Secure boot refers to preventing malicious software applications and “unauthorized” operating systems from loading during the system start-up process. The goal is to protect users from rootkits and other low-level malware attacks. Modern bootloaders come with features that can be used to enable secure boot in the system.

Boot Hardening: Steps/requirements to configure the boot sequence, in order to restrict the device from executing anything other than the approved software image.

In this part, we will see a series of settings that will allow us to improve security during boot phase. For the purposes of reference and explanation, we are providing guidance on how to configure an embedded device that runs with a 3.10.17 Linux kernel. If the integrity is not checked or if a critical error occurs, the system must boot on a very stable backup image.

Requirements: These requirements must be met even if an alternative version of the Linux kernel is chosen.

Recommendations: Detailed best practices that should be applied in order to secure a device. Although they are not currently listed as hard requirements, they may be upgraded to requirements status in the future. In addition, specific operators may change some of these recommendations into requirements based on their specific needs and objectives.

Boot loader: The boot loader consists of the Primary boot loader residing in OTP memory, sboot, U-Boot and Secure loader residing in external flash (NAND or SPI/NOR flash memory). The CPU on power on or reset executes the primary boot loader. The OTP primary boot loader makes the necessary initial system configuration and then loads the secondary boot loader sboot from external flash memory to ram memory. The sboot then loads the U-Boot along with the Secure loader. U-Boot then verifies the Kernel/system image integrity, then loads the Kernel/system image before passing control to it.

Acronyms and Abbreviations

The following table lists the terms utilized within this part of the document.

Image

Image selection

The boot process shall be uninterruptible and shall irrevocably boot the image as specified in the boot environment.

In U-Boot set the "bootdelay" environment variable and/or define CONFIG_BOOTDELAY to -2.

Image authenticity

It shall not be possible to boot from an unverified image. The secure boot feature in U-Boot shall be enabled. The secure boot feature is available from U-Boot 2013.07 version. To enable the secure boot feature, enable the following features:

CONFIG_FIT: Enables support for Flat Image Tree (FIT) uImage format.
CONFIG_FIT_SIGNATURE: Enables signature verification of FIT images.
CONFIG_RSA: Enables RSA algorithm used for FIT image verification.
CONFIG_OF_CONTROL: Enables Flattened Device Tree (FDT) configuration.
CONFIG_OF_SEPARATE: Enables separate build of u-Boot from the device tree.
CONFIG_DEFAULT_DEVICE_TREE: Specifies the default Device Tree used for the run-time configuration of U-Boot.

Generate the U-Boot image with public keys to validate and load the image. It shall use RSA2048 and SHA256 for authentication.

Communication modes

Disable USB, Serial and DOCSIS Support

To disable USB support in U-Boot, following config's shall not be defined:

CONFIG_CMD_USB: Enables basic USB support and the usb command.
CONFIG_USB_UHCI: Defines the lowlevel part.
CONFIG_USB_KEYBOARD: Enables the USB Keyboard.
CONFIG_USB_STORAGE: Enables the USB storage devices.
CONFIG_USB_HOST_ETHER: Enables USB Ethernet adapter support.

In addition, disable unnecessary communication modes like Ethernet, Serial ports, DOCSIS in U-Boot and sboot that are not necessary.

Linux Kernel support for USB should be compiled-out if not required. If it is needed, the Linux Kernel should be configured to only enable the minimum required USB devices. User-initiated USB-filesystems should be treated with special care. Whether or not the filesystems are mounted in userspace (FUSE), restricted mount options should be observed.

Disable all unused Network Interfaces

Only used network interfaces should be enabled. Where possible, services should also be limited to those necessary.

Remove or Disable Unnecessary Services, Ports, and Devices

Restrict the services, ports and devices to those used.

Disable flash access

Recommendation:

In U-Boot following flash memory commands shall be disabled:

NAND: Support for nand flash access available through do_nand has to be disabled.

Similarly sboot should disable flash access support through command line if any.

Consoles

Disable serial console

Serial console output shall be disabled. To disable console output in U-Boot, set the following macros:

And set "silent" environment variable. For the Secure loader, disable the traces by not defining the below macro:

For sboot proper configuration needs to be done to disable the serial console.

Immutable environment variables

In U-Boot, ensure Kernel command line, boot commands, boot delay and other environment variables are immutable. This will prevent side-loading of alternate images, by restricting the boot selection to only the image in FLASH.

The environment variables shall be part of the text region in U-Boot as default environment variable and not in non-volatile memory.

Remove configuration options related to non-volatile memory, such as:

(Recommendation) Removal of memory dump commands

In U-Boot, following commands shall be disabled to avoid memory dumps:

md : Memory Display command.
mm : Memory modify command - auto incrementing address.
nm : Memory modify command - constant address.
mw : Memory write.
cp : Memory copy.
mwc : Memory write cyclic.
mdc : Memory display cyclic.
mtest : Simple ram read/write test.
loopw : Infinite write loop on address range.

Similarly, memory dump support shall be disabled from sboot.