zephyr: Add CONFIG_MCUBOOT_CLEANUP_RAM

Add Kconfig option to cleanup RAM in MCUboot before passing control
to an application.

Signed-off-by: Dominik Ermel <[email protected]>

boot/zephyr/Kconfig

@@ -219,6 +219,12 @@ config MCUBOOT_CLEANUP_ARM_CORE
 	  start-up code which can cause a module fault and potentially make the
 	  module irrecoverable.
 
+config MCUBOOT_CLEANUP_RAM
+	bool "Perform RAM cleanup"
+	depends on CPU_CORTEX_M
+	help
+	  Sets contents of memory to 0 before jumping to application.
+
 config MBEDTLS_CFG_FILE
 	default "mcuboot-mbedtls-cfg.h"
 

boot/zephyr/main.c

@@ -236,7 +236,33 @@ static void do_boot(struct boot_rsp *rsp)
     __set_CONTROL(0x00); /* application will configures core on its own */
     __ISB();
 #endif
+#if CONFIG_MCUBOOT_CLEANUP_RAM
+    __asm__ volatile (
+        /* rt->reset -> r0 */
+        "   mov     r0, %0\n"
+        /* base to write -> r1 */
+        "   mov     r1, %1\n"
+        /* size to write -> r2 */
+        "   mov     r2, %2\n"
+        /* value to write -> r3 */
+        "   mov     r3, %3\n"
+        "clear:\n"
+        "   str     r3, [r1]\n"
+        "   add     r1, r1, %4\n"
+        "   sub     r2, r2, %4\n"
+        "   cbz     r2, out\n"
+        "   b       clear\n"
+        "out:\n"
+        /* jump to reset vector of an app */
+        "   bx      r0\n"
+        :
+        : "r" (vt->reset), "i" (CONFIG_SRAM_BASE_ADDRESS),
+          "i" (CONFIG_SRAM_SIZE * 1024), "i" (0), "i" (sizeof(uint32_t))
+        : "r0", "r1", "r2", "r3", "memory"
+    );
+#else
     ((void (*)(void))vt->reset)();
+#endif
 }
 
 #elif defined(CONFIG_XTENSA) || defined(CONFIG_RISCV)