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flash encryption: Add config option to disable any plaintext reflashes

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Enabled by default when Secure Boot is on, so Flash Encryption protection
is always available in case of a Secure Boot bypass.
This commit is contained in:
Angus Gratton
2019-06-12 11:03:42 +10:00
committed by Angus Gratton
parent cdabee59ef
commit 8df3c666db
5 changed files with 53 additions and 17 deletions
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18
components/bootloader/Kconfig.projbuild
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@@ -251,4 +251,22 @@ config SECURE_BOOT_TEST_MODE
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

5
components/bootloader_support/include/esp_flash_encrypt.h
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@@ -102,8 +102,9 @@ esp_err_t esp_flash_encrypt_region(uint32_t src_addr, size_t data_length);
/** @brief Write protect FLASH_CRYPT_CNT
*
* Intended to be called as a part of boot process if flash encryption
* is enabled but secure boot is not used. This should protect against
* serial re-flashing of an unauthorised code in absence of secure boot.
* should be permanently enabled. This should protect against serial
* re-flashing of an unauthorised code in absence of secure boot or if
* secure boot protection is bypassed.
*
* @return
*/

8
components/bootloader_support/src/flash_encrypt.c
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@@ -210,6 +210,14 @@ 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");

6
components/esp32/cpu_start.c
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@@ -67,6 +67,7 @@
#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"
@@ -301,6 +302,11 @@ 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();