Merge branch 'docs/security' into 'master'

docs: Added more wordings to capture secure boot and flash encryption dependency.

See merge request idf/esp-idf!2947

components/bootloader_support/include/esp_flash_encrypt.h

@@ -99,4 +99,14 @@ esp_err_t esp_flash_encrypt_check_and_update(void);
  */
 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.
+ *
+ * @return 
+ */
+void esp_flash_write_protect_crypt_cnt();
+
 #endif

components/bootloader_support/src/flash_encrypt.c

@@ -337,3 +337,13 @@ esp_err_t esp_flash_encrypt_region(uint32_t src_addr, size_t data_length)
     ESP_LOGE(TAG, "flash operation failed: 0x%x", err);
     return err;
 }
+
+void esp_flash_write_protect_crypt_cnt() 
+{
+    uint32_t efuse_blk0 = REG_READ(EFUSE_BLK0_RDATA0_REG);
+    bool flash_crypt_wr_dis = efuse_blk0 & EFUSE_WR_DIS_FLASH_CRYPT_CNT;
+    if(!flash_crypt_wr_dis) { 
+        REG_WRITE(EFUSE_BLK0_WDATA0_REG, EFUSE_WR_DIS_FLASH_CRYPT_CNT);
+        esp_efuse_burn_new_values();
+    }
+}