[PATCH v4 0/3] boot: fit: authenticate the dm-verity roothash

[PATCH v4 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.

v4: fix the CI failure Tom Rini reported
 * test_fit_verity_roothash_signed() built a FIT with a dm-verity
   subnode, which makes mkimage shell out to veritysetup internally to
   compute the Merkle tree/roothash, but the test only declared dtc,
   fdtget, fdtput and openssl as required tools. On the CI image, which
   doesn't have veritysetup installed, mkimage's internal exec of it
   failed silently (the diagnostic goes to a private pipe mkimage uses
   to parse veritysetup's output, not to stderr) and the test hard
   failed with "Can't add verification data ... (Input/output error)"
   instead of skipping, the way test_fit_verity.py's veritysetup-using
   tests already do via the same marker.
 * add @pytest.mark.requiredtool('veritysetup') to
   test_fit_verity_roothash_signed(), matching test_fit_verity.py
 * collect Reviewed-by tags from Simon Glass on all three patches.
   Simon also asked, on this cover letter, whether the series should
   spell out the compatibility break explicitly (an image with a
   dm-verity subnode signed by an older mkimage stops verifying, and
   vice versa); resolved in-thread since there are no real-world
   deployments of this mechanism yet, only the upcoming OpenWrt bootstd
   use case -- no documentation change needed for this revision
v3: address comments by Simon Glass
 * factor tools/image-host.c's node-path collection into patch 1 too,
   mirroring the boot-side helper, so both the sign side and the verify
   side share the same shape and stay easy to compare
 * use present tense for the pre-patch code description in patch 2;
   document the dm-verity subnode in the rebuilt node list in
   doc/usage/fit/signature.rst and note the signature coverage in
   doc/usage/fit/dm-verity.rst
 * in the unit test, bound-check fdt_find_regions()'s returned count
   the same way fit_config_check_sig() does, tamper the digest through
   the buffer instead of casting away const, and note in a comment that
   the digest check stands in for the whole dm-verity node
 * fix test_fit_verity_roothash_signed(), which never actually ran: its
   ITS baked the tmpdir prefix into the /incbin/() paths while dtc also
   resolves incbin paths relative to the .its file's own directory, so
   the path was searched doubled and mkimage always failed
 * kept Reviewed-by tags on patches 1 and 2: neither change invalidates
   what was reviewed, the diffs are mechanical/cosmetic
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 +
 doc/usage/fit/dm-verity.rst                 |   5 +
 doc/usage/fit/signature.rst                 |   2 +-
 include/image.h                             |  23 +++
 test/boot/fit_verity.c                      | 200 +++++++++++++++++++
 test/py/tests/test_fit_verity_sign.py       | 203 ++++++++++++++++++++
 tools/image-host.c                          |  94 ++++++---
 8 files changed, 579 insertions(+), 66 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: b635d43bca429500cb8ef20aa151cb5773b9a8a5
-- 
2.55.0

[PATCH v4 1/3] boot: fit: factor out node-path collection in fit_config_add_hash()

[Daniel Golle]

Both the boot-side and host-side fit_config_add_hash() repeat 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(), in each file, with no
functional change.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
---
v4: collect Reviewed-by from Simon Glass
v3: also factor out tools/image-host.c's fit_config_add_hash(), which
    had the same duplication, per Simon Glass's review; this keeps the
    sign-side and verify-side node list construction easy to compare.
    Kept Tom's Reviewed-by: the boot-side code he reviewed is unchanged
    and the host-side addition mechanically mirrors the same pattern.
v2: no changes

 boot/image-fit-sig.c | 73 ++++++++++++++++++++++++++------------------
 tools/image-host.c   | 73 ++++++++++++++++++++++++++++----------------
 2 files changed, 91 insertions(+), 55 deletions(-)

diff --git a/boot/image-fit-sig.c b/boot/image-fit-sig.c
index fe7ca6e4ab5..3357ec92116 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;
diff --git a/tools/image-host.c b/tools/image-host.c
index 8f1e7be4066..fd2ef99d399 100644
--- a/tools/image-host.c
+++ b/tools/image-host.c
@@ -1183,6 +1183,41 @@ static const char *fit_config_get_image_list(const void *fit, int noffset,
 	return default_list;
 }
 
+/**
+ * fit_config_add_node() - Add a node's path to a list of nodes to hash
+ *
+ * @fit:	Pointer to the FIT format image header
+ * @noffset:	Offset of the node whose path should be added
+ * @node_inc:	List of nodes to add to
+ * @conf_name	Configuration-node name, child of /configurations node (only
+ *	used for error messages)
+ * @sig_name	Signature-node name (only used for error messages)
+ * @iname:	Name of image being processed (e.g. "kernel-1" (only used
+ *	for error messages)
+ */
+static int fit_config_add_node(const void *fit, int noffset,
+			       struct strlist *node_inc, const char *conf_name,
+			       const char *sig_name, const char *iname)
+{
+	char path[200];
+	int ret;
+
+	ret = fdt_get_path(fit, noffset, path, sizeof(path));
+	if (ret < 0) {
+		fprintf(stderr,
+			"Failed to get path for image '%s' in configuration '%s/%s': %s\n",
+			iname, conf_name, sig_name, fdt_strerror(ret));
+		return -ENOENT;
+	}
+	if (strlist_add(node_inc, path)) {
+		fprintf(stderr, "Out of memory processing configuration '%s/%s'\n",
+			conf_name, sig_name);
+		return -ENOMEM;
+	}
+
+	return 0;
+}
+
 /**
  * fit_config_add_hash() - Add a list of nodes to hash for an image
  *
@@ -1202,16 +1237,14 @@ static int fit_config_add_hash(const void *fit, int image_noffset,
 			       struct strlist *node_inc, const char *conf_name,
 			       const char *sig_name, const char *iname)
 {
-	char path[200];
 	int noffset;
 	int hash_count;
 	int ret;
 
-	ret = fdt_get_path(fit, image_noffset, path, sizeof(path));
-	if (ret < 0)
-		goto err_path;
-	if (strlist_add(node_inc, path))
-		goto err_mem;
+	ret = fit_config_add_node(fit, image_noffset, node_inc, conf_name,
+				  sig_name, iname);
+	if (ret)
+		return ret;
 
 	/* Add all this image's hashes */
 	hash_count = 0;
@@ -1223,11 +1256,10 @@ static int fit_config_add_hash(const void *fit, int image_noffset,
 		if (strncmp(name, FIT_HASH_NODENAME,
 			    strlen(FIT_HASH_NODENAME)))
 			continue;
-		ret = fdt_get_path(fit, noffset, path, sizeof(path));
-		if (ret < 0)
-			goto err_path;
-		if (strlist_add(node_inc, path))
-			goto err_mem;
+		ret = fit_config_add_node(fit, noffset, node_inc, conf_name,
+					  sig_name, iname);
+		if (ret)
+			return ret;
 		hash_count++;
 	}
 
@@ -1249,24 +1281,13 @@ static int fit_config_add_hash(const void *fit, int image_noffset,
 				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;
+		ret = fit_config_add_node(fit, noffset, node_inc, conf_name,
+					  sig_name, iname);
+		if (ret)
+			return ret;
 	}
 
 	return 0;
-
-err_mem:
-	fprintf(stderr, "Out of memory processing configuration '%s/%s'\n", conf_name,
-		sig_name);
-	return -ENOMEM;
-
-err_path:
-	fprintf(stderr, "Failed to get path for image '%s' in configuration '%s/%s': %s\n",
-		iname, conf_name, sig_name, fdt_strerror(ret));
-	return -ENOENT;
 }
 
 /**
-- 
2.55.0

[PATCH v4 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 adds 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>
Reviewed-by: Simon Glass <sjg@chromium.org>
---
v4: collect Reviewed-by from Simon Glass
v3: use present tense for the pre-patch code description, per Simon
    Glass's review; document the dm-verity subnode in the rebuilt node
    list in doc/usage/fit/signature.rst and note the signature coverage
    in doc/usage/fit/dm-verity.rst. The tools/image-host.c hunk now
    calls the patch 1 helper instead of duplicating the pattern inline
    (no functional change). Kept Tom's Reviewed-by: the functional
    change he reviewed is unchanged; the rest is doc/message wording
    and a mechanical refactor.
v2: no changes

 boot/image-fit-sig.c        | 23 +++++++++++++++++++----
 doc/usage/fit/dm-verity.rst |  5 +++++
 doc/usage/fit/signature.rst |  2 +-
 tools/image-host.c          | 21 +++++++++++++++++++++
 4 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/boot/image-fit-sig.c b/boot/image-fit-sig.c
index 3357ec92116..f7ab036dcb5 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/doc/usage/fit/dm-verity.rst b/doc/usage/fit/dm-verity.rst
index 800a18fceae..76030c751ae 100644
--- a/doc/usage/fit/dm-verity.rst
+++ b/doc/usage/fit/dm-verity.rst
@@ -209,6 +209,11 @@ typically be obtained from its output.
 The ``digest`` and ``salt`` byte arrays correspond to the hex-encoded
 ``Root hash`` and ``Salt`` printed by ``veritysetup format``.
 
+When the configuration is signed, ``digest`` and ``salt`` are covered by
+the configuration signature (see :doc:`signature`), so the roothash
+cannot be swapped out for a matching one without invalidating the
+signature.
+
 Optional boolean properties (when present, they are collected and appended
 as dm-verity optional parameters with hyphens converted to underscores):
 
diff --git a/doc/usage/fit/signature.rst b/doc/usage/fit/signature.rst
index da08cc75c3a..64bada2f58f 100644
--- a/doc/usage/fit/signature.rst
+++ b/doc/usage/fit/signature.rst
@@ -359,7 +359,7 @@ however, U-Boot does not read 'hashed-nodes'. Instead it rebuilds the node
 list from the configuration's own image references (kernel, fdt, ramdisk,
 etc.), since 'hashed-nodes' is not itself covered by the signature. The
 rebuilt list always includes the root node, the configuration node, each
-referenced image node and its hash/cipher subnodes.
+referenced image node and its hash, cipher and dm-verity subnodes.
 
 The image is walked in order and each tag processed as follows:
 
diff --git a/tools/image-host.c b/tools/image-host.c
index fd2ef99d399..16a5ad6c22d 100644
--- a/tools/image-host.c
+++ b/tools/image-host.c
@@ -1287,6 +1287,27 @@ 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) {
+			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 = fit_config_add_node(fit, noffset, node_inc, conf_name,
+					  sig_name, iname);
+		if (ret)
+			return ret;
+	}
+
 	return 0;
 }
 
-- 
2.55.0

[PATCH v4 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>
Reviewed-by: Simon Glass <sjg@chromium.org>
---
v4: fix the CI failure Tom Rini reported
 * test_fit_verity_roothash_signed() builds a FIT with a dm-verity
   subnode, which makes mkimage shell out to veritysetup internally to
   compute the Merkle tree/roothash, but the test only declared dtc,
   fdtget, fdtput and openssl as required tools. On the CI image, which
   doesn't have veritysetup installed, mkimage's internal exec of it
   fails silently (the diagnostic goes to a private pipe mkimage uses
   to parse veritysetup's output, not to stderr) and the test hard
   fails with "Can't add verification data ... (Input/output error)"
   instead of skipping, the way test_fit_verity.py's veritysetup-using
   tests already do via the same marker.
 * add @pytest.mark.requiredtool('veritysetup') to
   test_fit_verity_roothash_signed(), matching test_fit_verity.py.
   Verified locally both ways: with veritysetup hidden from PATH the
   test now skips instead of failing, and with it present it still
   passes.
 * collect Reviewed-by from Simon Glass
v3: address comments by Simon Glass
 * assert fdt_find_regions()'s returned count stays within the region
   array bound, matching the check fit_config_check_sig() enforces,
   instead of only checking it is non-zero
 * tamper the roothash through buf[digest_off] instead of casting away
   the const from fdt_getprop()'s return value
 * note in a comment that the digest tamper stands in for the whole
   dm-verity node, since salt lives in the same node and is covered by
   the same region
 * fix test_fit_verity_roothash_signed(), which never actually ran:
   its ITS baked the tmpdir prefix into the /incbin/() paths while dtc
   also resolves incbin paths relative to the .its file's own
   directory (same as tmpdir here), so the path was searched doubled
   ("tmpdir/tmpdir/kernel.bin") and mkimage always failed with "No
   such file or directory". Use bare filenames in the ITS and let dtc
   resolve them; drop the now-unused %(tmpdir)s substitution.
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                      | 200 +++++++++++++++++++
 test/py/tests/test_fit_verity_sign.py       | 203 ++++++++++++++++++++
 5 files changed, 443 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 f7ab036dcb5..a0c50bba4cf 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 4149ebbcce9..6edcb1995bf 100644
--- a/include/image.h
+++ b/include/image.h
@@ -1897,6 +1897,29 @@ 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..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 <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/("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)))
+
+
+@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')
+@pytest.mark.requiredtool('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')
-- 
2.55.0

Re: [PATCH v4 0/3] boot: fit: authenticate the dm-verity roothash

[Tom Rini]

On Tue, 28 Jul 2026 23:09:30 +0100, Daniel Golle wrote:

> 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.
> 
> [...]

Applied to u-boot/main, thanks!

[1/3] boot: fit: factor out node-path collection in fit_config_add_hash()
      commit: ba9ce23d21e3536b7de5e5722f20b8d6be695d3c
[2/3] boot: fit: cover the dm-verity roothash with the config signature
      commit: 2601d94691c00e0a05dc61241bb91d680519d49b
[3/3] test: fit: verify dm-verity roothash is covered by the config signature
      commit: fe9877c7d9dea740985edd11f7ff583e311568be
-- 
Tom