
+  FEATURES_REQUIRED += puf_sram
+endif
+
 ifneq (,$(filter random,$(USEMODULE)))
   USEMODULE += prng
   # select default prng

cpu/cortexm_common/ldscripts/cortexm_base.ld

@@ -108,6 +108,7 @@ SECTIONS
     /* stack section */
     .stack (NOLOAD):
     {
+        KEEP (*(.puf))
         . = ALIGN(8);
         _sstack = .;
         KEEP (*(.isr_stack))

cpu/cortexm_common/vectors_cortexm.c

@@ -31,6 +31,9 @@
 #include "mpu.h"
 #include "panic.h"
 #include "vectors_cortexm.h"
+#ifdef MODULE_PUF_SRAM
+#include "puf_sram.h"
+#endif
 
 #ifndef SRAM_BASE
 #define SRAM_BASE 0
@@ -77,6 +80,10 @@ void reset_handler_default(void)
     uint32_t *dst;
     uint32_t *src = &_etext;
 
+#ifdef MODULE_PUF_SRAM
+    puf_sram_init((uint8_t *)&_srelocate, SEED_RAM_LEN);
+#endif
+
     pre_startup();
 
 #ifdef DEVELHELP

sys/include/puf_sram.h

@@ -0,0 +1,92 @@
+/*
+ * Copyright (C) 2018 HAW Hamburg
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @defgroup    sys_puf_sram SRAM PUF
+ * @ingroup     sys
+ *
+ * @brief       SRAM based physically unclonable function (PUF)
+ * @experimental
+ *
+ * @{
+ * @file
+ *
+ * @author      Peter Kietzmann <peter.kietzmann@haw-hamburg.de>
+ */
+#ifndef PUF_SRAM_H
+#define PUF_SRAM_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "hashes.h"
+#include "thread.h"
+
+/**
+ * @brief SRAM length considered for seeding
+ */
+#ifndef SEED_RAM_LEN
+#define SEED_RAM_LEN     (2048 / sizeof(uint32_t))
+#endif
+
+/**
+ * @brief SRAM marker to detect reboot without power-off
+ *
+ * Source: https://www.random.org/bytes/
+ */
+#define PUF_SRAM_MARKER  (0xad3021ff)
+
+/**
+ * @brief Global seed variable, allocated in puf_sram.c
+ */
+extern uint32_t puf_sram_seed;
+
+/**
+ * @brief Global seed state, allocated in puf_sram.c
+ *        0 means seed was generated from SRAM pattern,
+          1 means missing power cycle detected,
+          2 means power cycle detected. The state will most likely
+            be overwritten with 0 in the next steps
+ */
+extern uint32_t puf_sram_state;
+
+/**
+ * @brief checks source of reboot by @p puf_sram_softreset and conditionally
+          calls @p puf_sram_generate
+ *
+ * @param[in] ram pointer to SRAM memory
+ * @param[in] len length of the memroy to consider
+ *
+ */
+void puf_sram_init(const uint8_t *ram, size_t len);
+
+/**
+ * @brief builds hash from @p SEED_RAM_LEN bytes uninitialized SRAM, writes it
+ *        to the global variable @p puf_sram_seed and returns the value
+ *
+ * @param[in] ram pointer to SRAM memory
+ * @param[in] len length of the memroy to consider
+ */
+void puf_sram_generate(const uint8_t *ram, size_t len);
+
+/**
+ * @brief checks for a memory marker to determine whether memory contains old data.
+          Otherwise it assumes a reboot from power down mode
+ *
+ * @return    0 when reset with power cycle was detected
+ * @return    1 when reset without power cycle was detected
+ */
+bool puf_sram_softreset(void);
+
+
+#ifdef __cplusplus
+}
+#endif
+/** @} */
+#endif /* PUF_SRAM_H */

sys/puf_sram/Makefile

@@ -0,0 +1,3 @@
+$(warning Attention! Module PUF SRAM requires power-off in order to generate a\
+	high-entropy seed)
+include $(RIOTBASE)/Makefile.base

sys/puf_sram/puf_sram.c

@@ -0,0 +1,55 @@
+/*
+ * Copyright (C) 2018 HAW Hamburg
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     sys_puf_sram
+ *
+ * @{
+ * @file
+ *
+ * @author      Peter Kietzmann <peter.kietzmann@haw-hamburg.de>
+ *
+ * @}
+ */
+#include "puf_sram.h"
+
+/* Allocation of the PUF seed variable */
+__attribute__((used,section(".puf"))) uint32_t puf_sram_seed;
+
+/* Allocation of the PUF seed state */
+__attribute__((used,section(".puf"))) uint32_t puf_sram_state;
+
+/* Allocation of the memory marker */
+__attribute__((used,section(".puf"))) uint32_t puf_sram_marker;
+
+void puf_sram_init(const uint8_t *ram, size_t len)
+{
+    /* generates a new seed value if power cycle was detected */
+    if (!puf_sram_softreset()) {
+        puf_sram_generate(ram, len);
+    }
+}
+void puf_sram_generate(const uint8_t *ram, size_t len)
+{
+    /* build hash from start-up pattern */
+    puf_sram_seed = dek_hash(ram, len);
+    /* write marker to a defined section for subsequent reset detection */
+    puf_sram_marker = PUF_SRAM_MARKER;
+    /* seting state to 0 means seed was generated from SRAM pattern */
+    puf_sram_state = 0;
+}
+
+bool puf_sram_softreset(void)
+{
+    if (puf_sram_marker != PUF_SRAM_MARKER) {
+        puf_sram_state = 2;
+        return 0;
+    }
+    puf_sram_state = 1;
+    return 1;
+}

sys/random/random.c

@@ -23,6 +23,9 @@
 #include "luid.h"
 #include "periph/cpuid.h"
 #include "random.h"
+#ifdef MODULE_PUF_SRAM
+#include "puf_sram.h"
+#endif
 
 #define ENABLE_DEBUG (0)
 #include "debug.h"
@@ -30,7 +33,13 @@
 void auto_init_random(void)
 {
     uint32_t seed;
-#ifdef MODULE_PERIPH_CPUID
+#ifdef MODULE_PUF_SRAM
+    /* TODO: hand state to application? */
+    if (puf_sram_state) {
+        LOG_WARNING("random: PUF SEED not fresh\n");
+    }
+    seed = puf_sram_seed;
+#elif defined (MODULE_PERIPH_CPUID)
     luid_get(&seed, 4);
 #else
     LOG_WARNING("random: NO SEED AVAILABLE!\n");