[PATCH 00/16] tools: Add support for signing devicetree blobs

Simon Glass sjg at chromium.org
Fri Nov 12 20:28:01 CET 2021

At present mkimage supports signing FITs, the standard U-Boot image type.

Various people are opposed to using FIT since:

a) it requires adding support for FIT into other bootloaders, notably
b) it requires packaging a kernel in this standard U-Boot format, meaning
   that distros must run 'mkimage' and deal with the kernel and initrd
   being inside a FIT

The kernel and initrd can be dealt with in other ways. But without FIT,
we have no standard way of signing and grouping FDT files. Instead we must
include them in the distro as separate files.

In particular, some sort of mechanism for verifying FDT files is needed.
One option would be to tack a signature on before or after the file,
processing it accordingly. But due to the nature of the FDT binary format,
it is possible to embed a signature inside the FDT itself, which is very

This series provides a tool, fdt_sign, which can add a signature to an
FDT. The signature can be checked later, preventing any change to the FDT,
other than in permitted nodes (e.g. /chosen).

This series also provides a fdt_check_sign tool, used to check signatures.

Both of these tools are stand-alone do not require mkimage or FIT.

As with FIT signing, multiple signatures are possible, but in this case
that requires that fit_sign be called once for each signature. To make the
check fail if a signature does not match, it should be marked as
'required' using the -r flag to fdt_sign.

Run-time support for checking FDT signatures could be added to U-Boot
fairly easily, but needs further discussion as the correct plumbing needs
to be determined.

For now there is absolutely no configurability in the signature mechanism.
It would of course be possible to adjust which nodes are signed, as is
done for FIT, but that needs further discussion also. The omission of the
/chosen node is implemented in h_exclude_nodes() like this:

   if (type == FDT_IS_NODE) {
      /* Ignore the chosen node as well as /signature and subnodes */
      if (!strcmp("/chosen", data) || !strncmp("/signature", data, 10))
         return 0;

Man pages are provided with example usage of the tools. Use this to view

   man -l doc/fdt_check_sign.1

This series also includes various clean-ups noticed along the way, as well
as refactoring to avoid code duplication with the new tools. The last four
patches are the new code.

This series is available at u-boot-dm/fdt-sign-working :


Simon Glass (16):
  rsa: Add debugging for failure cases
  fit_check_sign: Update help to mention the key is in a dtb
  tools: Move copyfile() into a common file
  tools: Avoid leaving extra data at the end of copied files
  tools: Improve comments in signing functions
  tools: Drop unused name in image-host
  tools: Avoid confusion between keys and signatures
  tools: Tidy up argument order in fit_config_check_sig()
  tools: Pass the key blob around
  image: Return destination node for add_verify_data() method
  tools: Pass public-key node through to caller
  tools: mkimage: Show where signatures/keys are written
  tools: Add a new tool to sign FDT blobs
  tools: Add a new tool to check FDT-blob signatures
  test: Add a test for FDT signing
  tools: Add man pages for fdt_sign and fdt_check_sign

 MAINTAINERS                      |   7 +
 boot/image-fit-sig.c             | 151 +++++++++----
 boot/image-fit.c                 |  12 +-
 common/spl/spl_fit.c             |   3 +-
 doc/fdt_check_sign.1             |  74 +++++++
 doc/fdt_sign.1                   | 111 ++++++++++
 include/image.h                  |  80 ++++++-
 include/u-boot/ecdsa.h           |   5 +-
 include/u-boot/rsa.h             |   5 +-
 lib/ecdsa/ecdsa-libcrypto.c      |   4 +-
 lib/rsa/rsa-sign.c               |   5 +-
 lib/rsa/rsa-verify.c             |  13 +-
 test/py/tests/test_fdt_sign.py   |  83 ++++++++
 test/py/tests/test_vboot.py      |  21 +-
 test/py/tests/vboot/sign-fdt.dts |  23 ++
 test/py/tests/vboot_comm.py      |  22 ++
 tools/Makefile                   |  10 +-
 tools/fdt-host.c                 | 353 +++++++++++++++++++++++++++++++
 tools/fdt_check_sign.c           |  85 ++++++++
 tools/fdt_host.h                 |  46 ++++
 tools/fdt_sign.c                 | 210 ++++++++++++++++++
 tools/fit_check_sign.c           |   4 +-
 tools/fit_common.c               |  69 ++++++
 tools/fit_common.h               |  23 ++
 tools/fit_image.c                |  59 +-----
 tools/image-fdt-sig.c            | 254 ++++++++++++++++++++++
 tools/image-host.c               | 155 +++++++++++---
 tools/imagetool.h                |   4 +
 tools/mkimage.c                  |   4 +
 29 files changed, 1714 insertions(+), 181 deletions(-)
 create mode 100644 doc/fdt_check_sign.1
 create mode 100644 doc/fdt_sign.1
 create mode 100644 test/py/tests/test_fdt_sign.py
 create mode 100644 test/py/tests/vboot/sign-fdt.dts
 create mode 100644 test/py/tests/vboot_comm.py
 create mode 100644 tools/fdt-host.c
 create mode 100644 tools/fdt_check_sign.c
 create mode 100644 tools/fdt_sign.c
 create mode 100644 tools/image-fdt-sig.c


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