· Veytron Technologies · Security · 2 min read
5 Steps to Secure Your Bootloader
A practical guide to implementing verified boot on embedded Linux systems — from U-Boot configuration to TPM-anchored trust chains.
Securing the boot process is one of the highest-leverage things you can do for an embedded product. A compromised bootloader means everything above it — kernel, userspace, your application — cannot be trusted regardless of other security measures.
Step 1: Understand your threat model
Before touching a line of code, answer: who are you protecting against? Remote attackers exploiting OTA? Physical attackers with JTAG access? Motivated cloners extracting firmware for resale?
Your threat model determines whether you need full verified boot, just signed images, or hardware-backed attestation.
Step 2: Enable U-Boot verified boot
U-Boot’s Verified Boot uses Flattened Image Tree (FIT) images with embedded signatures. The public key is compiled into U-Boot itself, meaning it’s validated at every boot.
Key configuration options:
- CONFIG_FIT_SIGNATURE=y
- CONFIG_RSA=y
- CONFIG_OF_CONTROL=y
Generate your keys, sign your FIT images in CI, and boot a test image in QEMU before burning fuses — you cannot undo enforcement on a closed device.
Step 3: Lock down the bootloader environment
A signed kernel is worthless if an attacker can interrupt U-Boot and boot their own image from TFTP. Disable the autoboot interrupt, lock environment variables to flash, and disable JTAG on production fuses.
Step 4: Anchor to hardware root of trust
Software-only secure boot can be bypassed by attackers who can modify flash. A hardware root of trust — via eFuses, Secure Boot fuses on i.MX, or TPM 2.0 — ensures the chain starts in hardware.
For i.MX processors: burn the public key hash to SRK fuses and set the SEC_CONFIG fuse to close the device. On TI AM62/AM64, the equivalent is OTP row programming via TI’s k3-image-gen tooling. For generic ARM platforms without vendor-specific fusing, TPM 2.0 with measured boot is the practical alternative.
Step 5: Validate the full chain in CI
Your secure boot implementation is only as good as your test coverage. Every CI build should:
- Build a signed FIT image
- Boot it in QEMU with verified boot enabled
- Deliberately corrupt the signature and verify rejection
- Run the signed image on real hardware in your hardware-in-the-loop pipeline
Without automated validation, silent regressions are inevitable.
Need help implementing this for your platform? We’ve done it on i.MX, AM62, and custom BSPs — get in touch.
