Re: [RFC PATCH v7 11/16] ipe: add support for dm-verity as a trust provider

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On 11/25/2021 1:37 AM, Roberto Sassu wrote:
From: deven.desai@xxxxxxxxxxxxxxxxxxx
[mailto:deven.desai@xxxxxxxxxxxxxxxxxxx]
Sent: Wednesday, October 13, 2021 9:07 PM
From: Deven Bowers <deven.desai@xxxxxxxxxxxxxxxxxxx>

..snip

diff --git a/security/ipe/modules/Makefile b/security/ipe/modules/Makefile
index e0045ec65434..84fadce85193 100644
--- a/security/ipe/modules/Makefile
+++ b/security/ipe/modules/Makefile
@@ -6,3 +6,5 @@
  #

  obj-$(CONFIG_IPE_PROP_BOOT_VERIFIED) += boot_verified.o
+obj-$(CONFIG_IPE_PROP_DM_VERITY_SIGNATURE) += dmverity_signature.o
+obj-$(CONFIG_IPE_PROP_DM_VERITY_ROOTHASH) += dmverity_roothash.o
diff --git a/security/ipe/modules/dmverity_roothash.c
b/security/ipe/modules/dmverity_roothash.c
new file mode 100644
index 000000000000..0f82bec3b842
--- /dev/null
+++ b/security/ipe/modules/dmverity_roothash.c
@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) Microsoft Corporation. All rights reserved.
+ */
+
+#include "ipe_module.h"
+
+#include <linux/fs.h>
+#include <linux/types.h>
+
+struct counted_array {
+	size_t	len;
+	u8     *data;
+};
+
+static int dvrh_parse(const char *valstr, void **value)
+{
+	int rv = 0;
+	struct counted_array *arr;
+
+	arr = kzalloc(sizeof(*arr), GFP_KERNEL);
+	if (!arr) {
+		rv = -ENOMEM;
+		goto err;
+	}
+
+	arr->len = (strlen(valstr) / 2);
+
+	arr->data = kzalloc(arr->len, GFP_KERNEL);
+	if (!arr->data) {
+		rv = -ENOMEM;
+		goto err;
+	}
+
+	rv = hex2bin(arr->data, valstr, arr->len);
+	if (rv != 0)
+		goto err2;
+
+	*value = arr;
+	return rv;
+err2:
+	kfree(arr->data);
+err:
+	kfree(arr);
+	return rv;
+}
+
+static bool dvrh_eval(const struct ipe_eval_ctx *ctx, const void *val)
+{
+	const u8 *src;
+	struct counted_array *expect = (struct counted_array *)val;
+
+	if (!ctx->ipe_bdev)
+		return false;
+
+	if (ctx->ipe_bdev->hashlen != expect->len)
+		return false;
+
+	src = ctx->ipe_bdev->hash;
+
+	return !memcmp(expect->data, src, expect->len);
Hi Deven

I was curious to see if determining the property at run-time
could apply also to dm-verity. It seems it could be done
(I omit some checks, I also keep the expected value in hex
format):

---
         md = dm_get_md(file_inode(ctx->file)->i_sb->s_dev);
         table = dm_get_live_table(md, &srcu_idx);
         num_targets = dm_table_get_num_targets(table);

         for (i = 0; i < num_targets; i++) {
                 struct dm_target *ti = dm_table_get_target(table, i);

                 if (strcmp(ti->type->name, "verity"))
                         continue;

                 ti->type->status(ti, STATUSTYPE_IMA, 0, result, sizeof(result));
         }

         dm_put_live_table(md, srcu_idx);
         dm_put(md);

         root_digest_ptr = strstr(result, "root_digest=");
         return !strncmp(expect->data, root_digest_ptr + 12, expect->len);
---

Only dm_table_get_target() is not exported yet, but I guess it could
be. dm_table_get_num_targets() is exported.

I had tried something similar in a very early draft of IPE. The issue
that comes with this is that when you compile device-mapper as a module
(CONFIG_BLK_DEV_DM=m) you start to get linking errors with this
approach.

Obviously, we can fix this in the IPE's module Kconfig by setting the
dependency to be =y, but it's something to highlight. My general
preference is to support the =m configuration by using these blobs.

The runtime approach does work with fs-verity, because fs-verity is a
file-system level feature that cannot be compiled as a module.

With this code, you would not have to manage security blobs
outside IPE. Maybe you could add a blob for the super block, so
that you verify the dm-verity property just once per filesystem.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Zhong Ronghua

+}
+
+static int dvrh_free(void **val)
+{
+	struct counted_array *expect = (struct counted_array *)val;
+
+	kfree(expect->data);
+	kfree(expect);
+
+	return 0;
+}
+
+IPE_MODULE(dvrh) = {
+	.name = "dmverity_roothash",
+	.version = 1,
+	.parse = dvrh_parse,
+	.free = dvrh_free,
+	.eval = dvrh_eval,
+};
diff --git a/security/ipe/modules/dmverity_signature.c
b/security/ipe/modules/dmverity_signature.c
new file mode 100644
index 000000000000..08746fcbcb3e
--- /dev/null
+++ b/security/ipe/modules/dmverity_signature.c
@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) Microsoft Corporation. All rights reserved.
+ */
+
+#include "ipe_module.h"
+
+#include <linux/fs.h>
+#include <linux/types.h>
+
+static bool dvv_eval(const struct ipe_eval_ctx *ctx, const void *val)
+{
+	bool expect = (bool)val;
+	bool eval = ctx->ipe_bdev && (!!ctx->ipe_bdev->sigdata);
+
+	return expect == eval;
+}
+
+IPE_MODULE(dvv) = {
+	.name = "dmverity_signature",
+	.version = 1,
+	.parse = ipe_bool_parse,
+	.free = NULL,
+	.eval = dvv_eval,
+};
--
2.33.0

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