Merge patch series "boot: fit: authenticate the dm-verity roothash"

Daniel Golle <[email protected]> says:

A signed FIT configuration can delegate the integrity of a (potentially
large) root filesystem image to the kernel's dm-verity instead of having
U-Boot hash the whole payload at boot: the FIT carries a "dm-verity"
subnode with the roothash, salt and block parameters, U-Boot passes the
roothash to Linux through the dm-mod.create bootargs, and dm-verity then
validates the filesystem block by block against it.

For that to be safe the roothash has to be trusted, and in a signed
configuration the only thing that establishes trust is the configuration
signature. The roothash was not covered by it. fit_config_add_hash()
collected the image node, its hash subnodes and its cipher subnode into
the signed region, but not the dm-verity subnode, so the roothash, the
sole integrity anchor for the filesystem, was left unsigned.

The result is a verified-boot bypass for the root filesystem: an
attacker who can rewrite the boot medium can replace the filesystem,
recompute a matching dm-verity tree, write the new roothash into the
unsigned dm-verity subnode, and the configuration signature still
verifies. dm-verity then faithfully validates the malicious filesystem
against the attacker's roothash.

This series closes the gap.

Link: https://lore.kernel.org/r/[email protected]

2 files changed, 403 insertions, 0 deletions

diff --git a/test/boot/fit_verity.c b/test/boot/fit_verity.c
index 7459a9d6f81..4b5db839085 100644
--- a/test/boot/fit_verity.c
+++ b/test/boot/fit_verity.c
@@ -6,6 +6,11 @@
  */
 
 #include <image.h>
+#include <fdt_region.h>
+#include <malloc.h>
+#include <linux/kernel.h>
+#include <linux/libfdt.h>
+#include <u-boot/hash-checksum.h>
 #include <test/test.h>
 #include <test/ut.h>
 
@@ -304,3 +309,198 @@ static int fit_verity_test_bad_blocksize(struct unit_test_state *uts)
 	return 0;
 }
 FIT_VERITY_TEST(fit_verity_test_bad_blocksize, 0);
+
+#if CONFIG_IS_ENABLED(FIT_SIGNATURE)
+/**
+ * build_signed_verity_fit() - build a FIT with a signable verity config
+ * @buf:	output buffer (at least FIT_BUF_SIZE bytes)
+ *
+ * Like build_verity_fit(), but the filesystem image also carries a hash
+ * subnode (required for a configuration to be signable) so the config's
+ * signed-region node list can be built with fit_config_get_signed_nodes().
+ *
+ * Return: configuration node offset, or -ve on error
+ */
+static int build_signed_verity_fit(void *buf)
+{
+	int images_node, confs_node, conf_node, img_node, hash_node, verity_node;
+	fdt32_t val;
+	int ret;
+
+	ret = fdt_create_empty_tree(buf, FIT_BUF_SIZE);
+	if (ret)
+		return ret;
+
+	images_node = fdt_add_subnode(buf, 0, "images");
+	if (images_node < 0)
+		return images_node;
+
+	img_node = fdt_add_subnode(buf, images_node, "rootfs");
+	if (img_node < 0)
+		return img_node;
+	ret = fdt_setprop_string(buf, img_node, FIT_TYPE_PROP, "filesystem");
+	if (ret)
+		return ret;
+
+	hash_node = fdt_add_subnode(buf, img_node, "hash-1");
+	if (hash_node < 0)
+		return hash_node;
+	ret = fdt_setprop_string(buf, hash_node, FIT_ALGO_PROP, "sha256");
+	if (ret)
+		return ret;
+	ret = fdt_setprop(buf, hash_node, FIT_VALUE_PROP, test_digest,
+			  sizeof(test_digest));
+	if (ret)
+		return ret;
+
+	verity_node = fdt_add_subnode(buf, img_node, FIT_VERITY_NODENAME);
+	if (verity_node < 0)
+		return verity_node;
+	ret = fdt_setprop_string(buf, verity_node, FIT_VERITY_ALGO_PROP,
+				 "sha256");
+	if (ret)
+		return ret;
+	val = cpu_to_fdt32(4096);
+	ret = fdt_setprop(buf, verity_node, FIT_VERITY_DBS_PROP, &val,
+			  sizeof(val));
+	if (ret)
+		return ret;
+	ret = fdt_setprop(buf, verity_node, FIT_VERITY_HBS_PROP, &val,
+			  sizeof(val));
+	if (ret)
+		return ret;
+	val = cpu_to_fdt32(100);
+	ret = fdt_setprop(buf, verity_node, FIT_VERITY_NBLK_PROP, &val,
+			  sizeof(val));
+	if (ret)
+		return ret;
+	ret = fdt_setprop(buf, verity_node, FIT_VERITY_HBLK_PROP, &val,
+			  sizeof(val));
+	if (ret)
+		return ret;
+	ret = fdt_setprop(buf, verity_node, FIT_VERITY_DIGEST_PROP, test_digest,
+			  sizeof(test_digest));
+	if (ret)
+		return ret;
+	ret = fdt_setprop(buf, verity_node, FIT_VERITY_SALT_PROP, test_salt,
+			  sizeof(test_salt));
+	if (ret)
+		return ret;
+
+	confs_node = fdt_add_subnode(buf, 0, "configurations");
+	if (confs_node < 0)
+		return confs_node;
+	conf_node = fdt_add_subnode(buf, confs_node, "conf-1");
+	if (conf_node < 0)
+		return conf_node;
+	ret = fdt_setprop_string(buf, conf_node, FIT_LOADABLE_PROP, "rootfs");
+	if (ret)
+		return ret;
+
+	return conf_node;
+}
+
+/*
+ * Test: the dm-verity roothash and salt are inside the region covered by the
+ * configuration signature.
+ *
+ * A dm-verity filesystem image is not hashed by U-Boot; its integrity is
+ * delegated to the kernel, which trusts the roothash from the FIT dm-verity
+ * subnode. That roothash must therefore be part of the signed region, so that
+ * an attacker cannot swap both the filesystem and the roothash while keeping
+ * the configuration signature valid.
+ *
+ * This checks the property without a private key, so it also runs on real
+ * devices: it builds the exact node list the signature is computed over
+ * (fit_config_get_signed_nodes), turns it into hashed regions, and verifies both
+ * that the roothash bytes fall inside a region and that tampering them changes
+ * the hash. It uses the same hash path a device would (crypto accelerated where
+ * available).
+ */
+static int fit_verity_test_roothash_signed(struct unit_test_state *uts)
+{
+	char buf[FIT_BUF_SIZE];
+	char *node_inc[32];
+	char path_buf[256];
+	char region_path[256];
+	struct fdt_region fdt_regions[64];
+	struct image_region *region = NULL;
+	int conf_node, verity_node;
+	int count, i, digest_len;
+	const void *digest;
+	ulong digest_off, region_off;
+	bool covered = false;
+	u8 hash_clean[32], hash_tampered[32], hash_control[32];
+
+	conf_node = build_signed_verity_fit(buf);
+	ut_assert(conf_node >= 0);
+
+	verity_node = fdt_path_offset(buf, "/images/rootfs/dm-verity");
+	ut_assert(verity_node >= 0);
+
+	/* Build the node list the configuration signature is computed over. */
+	count = fit_config_get_signed_nodes(buf, conf_node, node_inc,
+					    ARRAY_SIZE(node_inc), path_buf,
+					    sizeof(path_buf));
+	ut_assert(count > 0);
+
+	/*
+	 * Turn the node list into hashed regions. No exclude list is needed:
+	 * the excluded properties (data, data-size, data-offset,
+	 * data-position) never include the dm-verity digest or salt, so the
+	 * coverage answer is the same with or without it.
+	 */
+	count = fdt_find_regions(buf, node_inc, count, NULL, 0, fdt_regions,
+				 ARRAY_SIZE(fdt_regions) - 1, region_path,
+				 sizeof(region_path), 0);
+	ut_assert(count > 0);
+	/* Region array exhausted: mirror the bound fit_config_check_sig() enforces. */
+	ut_assert(count < ARRAY_SIZE(fdt_regions) - 1);
+
+	region = fit_region_make_list(buf, fdt_regions, count, NULL);
+	ut_assertnonnull(region);
+
+	digest = fdt_getprop(buf, verity_node, FIT_VERITY_DIGEST_PROP,
+			     &digest_len);
+	ut_assertnonnull(digest);
+	ut_assert(digest_len > 0);
+	digest_off = (ulong)((const char *)digest - (const char *)buf);
+
+	/*
+	 * Control: the hash covers a non-empty region and reacts to a change
+	 * inside it. Flip a byte of the (signed) image hash value and confirm
+	 * the computed hash differs, proving the region set and hash work.
+	 */
+	ut_assertok(hash_calculate("sha256", region, count, hash_clean));
+	for (i = 0; i < count; i++) {
+		region_off = (ulong)((const char *)region[i].data -
+				     (const char *)buf);
+		if (digest_off >= region_off &&
+		    digest_off + digest_len <= region_off + region[i].size) {
+			covered = true;
+			break;
+		}
+	}
+
+	/* The roothash must be covered by the configuration signature. */
+	ut_assert(covered);
+
+	/*
+	 * Tampering the roothash must change the signed hash. Only the digest
+	 * is flipped here; salt sits in the same dm-verity node, so coverage
+	 * of one implies coverage of the other.
+	 */
+	buf[digest_off] ^= 0xff;
+	ut_assertok(hash_calculate("sha256", region, count, hash_tampered));
+	buf[digest_off] ^= 0xff;
+	ut_assert(memcmp(hash_clean, hash_tampered, sizeof(hash_clean)) != 0);
+
+	/* Sanity: with the byte restored the hash matches the clean value. */
+	ut_assertok(hash_calculate("sha256", region, count, hash_control));
+	ut_asserteq_mem(hash_clean, hash_control, sizeof(hash_clean));
+
+	free(region);
+	return 0;
+}
+FIT_VERITY_TEST(fit_verity_test_roothash_signed, 0);
+#endif /* FIT_SIGNATURE */
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')