[PATCH v2 0/3] boot: fit: authenticate the dm-verity roothash

[PATCH v2 0/3] boot: fit: authenticate the dm-verity roothash

[Daniel Golle]

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.

v2: address comments by Tom Rini
 * drop the VISIBLE_IF_UT visibility macro; fit_config_get_signed_nodes()
   is now simply non-static (previously the function would end up being
   inlined, so there *is* a real cost to this)
 * document test_fit_verity_sign.py with pydoc docstrings including an
   ITS example, and add a page under doc/develop/pytest/ so the module
   is rendered in the generated documentation
 * collect Reviewed-by tags on patches 1 and 2

Daniel Golle (3):
  boot: fit: factor out node-path collection in fit_config_add_hash()
  boot: fit: cover the dm-verity roothash with the config signature
  test: fit: verify dm-verity roothash is covered by the config
    signature

 boot/image-fit-sig.c                        | 108 +++++++----
 doc/develop/pytest/test_fit_verity_sign.rst |  10 +
 include/image.h                             |  23 +++
 test/boot/fit_verity.c                      | 194 +++++++++++++++++++
 test/py/tests/test_fit_verity_sign.py       | 202 ++++++++++++++++++++
 tools/image-host.c                          |  22 +++
 6 files changed, 520 insertions(+), 39 deletions(-)
 create mode 100644 doc/develop/pytest/test_fit_verity_sign.rst
 create mode 100644 test/py/tests/test_fit_verity_sign.py


base-commit: 6741b0dfb41dc82a284ab1cff4c58af6ef2f3f9c
-- 
2.55.0

[PATCH v2 1/3] boot: fit: factor out node-path collection in fit_config_add_hash()

[Daniel Golle]

fit_config_add_hash() repeats the same sequence to append a node's path
to the hashed-node list three times: for the image node, for each hash
subnode and for the cipher subnode. Extract it into a helper,
fit_config_add_node(), with no functional change.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
---
v2: no changes

 boot/image-fit-sig.c | 73 ++++++++++++++++++++++++++------------------
 1 file changed, 44 insertions(+), 29 deletions(-)

diff --git a/boot/image-fit-sig.c b/boot/image-fit-sig.c
index 9b5ab754561..513ee9cc16a 100644
--- a/boot/image-fit-sig.c
+++ b/boot/image-fit-sig.c
@@ -230,6 +230,37 @@ int fit_image_verify_required_sigs(const void *fit, int image_noffset,
 	return 0;
 }
 
+/**
+ * fit_config_add_node() - Append one node's path to the hashed-node list
+ *
+ * @fit:		FIT blob
+ * @noffset:		Offset of the node whose path should be added
+ * @node_inc:		Array of path pointers to fill
+ * @count:		Pointer to current count (updated on return)
+ * @max_nodes:		Maximum entries in @node_inc
+ * @buf:		Buffer for packed path strings
+ * @buf_used:		Pointer to bytes used in @buf (updated on return)
+ * @buf_len:		Total size of @buf
+ * Return: 0 on success, -ve on error
+ */
+static int fit_config_add_node(const void *fit, int noffset, char **node_inc,
+			       int *count, int max_nodes, char *buf,
+			       int *buf_used, int buf_len)
+{
+	int ret, len;
+
+	if (*count >= max_nodes)
+		return -ENOSPC;
+	ret = fdt_get_path(fit, noffset, buf + *buf_used, buf_len - *buf_used);
+	if (ret < 0)
+		return -ENOENT;
+	len = strlen(buf + *buf_used) + 1;
+	node_inc[(*count)++] = buf + *buf_used;
+	*buf_used += len;
+
+	return 0;
+}
+
 /**
  * fit_config_add_hash() - Add hash nodes for one image to the node list
  *
@@ -250,18 +281,12 @@ static int fit_config_add_hash(const void *fit, int image_noffset,
 			       char **node_inc, int *count, int max_nodes,
 			       char *buf, int *buf_used, int buf_len)
 {
-	int noffset, hash_count, ret, len;
+	int noffset, hash_count, ret;
 
-	if (*count >= max_nodes)
-		return -ENOSPC;
-
-	ret = fdt_get_path(fit, image_noffset, buf + *buf_used,
-			   buf_len - *buf_used);
-	if (ret < 0)
-		return -ENOENT;
-	len = strlen(buf + *buf_used) + 1;
-	node_inc[(*count)++] = buf + *buf_used;
-	*buf_used += len;
+	ret = fit_config_add_node(fit, image_noffset, node_inc, count,
+				  max_nodes, buf, buf_used, buf_len);
+	if (ret)
+		return ret;
 
 	/* Add all this image's hash subnodes */
 	hash_count = 0;
@@ -273,15 +298,10 @@ static int fit_config_add_hash(const void *fit, int image_noffset,
 		if (strncmp(name, FIT_HASH_NODENAME,
 			    strlen(FIT_HASH_NODENAME)))
 			continue;
-		if (*count >= max_nodes)
-			return -ENOSPC;
-		ret = fdt_get_path(fit, noffset, buf + *buf_used,
-				   buf_len - *buf_used);
-		if (ret < 0)
-			return -ENOENT;
-		len = strlen(buf + *buf_used) + 1;
-		node_inc[(*count)++] = buf + *buf_used;
-		*buf_used += len;
+		ret = fit_config_add_node(fit, noffset, node_inc, count,
+					  max_nodes, buf, buf_used, buf_len);
+		if (ret)
+			return ret;
 		hash_count++;
 	}
 
@@ -296,15 +316,10 @@ static int fit_config_add_hash(const void *fit, int image_noffset,
 	if (noffset != -FDT_ERR_NOTFOUND) {
 		if (noffset < 0)
 			return -EIO;
-		if (*count >= max_nodes)
-			return -ENOSPC;
-		ret = fdt_get_path(fit, noffset, buf + *buf_used,
-				   buf_len - *buf_used);
-		if (ret < 0)
-			return -ENOENT;
-		len = strlen(buf + *buf_used) + 1;
-		node_inc[(*count)++] = buf + *buf_used;
-		*buf_used += len;
+		ret = fit_config_add_node(fit, noffset, node_inc, count,
+					  max_nodes, buf, buf_used, buf_len);
+		if (ret)
+			return ret;
 	}
 
 	return 0;
-- 
2.55.0

[PATCH v2 2/3] boot: fit: cover the dm-verity roothash with the config signature

[Daniel Golle]

A dm-verity protected filesystem image is not hashed by U-Boot when it
is loaded; its integrity is delegated to the kernel, which validates the
filesystem on the fly against the roothash taken from the FIT dm-verity
subnode. The roothash is therefore the sole integrity anchor for the
filesystem, yet fit_config_add_hash() only added the image node, its
hash subnodes and its cipher subnode to the signed region, leaving the
dm-verity subnode (roothash, salt and block parameters) unsigned.

An attacker able to rewrite the boot medium could then replace both the
filesystem and the roothash, recompute a matching dm-verity tree and
keep the configuration signature valid, defeating verified boot for the
root filesystem.

Add the dm-verity subnode to the list of nodes covered by the
configuration signature, both when signing (tools/image-host.c) and when
verifying (boot/image-fit-sig.c), so the roothash and salt are
authenticated together with the rest of the configuration.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
---
v2: no changes

 boot/image-fit-sig.c | 23 +++++++++++++++++++----
 tools/image-host.c   | 22 ++++++++++++++++++++++
 2 files changed, 41 insertions(+), 4 deletions(-)

diff --git a/boot/image-fit-sig.c b/boot/image-fit-sig.c
index 513ee9cc16a..c2f09885492 100644
--- a/boot/image-fit-sig.c
+++ b/boot/image-fit-sig.c
@@ -264,8 +264,8 @@ static int fit_config_add_node(const void *fit, int noffset, char **node_inc,
 /**
  * fit_config_add_hash() - Add hash nodes for one image to the node list
  *
- * Adds the image path, all its hash-* subnode paths, and its cipher
- * subnode path (if present) to the packed buffer.
+ * Adds the image path, all its hash-* subnode paths, and its cipher and
+ * dm-verity subnode paths (each if present) to the packed buffer.
  *
  * @fit:		FIT blob
  * @image_noffset:	Image node offset (e.g. /images/kernel-1)
@@ -322,6 +322,21 @@ static int fit_config_add_hash(const void *fit, int image_noffset,
 			return ret;
 	}
 
+	/*
+	 * Add this image's dm-verity node if present. Its roothash is the
+	 * only integrity anchor for a dm-verity filesystem image, so it must
+	 * be covered by the configuration signature.
+	 */
+	noffset = fdt_subnode_offset(fit, image_noffset, FIT_VERITY_NODENAME);
+	if (noffset != -FDT_ERR_NOTFOUND) {
+		if (noffset < 0)
+			return -EIO;
+		ret = fit_config_add_node(fit, noffset, node_inc, count,
+					  max_nodes, buf, buf_used, buf_len);
+		if (ret)
+			return ret;
+	}
+
 	return 0;
 }
 
@@ -329,8 +344,8 @@ static int fit_config_add_hash(const void *fit, int image_noffset,
  * fit_config_get_hash_list() - Build the list of nodes to hash
  *
  * Works through every image referenced by the configuration and collects the
- * node paths: root + config + all referenced images with their hash and
- * cipher subnodes.
+ * node paths: root + config + all referenced images with their hash,
+ * cipher and dm-verity subnodes.
  *
  * Properties known not to be image references (description, compatible,
  * default, load-only) are skipped, so any new image type is covered by default.
diff --git a/tools/image-host.c b/tools/image-host.c
index 8f1e7be4066..235f7643d08 100644
--- a/tools/image-host.c
+++ b/tools/image-host.c
@@ -1256,6 +1256,28 @@ static int fit_config_add_hash(const void *fit, int image_noffset,
 			goto err_mem;
 	}
 
+	/*
+	 * Add this image's dm-verity node if present. Its roothash is the
+	 * only integrity anchor for a dm-verity filesystem image, so it must
+	 * be covered by the configuration signature.
+	 */
+	noffset = fdt_subnode_offset(fit, image_noffset,
+				     FIT_VERITY_NODENAME);
+	if (noffset != -FDT_ERR_NOTFOUND) {
+		if (noffset < 0) {
+			fprintf(stderr,
+				"Failed to get dm-verity node in configuration '%s/%s' image '%s': %s\n",
+				conf_name, sig_name, iname,
+				fdt_strerror(noffset));
+			return -EIO;
+		}
+		ret = fdt_get_path(fit, noffset, path, sizeof(path));
+		if (ret < 0)
+			goto err_path;
+		if (strlist_add(node_inc, path))
+			goto err_mem;
+	}
+
 	return 0;
 
 err_mem:
-- 
2.55.0

[PATCH v2 3/3] test: fit: verify dm-verity roothash is covered by the config signature

[Daniel Golle]

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. For that chain of trust to hold, the
roothash (and salt) must be part of the region covered by the
configuration signature, otherwise an attacker can replace both the
filesystem and the roothash while keeping the signature valid.

Add two independent checks of this property:

 - test/py/tests/test_fit_verity_sign.py signs a configuration that
   references a filesystem image carrying a dm-verity subnode, then
   confirms that tampering the roothash or the salt is rejected by
   fit_check_sign. A control that tampers a byte known to be signed
   proves the check can fail. A matching page is added under
   doc/develop/pytest/ so the module documentation is rendered with
   the rest of the generated docs.

 - test/boot/fit_verity.c gains a runtime unit test that builds the
   exact node list the configuration signature is computed over,
   turns it into hashed regions and checks both that the roothash
   bytes fall inside a signed region and that tampering them changes
   the hash. It needs no private key, so it also runs on real devices
   and uses the same hash path a device would.

To let the unit test build the signed-region node list, rename the
config node-list helper to fit_config_get_signed_nodes(), make it
non-static and declare it in image.h.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
---
v2: address comments by Tom Rini
 * drop the VISIBLE_IF_UT visibility macro and simply make
   fit_config_get_signed_nodes() non-static
 * convert the comment block in test_fit_verity_sign.py into a module
   docstring, add an ITS example and add a page under doc/develop/pytest/
   so the module is rendered in the generated documentation

 boot/image-fit-sig.c                        |  14 +-
 doc/develop/pytest/test_fit_verity_sign.rst |  10 +
 include/image.h                             |  23 +++
 test/boot/fit_verity.c                      | 194 +++++++++++++++++++
 test/py/tests/test_fit_verity_sign.py       | 202 ++++++++++++++++++++
 5 files changed, 436 insertions(+), 7 deletions(-)
 create mode 100644 doc/develop/pytest/test_fit_verity_sign.rst
 create mode 100644 test/py/tests/test_fit_verity_sign.py

diff --git a/boot/image-fit-sig.c b/boot/image-fit-sig.c
index c2f09885492..bfc6d2a8eed 100644
--- a/boot/image-fit-sig.c
+++ b/boot/image-fit-sig.c
@@ -341,7 +341,7 @@ static int fit_config_add_hash(const void *fit, int image_noffset,
 }
 
 /**
- * fit_config_get_hash_list() - Build the list of nodes to hash
+ * fit_config_get_signed_nodes() - Build the list of nodes to hash
  *
  * Works through every image referenced by the configuration and collects the
  * node paths: root + config + all referenced images with their hash,
@@ -358,9 +358,9 @@ static int fit_config_add_hash(const void *fit, int image_noffset,
  * @buf_len:	Size of @buf
  * Return: number of entries in @node_inc, or -ve on error
  */
-static int fit_config_get_hash_list(const void *fit, int conf_noffset,
-				    char **node_inc, int max_nodes,
-				    char *buf, int buf_len)
+int fit_config_get_signed_nodes(const void *fit, int conf_noffset,
+				char **node_inc, int max_nodes,
+				char *buf, int buf_len)
 {
 	const char *conf_name;
 	int image_count;
@@ -500,9 +500,9 @@ static int fit_config_check_sig(const void *fit, int noffset, int conf_noffset,
 	}
 
 	/* Build the node list from the config, ignoring hashed-nodes */
-	count = fit_config_get_hash_list(fit, conf_noffset,
-					 node_inc, IMAGE_MAX_HASHED_NODES,
-					 hash_buf, sizeof(hash_buf));
+	count = fit_config_get_signed_nodes(fit, conf_noffset,
+					    node_inc, IMAGE_MAX_HASHED_NODES,
+					    hash_buf, sizeof(hash_buf));
 	if (count < 0) {
 		*err_msgp = "Failed to build hash node list";
 		return -1;
diff --git a/doc/develop/pytest/test_fit_verity_sign.rst b/doc/develop/pytest/test_fit_verity_sign.rst
new file mode 100644
index 00000000000..94a39b08952
--- /dev/null
+++ b/doc/develop/pytest/test_fit_verity_sign.rst
@@ -0,0 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+test_fit_verity_sign
+====================
+
+.. automodule:: test_fit_verity_sign
+   :synopsis:
+   :member-order: bysource
+   :members:
+   :undoc-members:
diff --git a/include/image.h b/include/image.h
index 9c8a746d576..081508707ff 100644
--- a/include/image.h
+++ b/include/image.h
@@ -1884,6 +1884,29 @@ struct image_region *fit_region_make_list(const void *fit,
 		struct fdt_region *fdt_regions, int count,
 		struct image_region *region);
 
+/**
+ * fit_config_get_signed_nodes() - Build the list of nodes covered by a config
+ *				   signature
+ *
+ * Collects the paths of the nodes that the configuration signature is
+ * computed over: the root node, the configuration node, and for each image
+ * referenced by the configuration its node, its hash subnodes and its cipher
+ * and dm-verity subnodes. The result is the same node list used when creating
+ * and verifying the signature, and is suitable for passing to
+ * fdt_find_regions().
+ *
+ * @fit:	FIT blob
+ * @conf_noffset: Configuration node offset
+ * @node_inc:	Array to fill with pointers to packed path strings
+ * @max_nodes:	Number of entries in @node_inc
+ * @buf:	Buffer for the packed null-terminated path strings
+ * @buf_len:	Size of @buf
+ * Return: number of entries written to @node_inc, or -ve on error
+ */
+int fit_config_get_signed_nodes(const void *fit, int conf_noffset,
+				char **node_inc, int max_nodes,
+				char *buf, int buf_len);
+
 static inline int fit_image_check_target_arch(const void *fdt, int node)
 {
 #ifndef USE_HOSTCC
diff --git a/test/boot/fit_verity.c b/test/boot/fit_verity.c
index 7459a9d6f81..a0a423f6666 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 +309,192 @@ 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 = 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. */
+	((u8 *)digest)[0] ^= 0xff;
+	ut_assertok(hash_calculate("sha256", region, count, hash_tampered));
+	((u8 *)digest)[0] ^= 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..ccdeccc0149
--- /dev/null
+++ b/test/py/tests/test_fit_verity_sign.py
@@ -0,0 +1,202 @@
+# SPDX-License-Identifier: GPL-2.0
+# Copyright 2026 Daniel Golle <daniel@makrotopia.org>
+
+"""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/("%(tmpdir)skernel.bin");
+            type = "kernel";
+            arch = "arm64";
+            os = "linux";
+            compression = "none";
+            load = <0x40000000>;
+            entry = <0x40000000>;
+            hash-1 { algo = "sha256"; };
+        };
+        rootfs-1 {
+            description = "rootfs";
+            data = /incbin/("%(tmpdir)srootfs.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)))
+
+
+@pytest.mark.boardspec('sandbox')
+@pytest.mark.buildconfigspec('fit_signature')
+@pytest.mark.requiredtool('dtc')
+@pytest.mark.requiredtool('fdtget')
+@pytest.mark.requiredtool('fdtput')
+@pytest.mark.requiredtool('openssl')
+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 % {'tmpdir': tmpdir})
+
+    # 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')
-- 
2.55.0