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Diffstat (limited to 'test/py')
| -rw-r--r-- | test/py/tests/test_fit_verity_sign.py | 203 |
1 files changed, 203 insertions, 0 deletions
diff --git a/test/py/tests/test_fit_verity_sign.py b/test/py/tests/test_fit_verity_sign.py new file mode 100644 index 00000000000..3c75ef8558c --- /dev/null +++ b/test/py/tests/test_fit_verity_sign.py @@ -0,0 +1,203 @@ +# SPDX-License-Identifier: GPL-2.0 +# Copyright 2026 Daniel Golle <[email protected]> + +"""Verify that the dm-verity roothash is covered by the FIT configuration +signature. + +A dm-verity protected filesystem image is not hashed by U-Boot; its integrity +is delegated to the kernel, which trusts the roothash taken from the FIT +``dm-verity`` subnode. That roothash must therefore be part of the signed +region of the configuration, otherwise an attacker can replace both the +filesystem and the roothash while keeping the configuration signature valid. + +This test signs a configuration referencing a filesystem image that carries a +``dm-verity`` subnode, then flips one byte of the roothash and of the salt and +checks that verification rejects the image. A control tampering a byte that is +known to be signed confirms that the check is able to detect a broken region. + +The FIT pairs a signed configuration with a filesystem image carrying a +``dm-verity`` subnode: + +.. code-block:: devicetree + + images { + rootfs-1 { + data = /incbin/("rootfs.bin"); + type = "filesystem"; + compression = "none"; + hash-1 { + algo = "sha256"; + }; + dm-verity { + algo = "sha256"; + data-block-size = <4096>; + hash-block-size = <4096>; + num-data-blocks = <16>; + hash-start-block = <16>; + }; + }; + }; + + configurations { + conf-1 { + kernel = "kernel-1"; + loadables = "rootfs-1"; + signature-1 { + algo = "sha256,rsa2048"; + key-name-hint = "dev"; + sign-images = "kernel", "loadables"; + }; + }; + }; + +mkimage builds the dm-verity hash tree when assembling the image and records +the resulting roothash and salt in the ``dm-verity`` subnode; fit_check_sign +must reject an image where either was modified after signing. +""" + +import os +import pytest +import utils + +# 16 blocks of 4096 bytes, matching num-data-blocks/data-block-size below. +ROOTFS_SIZE = 16 * 4096 + +ITS = ''' +/dts-v1/; +/ { + description = "verity roothash signing coverage test"; + #address-cells = <1>; + + images { + kernel-1 { + description = "kernel"; + data = /incbin/("kernel.bin"); + type = "kernel"; + arch = "arm64"; + os = "linux"; + compression = "none"; + load = <0x40000000>; + entry = <0x40000000>; + hash-1 { algo = "sha256"; }; + }; + rootfs-1 { + description = "rootfs"; + data = /incbin/("rootfs.bin"); + type = "filesystem"; + arch = "arm64"; + compression = "none"; + hash-1 { algo = "sha256"; }; + dm-verity { + algo = "sha256"; + data-block-size = <4096>; + hash-block-size = <4096>; + num-data-blocks = <16>; + hash-start-block = <16>; + }; + }; + }; + + configurations { + default = "conf-1"; + conf-1 { + description = "signed config"; + kernel = "kernel-1"; + loadables = "rootfs-1"; + signature-1 { + algo = "sha256,rsa2048"; + key-name-hint = "dev"; + sign-images = "kernel", "loadables"; + }; + }; + }; +}; +''' + +VERITY_NODE = '/images/rootfs-1/dm-verity' +ROOTFS_HASH_NODE = '/images/rootfs-1/hash-1' + + +def flip_prop_byte(ubman, fit, node, prop): + """Flip the first byte of a byte-array property in a FIT, in place. + + The property is rewritten with the same length so that no node is + relaid out and the signed regions keep their offsets. + """ + val = utils.run_and_log(ubman, 'fdtget -t bx %s %s %s' % (fit, node, prop)) + bytelist = val.split() + bytelist[0] = '%x' % (int(bytelist[0], 16) ^ 0xff) + utils.run_and_log(ubman, 'fdtput -t bx %s %s %s %s' % + (fit, node, prop, ' '.join(bytelist))) + + [email protected]('sandbox') [email protected]('fit_signature') [email protected]('dtc') [email protected]('fdtget') [email protected]('fdtput') [email protected]('openssl') [email protected]('veritysetup') +def test_fit_verity_roothash_signed(ubman): + """The dm-verity roothash must be inside the signed configuration region.""" + tmpdir = os.path.join(ubman.config.result_dir, 'verity-sign') + '/' + if not os.path.exists(tmpdir): + os.makedirs(tmpdir) + mkimage = ubman.config.build_dir + '/tools/mkimage' + fit_check_sign = ubman.config.build_dir + '/tools/fit_check_sign' + dtc_args = '-I dts -O dtb -i %s' % tmpdir + its = tmpdir + 'verity.its' + fit = tmpdir + 'verity.itb' + dtb = tmpdir + 'control.dtb' + + # Signing key and empty control dtb to receive the public key. + utils.run_and_log(ubman, 'openssl genpkey -algorithm RSA -out %sdev.key ' + '-pkeyopt rsa_keygen_bits:2048 ' + '-pkeyopt rsa_keygen_pubexp:65537' % tmpdir) + utils.run_and_log(ubman, 'openssl req -batch -new -x509 -key %sdev.key ' + '-out %sdev.crt' % (tmpdir, tmpdir)) + with open(tmpdir + 'control.dts', 'w') as f: + f.write('/dts-v1/; / { model = "verity-test"; };\n') + utils.run_and_log(ubman, 'dtc -O dtb -o %s %scontrol.dts' % (dtb, tmpdir)) + + # Payloads. The rootfs must be a whole number of data blocks so mkimage can + # build the dm-verity hash tree and compute the roothash. + with open(tmpdir + 'rootfs.bin', 'wb') as f: + f.write(b'R' * ROOTFS_SIZE) + with open(tmpdir + 'kernel.bin', 'wb') as f: + f.write(b'KERNEL') + + with open(its, 'w') as f: + f.write(ITS) + + # Build and sign. -E keeps the (large) rootfs external, as on a real device. + utils.run_and_log(ubman, [mkimage, '-D', dtc_args, '-E', '-f', its, + '-k', tmpdir, '-K', dtb, '-r', fit]) + + # Baseline: the freshly signed image must verify. + utils.run_and_log(ubman, [fit_check_sign, '-f', fit, '-k', dtb]) + + # Control: tampering a byte that is signed (the filesystem image hash value) + # must be detected. This proves the check can fail. + control = tmpdir + 'control.itb' + utils.run_and_log(ubman, 'cp %s %s' % (fit, control)) + flip_prop_byte(ubman, control, ROOTFS_HASH_NODE, 'value') + utils.run_and_log_expect_exception( + ubman, [fit_check_sign, '-f', control, '-k', dtb], + 1, 'Failed to verify required signature') + + # Roothash: tampering the dm-verity digest must be rejected. If the digest + # is outside the signed region this check passes and boot is compromised. + tampered = tmpdir + 'tamper-digest.itb' + utils.run_and_log(ubman, 'cp %s %s' % (fit, tampered)) + flip_prop_byte(ubman, tampered, VERITY_NODE, 'digest') + utils.run_and_log_expect_exception( + ubman, [fit_check_sign, '-f', tampered, '-k', dtb], + 1, 'Failed to verify required signature') + + # Salt: likewise, the salt feeds the dm-verity target and must be signed. + tampered = tmpdir + 'tamper-salt.itb' + utils.run_and_log(ubman, 'cp %s %s' % (fit, tampered)) + flip_prop_byte(ubman, tampered, VERITY_NODE, 'salt') + utils.run_and_log_expect_exception( + ubman, [fit_check_sign, '-f', tampered, '-k', dtb], + 1, 'Failed to verify required signature') |