[PATCH v5 6/6] test/py: ecdsa: Add test for mkimage ECDSA signing

[Simon Glass]

Hi Alexandru,

On Thu, 28 Jan 2021 at 08:52, Alexandru Gagniuc <mr.nuke.me at gmail.com> wrote:
>
> Add a test to make sure that the ECDSA signatures generated by
> mkimage can be verified successfully. pyCryptodomex was chosen as the
> crypto library because it integrates much better with python code.
> Using openssl would have been unnecessarily painful.
>
> Signed-off-by: Alexandru Gagniuc <mr.nuke.me at gmail.com>
> Reviewed-by: Simon Glass <sjg at chromium.org>
> ---
>  test/py/tests/test_fit_ecdsa.py | 111 ++++++++++++++++++++++++++++++++
>  1 file changed, 111 insertions(+)
>  create mode 100644 test/py/tests/test_fit_ecdsa.py
>
> diff --git a/test/py/tests/test_fit_ecdsa.py b/test/py/tests/test_fit_ecdsa.py
> new file mode 100644
> index 0000000000..6cadb0cbb5
> --- /dev/null
> +++ b/test/py/tests/test_fit_ecdsa.py
> @@ -0,0 +1,111 @@
> +# SPDX-License-Identifier:     GPL-2.0+
> +#
> +# Copyright (c) 2020,2021 Alexandru Gagniuc <mr.nuke.me at gmail.com>
> +
> +"""
> +Test ECDSA signing of FIT images
> +
> +This test uses mkimage to sign an existing FIT image with an ECDSA key. The
> +signature is then extracted, and verified against pyCryptodome.
> +This test doesn't run the sandbox. It only checks the host tool 'mkimage'
> +"""
> +
> +import pytest
> +import u_boot_utils as util
> +from Cryptodome.Hash import SHA256
> +from Cryptodome.PublicKey import ECC
> +from Cryptodome.Signature import DSS
> +
> +class SignableFitImage(object):
> +    """ Helper to manipulate a FIT image on disk """
> +    def __init__(self, cons, file_name):
> +        self.fit = file_name
> +        self.cons = cons
> +        self.signable_nodes = set()
> +
> +    def __fdt_list(self, path):
> +        return util.run_and_log(self.cons, f'fdtget -l {self.fit} {path}')
> +
> +    def __fdt_set(self, node, **prop_value):
> +        for prop, value in prop_value.items():
> +            util.run_and_log(self.cons, f'fdtput -ts {self.fit} {node} {prop} {value}')
> +
> +    def __fdt_get_binary(self, node, prop):
> +        numbers = util.run_and_log(self.cons, f'fdtget -tbi {self.fit} {node} {prop}')
> +
> +        bignum = bytearray()
> +        for little_num in numbers.split():
> +            bignum.append(int(little_num))
> +
> +        return bignum
> +
> +    def find_signable_image_nodes(self):
> +        for node in self.__fdt_list('/images').split():
> +            image = f'/images/{node}'
> +            if 'signature' in self.__fdt_list(image):
> +                self.signable_nodes.add(image)
> +
> +        return self.signable_nodes
> +
> +    def change_signature_algo_to_ecdsa(self):
> +        for image in self.signable_nodes:
> +            self.__fdt_set(f'{image}/signature', algo='sha256,ecdsa256')
> +
> +    def sign(self, mkimage, key_file):
> +        util.run_and_log(self.cons, [mkimage, '-F', self.fit, f'-k{key_file}'])
> +
> +    def check_signatures(self, key):
> +        for image in self.signable_nodes:
> +            raw_sig = self.__fdt_get_binary(f'{image}/signature', 'value')
> +            raw_bin = self.__fdt_get_binary(image, 'data')
> +
> +            sha = SHA256.new(raw_bin)
> +            verifier = DSS.new(key, 'fips-186-3')
> +            verifier.verify(sha, bytes(raw_sig))
> +
> +
> + at pytest.mark.buildconfigspec('fit_signature')
> + at pytest.mark.requiredtool('dtc')
> + at pytest.mark.requiredtool('fdtget')
> + at pytest.mark.requiredtool('fdtput')
> +def test_fit_ecdsa(u_boot_console):
> +    """ Test that signatures generated by mkimage are legible. """
> +    def generate_ecdsa_key():
> +        return ECC.generate(curve='prime256v1')
> +
> +    def assemble_fit_image(dest_fit, its, destdir):
> +        dtc_args = f'-I dts -O dtb -i {destdir}'
> +        util.run_and_log(cons, [mkimage, '-D', dtc_args, '-f', its, dest_fit])
> +
> +    def dtc(dts):
> +        dtb = dts.replace('.dts', '.dtb')
> +        util.run_and_log(cons, f'dtc {datadir}/{dts} -O dtb -o {tempdir}/{dtb}')
> +
> +    cons = u_boot_console
> +    mkimage = cons.config.build_dir + '/tools/mkimage'
> +    datadir = cons.config.source_dir + '/test/py/tests/vboot/'
> +    tempdir = cons.config.result_dir
> +    key_file = f'{tempdir}/ecdsa-test-key.pem'
> +    fit_file = f'{tempdir}/test.fit'
> +    dtc('sandbox-kernel.dts')
> +
> +    key = generate_ecdsa_key()
> +
> +    # Create a fake kernel image -- zeroes will do just fine
> +    with open(f'{tempdir}/test-kernel.bin', 'w') as fd:
> +        fd.write(500 * chr(0))
> +
> +    # invokations of mkimage expect to read the key from disk
> +    with open(key_file, 'w') as f:
> +        f.write(key.export_key(format='PEM'))
> +
> +    assemble_fit_image(fit_file, f'{datadir}/sign-images-sha256.its', tempdir)
> +
> +    fit = SignableFitImage(cons, fit_file)
> +    nodes = fit.find_signable_image_nodes()
> +    if len(nodes) == 0:
> +        raise ValueError('FIT image has no "/image" nodes with "signature"')
> +
> +    fit.change_signature_algo_to_ecdsa()
> +    fit.sign(mkimage, key_file)
> +    fit.check_signatures(key)
> --
> 2.26.2
>

As mentioned earlier, this does need a test that checks the U-Boot
code paths. This just seems to be checking the signing process. This
likely involves implementing the verification (or a fake of it) in
sandbox.

If I am missing something, please let me know.

Regards,
Simon