The bootloaders checks for the MCU Security Bit.

If Security Bit is set, it will not let a client read from flash memory, and will always erase the full sketch flash memory, when flashing, to avoid anyone trying to perform a partial flash.

If BOOTPROT is set to 2 and Security Bit is set, the bootloader will be fully protected, and it should not be trivial to read the sketch out of the MCU.

Without these changes, the bootloader ignores the Security Bit, and let clients like bossac.exe read the entire flash memory.

Protection may also be turned on by setting the SECURE_BY_DEFAULT compile flag.

To allow the MKR VIDOR 4000 loader to build to less than 8 KByte, the string handling of compile time DATE and TIME had to be concatinated in source code.

Author: ksmith3036

Diff for: bootloaders/zero/Makefile

@@ -71,11 +71,14 @@ else
   CFLAGS+=-Os -DDEBUG=0 -flto
 endif
 
+ifdef SECURE_BY_DEFAULT
+  CFLAGS+=-DSECURE_BY_DEFAULT=1
+endif
+
 ELF=$(NAME).elf
 BIN=$(NAME).bin
 HEX=$(NAME).hex
 
-
 INCLUDES=-I"$(MODULE_PATH)/tools/CMSIS/4.5.0/CMSIS/Include/" -I"$(MODULE_PATH)/tools/CMSIS-Atmel/1.2.0/CMSIS/Device/ATMEL/"
 
 # -----------------------------------------------------------------------------

Diff for: bootloaders/zero/board_definitions.h

@@ -48,3 +48,6 @@
 
 #define BOOT_PIN_MASK (1U << (BOOT_LOAD_PIN & 0x1f))
 
+// Set this flag to let the bootloader enforce read restrictions of flash memory, even if security bit is not set
+//#define SECURE_BY_DEFAULT
+

Diff for: bootloaders/zero/sam_ba_monitor.c

@@ -30,7 +30,8 @@
 #include <stdlib.h>
 
 const char RomBOOT_Version[] = SAM_BA_VERSION;
-const char RomBOOT_ExtendedCapabilities[] = "[Arduino:XYZ]";
+// X = Chip Erase, Y = Write Buffer, Z = Checksum Buffer, P = Secure Bit Aware
+const char RomBOOT_ExtendedCapabilities[] = "[Arduino:XYZP]";
 
 /* Provides one common interface to handle both USART and USB-CDC */
 typedef struct
@@ -83,6 +84,12 @@ const t_monitor_if usbcdc_if =
 /* The pointer to the interface object use by the monitor */
 t_monitor_if * ptr_monitor_if;
 
+#ifdef SECURE_BY_DEFAULT
+  bool b_security_enabled = true;
+#else
+  bool b_security_enabled = false;
+#endif
+
 /* b_terminal_mode mode (ascii) or hex mode */
 volatile bool b_terminal_mode = false;
 volatile bool b_sam_ba_interface_usart = false;
@@ -222,9 +229,12 @@ void sam_ba_putdata_term(uint8_t* data, uint32_t length)
   return;
 }
 
+#ifndef SECURE_BY_DEFAULT
 volatile uint32_t sp;
 void call_applet(uint32_t address)
 {
+  if (b_security_enabled) return;
+
   uint32_t app_start_address;
 
   __disable_irq();
@@ -240,8 +250,10 @@ void call_applet(uint32_t address)
   /* Jump to application Reset Handler in the application */
   asm("bx %0"::"r"(app_start_address));
 }
+#endif
 
 uint32_t current_number;
+uint32_t erased_from = 0;
 uint32_t i, length;
 uint8_t command, *ptr_data, *ptr, data[SIZEBUFMAX];
 uint8_t j;
@@ -264,11 +276,26 @@ static void put_uint32(uint32_t n)
   sam_ba_putdata( ptr_monitor_if, buff, 8);
 }
 
+static void eraseFlash(uint32_t dst_addr)
+{
+	erased_from = dst_addr;
+	while (dst_addr < MAX_FLASH)
+	{
+		// Execute "ER" Erase Row
+		NVMCTRL->ADDR.reg = dst_addr / 2;
+		NVMCTRL->CTRLA.reg = NVMCTRL_CTRLA_CMDEX_KEY | NVMCTRL_CTRLA_CMD_ER;
+		while (NVMCTRL->INTFLAG.bit.READY == 0)
+			;
+		dst_addr += PAGE_SIZE * 4; // Skip a ROW
+	}
+}
+
 #ifdef ENABLE_JTAG_LOAD
 static uint32_t offset = __UINT32_MAX__;
 static bool flashNeeded = false;
 #endif
 
+
 static void sam_ba_monitor_loop(void)
 {
   length = sam_ba_getdata(ptr_monitor_if, data, SIZEBUFMAX);
@@ -284,7 +311,7 @@ static void sam_ba_monitor_loop(void)
       {
         sam_ba_putdata(ptr_monitor_if, "\n\r", 2);
       }
-      if (command == 'S')
+      if (command == 'S') // Write memory (normally RAM, but might be flash, if client handles the Flash MCU commands?)
       {
         //Check if some data are remaining in the "data" buffer
         if(length>i)
@@ -318,37 +345,65 @@ static void sam_ba_monitor_loop(void)
 
         __asm("nop");
       }
-      else if (command == 'R')
+      else if (command == 'R') // Read memory (flash or RAM)
       {
-        sam_ba_putdata_xmd(ptr_monitor_if, ptr_data, current_number);
+		// Flash memory starts at address 0 and runs to flash size 0x40000 (256 KByte)
+
+		// Internal RWW section is at adress 0x400000. RWW is flash used for EEPROM emulation. Will not let anyone read that, when in secure mode, either.
+		// Bootloader ends at 0x1FFF, so user programs start at 0x2000
+		// RAM starts at 0x20000000, so redirect FLASH reads into RAM reads, when in secure mode
+		if (b_security_enabled && ((uint32_t)ptr_data >= 0x0000 && (uint32_t)ptr_data < 0x20000000))
+		  ptr_data = (uint8_t *)0x20005000;
+		sam_ba_putdata_xmd(ptr_monitor_if, ptr_data, current_number);
       }
-      else if (command == 'O')
+      else if (command == 'O') // write byte
       {
         *ptr_data = (char) current_number;
       }
-      else if (command == 'H')
+      else if (command == 'H') // Write half word
       {
         *((uint16_t *) ptr_data) = (uint16_t) current_number;
       }
-      else if (command == 'W')
+      else if (command == 'W') // Write word
       {
         *((int *) ptr_data) = current_number;
       }
-      else if (command == 'o')
+      else if (command == 'o') // Read byte
       {
-        sam_ba_putdata_term(ptr_data, 1);
+		// Flash memory starts at address 0 and runs to flash size 0x40000 (256 KByte). RAM starts at 0x20000000.
+		// Intern RWW section is at adress 0x400000. RWW is flash used for EEPROM emulation. Will not let anyone read that, when in secure mode, either.
+		// BOSSA reads address 0 to check something, but using read word instead of read byte, but in any case allow reading first byte
+		// Bootloader ends at 0x1FFF, so user programs start at 0x2000
+		if (b_security_enabled && ((uint32_t)ptr_data > 0x0003 && (uint32_t)ptr_data < 0x20000000))
+		    ptr_data = (uint8_t*) &current_number;
+	    sam_ba_putdata_term(ptr_data, 1);
       }
-      else if (command == 'h')
+      else if (command == 'h') // Read half word
       {
-        current_number = *((uint16_t *) ptr_data);
+		// Flash memory starts at address 0 and runs to flash size 0x40000 (256 KByte). RAM starts at 0x20000000.
+		// Intern RWW section is at adress 0x400000. RWW is flash used for EEPROM emulation. Will not let anyone read that, when in secure mode, either.
+		// BOSSA reads address 0 to check something, but using read word instead of read byte, but in any case allow reading first byte
+		// Bootloader ends at 0x1FFF, so user programs start at 0x2000
+		if (b_security_enabled && ((uint32_t)ptr_data > 0x0003 && (uint32_t)ptr_data < 0x20000000))
+	        current_number = 0;
+		else
+	        current_number = *((uint16_t *) ptr_data);
         sam_ba_putdata_term((uint8_t*) &current_number, 2);
       }
-      else if (command == 'w')
+      else if (command == 'w') // Read word
       {
-        current_number = *((uint32_t *) ptr_data);
+		// Flash memory starts at address 0 and runs to flash size 0x40000 (256 KByte). RAM starts at 0x20000000.
+		// Intern RWW section is at adress 0x400000. RWW is flash used for EEPROM emulation. Will not let anyone read that, when in secure mode, either.
+		// BOSSA reads address 0 to check something, but using read word instead of read byte, but in any case allow reading first byte
+		// Bootloader ends at 0x1FFF, so user programs start at 0x2000
+		if (b_security_enabled && ((uint32_t)ptr_data > 0x0003 && (uint32_t)ptr_data < 0x20000000))
+	        current_number = 0;
+		else
+	        current_number = *((uint32_t *) ptr_data);
         sam_ba_putdata_term((uint8_t*) &current_number, 4);
       }
-      else if (command == 'G')
+#ifndef SECURE_BY_DEFAULT
+      else if (!b_security_enabled && command == 'G')  // Execute code. Will not allow when security is enabled.
       {
         call_applet(current_number);
         /* Rebase the Stack Pointer */
@@ -358,40 +413,30 @@ static void sam_ba_monitor_loop(void)
           ptr_monitor_if->put_c(0x6);
         }
       }
-      else if (command == 'T')
+#endif
+      else if (command == 'T') // Turn on terminal mode
       {
         b_terminal_mode = 1;
         sam_ba_putdata(ptr_monitor_if, "\n\r", 2);
       }
-      else if (command == 'N')
+      else if (command == 'N') // Turn off terminal mode
       {
         if (b_terminal_mode == 0)
         {
           sam_ba_putdata( ptr_monitor_if, "\n\r", 2);
         }
         b_terminal_mode = 0;
       }
-      else if (command == 'V')
+      else if (command == 'V') // Read version information
       {
         sam_ba_putdata( ptr_monitor_if, "v", 1);
         sam_ba_putdata( ptr_monitor_if, (uint8_t *) RomBOOT_Version, strlen(RomBOOT_Version));
         sam_ba_putdata( ptr_monitor_if, " ", 1);
         sam_ba_putdata( ptr_monitor_if, (uint8_t *) RomBOOT_ExtendedCapabilities, strlen(RomBOOT_ExtendedCapabilities));
-        sam_ba_putdata( ptr_monitor_if, " ", 1);
-        ptr = (uint8_t*) &(__DATE__);
-        i = 0;
-        while (*ptr++ != '\0')
-          i++;
-        sam_ba_putdata( ptr_monitor_if, (uint8_t *) &(__DATE__), i);
-        sam_ba_putdata( ptr_monitor_if, " ", 1);
-        i = 0;
-        ptr = (uint8_t*) &(__TIME__);
-        while (*ptr++ != '\0')
-          i++;
-        sam_ba_putdata( ptr_monitor_if, (uint8_t *) &(__TIME__), i);
-        sam_ba_putdata( ptr_monitor_if, "\n\r", 2);
+        ptr = (uint8_t*) &(" " __DATE__ " " __TIME__ "\n\r");
+        sam_ba_putdata( ptr_monitor_if, ptr, strlen(ptr));
       }
-      else if (command == 'X')
+      else if (command == 'X') // Erase flash
       {
         // Syntax: X[ADDR]#
         // Erase the flash memory starting from ADDR to the end of flash.
@@ -400,22 +445,13 @@ static void sam_ba_monitor_loop(void)
         //       Even if the starting address is the last byte of a ROW the entire
         //       ROW is erased anyway.
 
-        uint32_t dst_addr = current_number; // starting address
-
-        while (dst_addr < MAX_FLASH)
-        {
-          // Execute "ER" Erase Row
-          NVMCTRL->ADDR.reg = dst_addr / 2;
-          NVMCTRL->CTRLA.reg = NVMCTRL_CTRLA_CMDEX_KEY | NVMCTRL_CTRLA_CMD_ER;
-          while (NVMCTRL->INTFLAG.bit.READY == 0)
-            ;
-          dst_addr += PAGE_SIZE * 4; // Skip a ROW
-        }
-
+		// BOSSAC.exe always erase with 0x2000 as argument, but an attacker might try to erase just parts of the flash, to be able to copy or analyze the untouched parts.
+		// To mitigate this, always erase all sketch flash, that is, starting from address 0x2000. This butloader always assume 8 KByte for itself, and sketch starting at 0x2000.
+		eraseFlash(b_security_enabled ? 0x2000 : current_number);
         // Notify command completed
         sam_ba_putdata( ptr_monitor_if, "X\n\r", 3);
       }
-      else if (command == 'Y')
+      else if (command == 'Y') // Write buffer to flash
       {
         // This command writes the content of a buffer in SRAM into flash memory.
 
@@ -435,6 +471,13 @@ static void sam_ba_monitor_loop(void)
         }
         else
         {
+          if (b_security_enabled) {
+            // To mitigate that an attacker might not use the ordinary BOSSA method of erasing flash before programming,
+            // always erase flash, if it hasn't been done already.
+            if (erased_from != 0x2000)
+              eraseFlash(0x2000);
+          }
+
           // Write to flash
           uint32_t size = current_number/4;
           uint32_t *src_addr = src_buff_addr;
@@ -546,7 +589,7 @@ static void sam_ba_monitor_loop(void)
         // Notify command completed
         sam_ba_putdata( ptr_monitor_if, "Y\n\r", 3);
       }
-      else if (command == 'Z')
+      else if (command == 'Z') // Calculate CRC16
       {
         // This command calculate CRC for a given area of memory.
         // It's useful to quickly check if a transfer has been done
@@ -648,6 +691,12 @@ void sam_ba_monitor_run(void)
   PAGES = NVMCTRL->PARAM.bit.NVMP;
   MAX_FLASH = PAGE_SIZE * PAGES;
 
+#ifdef SECURE_BY_DEFAULT
+  b_security_enabled = true;
+#else
+  b_security_enabled = NVMCTRL->STATUS.bit.SB != 0;
+#endif
+
   ptr_data = NULL;
   command = 'z';
   while (1)