---
title: "Verified boot and file system integrity"
---

# Verified boot and file system integrity

FIT images checked per configuration against U-Boot's control DTB, how the build signs with keys in files or an HSM, a development copy signed, and dm-verity, IMA and EVM read from the build, the image and the board.

## FIT images and verified boot

The Yocto classes `uboot-sign` and `kernel-fitimage` sign the kernel FIT and U-Boot's FIT, with keys in files or in the programme's HSM through OpenSSL's pkcs11 engine.

**State:** modelled with U-Boot 2024.01's `mkimage`, `dumpimage` and `fit_check_sign`, and SoftHSM 2.6.1 through libp11 0.4.12 (OpenSSL 3.0.13): a two-configuration FIT signed with a file key and with the HSM key, checked good, tampered, unsigned, with the wrong key and without a key; signing refused without the PIN; real `bitbake -e` (BitBake 2.8.1, poky scarthgap, meta-freescale); the running app over a local sshd. Not run with a real kernel and U-Boot build (a stand-in kernel and control DTB were used), on a board refusing an unsigned FIT, with a real HSM, on Windows, against a real build server, on Wind River Linux, with SPL and U-Boot FIT signing or `FIT_SIGN_INDIVIDUAL` in a real build, or with ECDSA keys. Vendor secure boot chains (NXP HAB and AHAB, TI, AMD, ST) are not built.

### Check a FIT image

**Open FIT Image** picks a kernel or U-Boot FIT from the deploy folder. It is read with `dumpimage -l` on the build host and every configuration is checked with `fit_check_sign` against U-Boot's control DTB (`UBOOT_DTB_SYMLINK`; **Check FIT Image Against DTB...** picks another). The editor shows:

- per configuration: what it boots, its signature, the verdict, and whether the DTB requires the key (`required = "conf"`);
- the control DTB's keys, read by the module's own device tree reader;
- the images with their hashes;
- **How the Build Signs**, from `bitbake -e virtual/kernel` and `virtual/bootloader`: which key signs what, a key file or the PKCS#11 URI as mkimage forms it (never read, PIN removed), each variable with its file and line;
- findings: no required key, unsigned configurations, a different key, generated keys, a `pkcs11:` prefix in `UBOOT_SIGN_KEYDIR`.

### Sign a development copy

**Sign FIT Image (Development Build)...** signs a copy with the build's key as kernel-fitimage does. The confirmation shows the key and the exact command. A PKCS#11 PIN comes from `MKIMAGE_SIGN_PIN` in the build account's environment, set through the company's secret mechanism, never from the editor; without it, signing fails without asking for a PIN.

Agent tools: three read-only; signing always asks first.

## Root file system integrity: dm-verity, IMA and EVM

**State:** modelled with veritysetup 2.7.0 (the root hash computed equal to veritysetup's; verify good, a tampered block named, a wrong root hash, a mismatching boot script), real `bitbake -e` (BitBake 2.8.1, poky scarthgap), and a QEMU arm64 board with a verity-mapped root, 32 IMA measurements (6 signed), EVM state, a refused policy read and a tampered disk reported corrupted. **meta-security and meta-integrity were never fetched**, so their variable names and the `.verity.env` and `.verity-params` formats are unchecked against the real layers. No image with dm-verity was built by BitBake; a stand-in initramfs was used. Not run on a real i.MX board, with a TPM, with IMA appraisal enforcement or on Windows.

- **Read Root File System Integrity Settings** reads the build's dm-verity and IMA and EVM settings from `bitbake -e` with file and line, naming keys without reading them.
- **Open dm-verity Image** reads a built image: the verity superblock, the root hash computed independently, `veritysetup verify` (good, or the failing block named), and every partition, kernel command line, boot script or initramfs that carries the root hash, with a warning when one does not match. **Verify dm-verity Image With Root Hash...** checks against a root hash you give.
- **Show Root File System Integrity of Target** reads a board, read-only: `dmsetup table`, `veritysetup status`, `/proc/cmdline`, the IMA measurement list and signing key ids, the IMA policy and EVM state, and what the image lacks.
- **Export Root File System Integrity Record...** writes Markdown or JSON for the security case (ISO 21434, IEC 62443).

Agent tools: five, all read-only.

Next: [SBOM and CVEs](https://neuralinverse.com/docs/modules/embedded-linux/sbom-and-cves/).