[RFC PATCH v9 10/16] dm-verity: consume root hash digest and signature data via LSM hook

From: Fan Wu
Date: Mon Jan 30 2023 - 18:00:02 EST

From: Deven Bowers <deven.desai@xxxxxxxxxxxxxxxxxxx>

dm-verity provides a strong guarantee of a block device's integrity. As
a generic way to check the integrity of a block device, it provides
those integrity guarantees to its higher layers, including the filesystem
level.

An LSM that control access to a resource on the system based on the
available integrity claims can use this transitive property of
dm-verity, by querying the underlying block_device of a particular
file.

The digest and signature information need to be stored in the block
device to fulfill the next requirement of authorization via LSM policy.
This will enable the LSM to perform revocation of devices that are still
mounted, prohibiting execution of files that are no longer authorized
by the LSM in question.

Signed-off-by: Deven Bowers <deven.desai@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx>
---
v2:
+ No Changes

v3:
+ No changes

v4:
+ No changes

v5:
+ No changes

v6:
+ Fix an improper cleanup that can result in
a leak

v7:
+ Squash patch 08/12, 10/12 to [11/16]
+ Use part0 for block_device, to retrieve the block_device, when
calling security_bdev_setsecurity

v8:
+ Undo squash of 08/12, 10/12 - separating drivers/md/ from
security/ & block/
+ Use common-audit function for dmverity_signature.
+ Change implementation for storing the dm-verity digest to use the
newly introduced dm_verity_digest structure introduced in patch
14/20.
+ Create new structure, dm_verity_digest, containing digest algorithm,
size, and digest itself to pass to the LSM layer. V7 was missing the
algorithm.
+ Create an associated public header containing this new structure and
the key values for the LSM hook, specific to dm-verity.
+ Additional information added to commit, discussing the layering of
the changes and how the information passed will be used.

v9:
+ No changes
---
drivers/md/dm-verity-target.c | 25 +++++++++++++++++++++++--
drivers/md/dm-verity-verify-sig.c | 16 +++++++++++++---
drivers/md/dm-verity-verify-sig.h | 10 ++++++----
include/linux/dm-verity.h | 19 +++++++++++++++++++
4 files changed, 61 insertions(+), 9 deletions(-)
create mode 100644 include/linux/dm-verity.h

diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index ccf5b852fbf7..afea61eed4ec 100644
--- a/drivers/md/dm-verity-target.c
+++ b/drivers/md/dm-verity-target.c
@@ -13,6 +13,7 @@
* access behavior.
*/

+#include "dm-core.h"
#include "dm-verity.h"
#include "dm-verity-fec.h"
#include "dm-verity-verify-sig.h"
@@ -21,6 +22,9 @@
#include <linux/scatterlist.h>
#include <linux/string.h>
#include <linux/jump_label.h>
+#include <linux/security.h>
+#include <linux/dm-verity.h>
+#include <crypto/hash_info.h>

#define DM_MSG_PREFIX "verity"

@@ -1169,6 +1173,8 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
sector_t hash_position;
char dummy;
char *root_hash_digest_to_validate;
+ struct block_device *bdev;
+ struct dm_verity_digest root_digest;

v = kzalloc(sizeof(struct dm_verity), GFP_KERNEL);
if (!v) {
@@ -1211,6 +1217,13 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
}
v->version = num;

+ bdev = dm_table_get_md(ti->table)->disk->part0;
+ if (!bdev) {
+ ti->error = "Mapped device lookup failed";
+ r = -ENOMEM;
+ goto bad;
+ }
+
r = dm_get_device(ti, argv[1], FMODE_READ, &v->data_dev);
if (r) {
ti->error = "Data device lookup failed";
@@ -1343,7 +1356,7 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
}

/* Root hash signature is a optional parameter*/
- r = verity_verify_root_hash(root_hash_digest_to_validate,
+ r = verity_verify_root_hash(bdev, root_hash_digest_to_validate,
strlen(root_hash_digest_to_validate),
verify_args.sig,
verify_args.sig_size);
@@ -1428,12 +1441,20 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
ti->per_io_data_size = roundup(ti->per_io_data_size,
__alignof__(struct dm_verity_io));

+ root_digest.digest = v->root_digest;
+ root_digest.digest_len = v->digest_size;
+ root_digest.algo = v->alg_name;
+
+ r = security_bdev_setsecurity(bdev, DM_VERITY_ROOTHASH_SEC_NAME, &root_digest,
+ sizeof(root_digest));
+ if (r)
+ goto bad;
+
verity_verify_sig_opts_cleanup(&verify_args);

return 0;

bad:
-
verity_verify_sig_opts_cleanup(&verify_args);
verity_dtr(ti);

diff --git a/drivers/md/dm-verity-verify-sig.c b/drivers/md/dm-verity-verify-sig.c
index db61a1f43ae9..5a73b91157d5 100644
--- a/drivers/md/dm-verity-verify-sig.c
+++ b/drivers/md/dm-verity-verify-sig.c
@@ -9,6 +9,9 @@
#include <linux/verification.h>
#include <keys/user-type.h>
#include <linux/module.h>
+#include <linux/security.h>
+#include <linux/dm-verity.h>
+#include "dm-core.h"
#include "dm-verity.h"
#include "dm-verity-verify-sig.h"

@@ -97,14 +100,17 @@ int verity_verify_sig_parse_opt_args(struct dm_arg_set *as,
* verify_verify_roothash - Verify the root hash of the verity hash device
* using builtin trusted keys.
*
+ * @bdev: block_device representing the device-mapper created block device.
+ * Used by the security hook, to set information about the block_device.
* @root_hash: For verity, the roothash/data to be verified.
* @root_hash_len: Size of the roothash/data to be verified.
* @sig_data: The trusted signature that verifies the roothash/data.
* @sig_len: Size of the signature.
*
*/
-int verity_verify_root_hash(const void *root_hash, size_t root_hash_len,
- const void *sig_data, size_t sig_len)
+int verity_verify_root_hash(struct block_device *bdev, const void *root_hash,
+ size_t root_hash_len, const void *sig_data,
+ size_t sig_len)
{
int ret;

@@ -126,8 +132,12 @@ int verity_verify_root_hash(const void *root_hash, size_t root_hash_len,
NULL,
#endif
VERIFYING_UNSPECIFIED_SIGNATURE, NULL, NULL);
+ if (ret)
+ return ret;

- return ret;
+ return security_bdev_setsecurity(bdev,
+ DM_VERITY_SIGNATURE_SEC_NAME,
+ sig_data, sig_len);
}

void verity_verify_sig_opts_cleanup(struct dm_verity_sig_opts *sig_opts)
diff --git a/drivers/md/dm-verity-verify-sig.h b/drivers/md/dm-verity-verify-sig.h
index 3987c7141f79..31692fff92e4 100644
--- a/drivers/md/dm-verity-verify-sig.h
+++ b/drivers/md/dm-verity-verify-sig.h
@@ -20,8 +20,9 @@ struct dm_verity_sig_opts {

#define DM_VERITY_ROOT_HASH_VERIFICATION_OPTS 2

-int verity_verify_root_hash(const void *data, size_t data_len,
- const void *sig_data, size_t sig_len);
+int verity_verify_root_hash(struct block_device *bdev, const void *data,
+ size_t data_len, const void *sig_data,
+ size_t sig_len);
bool verity_verify_is_sig_opt_arg(const char *arg_name);

int verity_verify_sig_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
@@ -34,8 +35,9 @@ void verity_verify_sig_opts_cleanup(struct dm_verity_sig_opts *sig_opts);

#define DM_VERITY_ROOT_HASH_VERIFICATION_OPTS 0

-static inline int verity_verify_root_hash(const void *data, size_t data_len,
- const void *sig_data, size_t sig_len)
+int verity_verify_root_hash(struct block_device *bdev, const void *data,
+ size_t data_len, const void *sig_data,
+ size_t sig_len)
{
return 0;
}
diff --git a/include/linux/dm-verity.h b/include/linux/dm-verity.h
new file mode 100644
index 000000000000..bb0413d55d72
--- /dev/null
+++ b/include/linux/dm-verity.h
@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _LINUX_DM_VERITY_H
+#define _LINUX_DM_VERITY_H
+
+#include <linux/types.h>
+#include <crypto/hash_info.h>
+#include <linux/device-mapper.h>
+
+struct dm_verity_digest {
+ const char *algo;
+ const u8 *digest;
+ size_t digest_len;
+};
+
+#define DM_VERITY_SIGNATURE_SEC_NAME DM_NAME ".verity-signature"
+#define DM_VERITY_ROOTHASH_SEC_NAME DM_NAME ".verity-roothash"
+
+#endif /* _LINUX_DM_VERITY_H */
--
2.39.0