flash encryption: Add config option to disable any plaintext reflashes

Enabled by default when Secure Boot is on, so Flash Encryption protection
is always available in case of a Secure Boot bypass.

components/bootloader/Kconfig.projbuild

@@ -505,4 +505,22 @@ menu "Security features"
                 Only set this option in testing environments.
 
     endmenu  # Potentially Insecure
+
+    config FLASH_ENCRYPTION_DISABLE_PLAINTEXT
+        bool "Disable serial reflashing of plaintext firmware"
+        depends on FLASH_ENCRYPTION_ENABLED
+        default y if SECURE_BOOT_ENABLED
+        default n if !SECURE_BOOT_ENABLED
+        help
+            If this option is enabled, flash encryption is permanently enabled after first boot by write-protecting
+            the FLASH_CRYPT_CNT efuse. This is the recommended configuration for a secure production system.
+
+            If this option is disabled, FLASH_CRYPT_CNT is left writeable and up to 4 plaintext re-flashes are allowed.
+            An attacker with physical access will be able to read out encrypted flash contents until all plaintext
+            re-flashes have been used up.
+
+            If this option is disabled and hardware Secure Boot is enabled, Secure Boot must be configured in
+            Reflashable mode so that a new Secure Boot digest can be flashed at the same time as plaintext firmware.
+            This combination is not secure and should not be used for a production system.
+
 endmenu  # Security features

components/bootloader_support/src/flash_encrypt.c

@@ -205,6 +205,14 @@ static esp_err_t encrypt_flash_contents(uint32_t flash_crypt_cnt, bool flash_cry
     uint32_t new_flash_crypt_cnt = flash_crypt_cnt + (1 << (ffs_inv - 1));
     ESP_LOGD(TAG, "FLASH_CRYPT_CNT 0x%x -> 0x%x", flash_crypt_cnt, new_flash_crypt_cnt);
     REG_SET_FIELD(EFUSE_BLK0_WDATA0_REG, EFUSE_FLASH_CRYPT_CNT, new_flash_crypt_cnt);
+
+#ifdef CONFIG_FLASH_ENCRYPTION_DISABLE_PLAINTEXT
+    ESP_LOGI(TAG, "Write protecting FLASH_CRYPT_CNT efuse...");
+    REG_SET_BIT(EFUSE_BLK0_WDATA0_REG, EFUSE_WR_DIS_FLASH_CRYPT_CNT);
+#else
+    ESP_LOGW(TAG, "Not disabling FLASH_CRYPT_CNT - plaintext flashing is still possible");
+#endif
+
     esp_efuse_burn_new_values();
 
     ESP_LOGI(TAG, "Flash encryption completed");

components/esp32/cpu_start.c

@@ -69,6 +69,7 @@
 #include "esp_clk_internal.h"
 #include "esp_timer.h"
 #include "esp_pm.h"
+#include "esp_flash_encrypt.h"
 #include "pm_impl.h"
 #include "trax.h"
 #include "esp_ota_ops.h"
@@ -347,6 +348,11 @@ void start_cpu0_default(void)
 #endif
 #if CONFIG_DISABLE_BASIC_ROM_CONSOLE
     esp_efuse_disable_basic_rom_console();
+#endif
+#ifdef CONFIG_FLASH_ENCRYPTION_DISABLE_PLAINTEXT
+    if (esp_flash_encryption_enabled()) {
+        esp_flash_write_protect_crypt_cnt();
+    }
 #endif
     rtc_gpio_force_hold_dis_all();
     esp_vfs_dev_uart_register();