如何通过源码锁定U-boot启动加载器（基于Amlogic S905x）

Great question—locking down U-Boot on the Amlogic S905x to prevent boot interrupts and firmware tampering involves a mix of software tweaks and hardware-level safeguards. Let’s break this down step by step, plus address your question about verified boot and kernel signing:

Disabling Auto-Boot Interrupts & Locking U-Boot #

1. Eliminate Boot Delay & Key-Based Interrupts #

First, remove any window for users to interrupt the boot process:

• Set bootdelay to 0 to skip the countdown where keys like spacebar/enter can drop you into the U-Boot shell:

  setenv bootdelay 0
  saveenv
  

• This alone isn’t enough—you need to disable key detection entirely. Most Amlogic U-Boot builds check for serial input during boot; modify the source code (look in board/amlogic/s905x/s905x.c or common boot logic) to comment out sections that listen for interrupt keys.

2. Lock U-Boot Environment Variables #

Prevent unauthorized changes to critical U-Boot settings:

• If your U-Boot version supports it, lock specific variables (or all variables) with:

  env lock bootdelay bootcmd
  saveenv
  

• For stronger protection, compile U-Boot with hardcoded environment variables (enable CONFIG_ENV_IS_EMBEDDED and disable CONFIG_ENV_IS_IN_MMC/SPI_FLASH). This makes the environment unmodifiable post-flash.

3. Hardware-Level Locking (Irreversible) #

The S905x has eFuses for permanent security:

• Burn eFuse bits to disable JTAG and serial debug access—this blocks anyone from connecting to modify U-Boot or interrupt boot.
• Burn the SECURE_BOOT eFuse to enforce verified boot (more below). Warning: eFuse burns are permanent—test your setup fully before proceeding.

Relation to Verified Boot & Kernel Image Signing #

This process is deeply tied to verified boot and kernel signing—they’re two parts of a complete firmware security chain:

• When secure boot is enabled via eFuses, U-Boot will only load kernel, device tree, and rootfs images signed with your private key (the public key is embedded in your custom U-Boot).
• Locking down U-Boot’s boot process ensures attackers can’t bypass signature checks by dropping into the shell and manually loading unsigned code. Without this lock, even secure boot can be defeated by interrupting the boot flow.

Critical Notes #

• Test first: Before burning eFuses or flashing a locked-down U-Boot, validate your custom build thoroughly. If U-Boot fails to boot after locking, recovery is impossible (especially if debug interfaces are disabled).
• Key management: For secure boot, generate an RSA key pair, sign your images with the private key, and embed the public key in U-Boot. Guard the private key closely—anyone with it can sign malicious images your board will accept.

内容的提问来源于stack exchange，提问作者Saleh