This patch set adds in-kernel pkcs7 signature checking for the roothash of
the dm-verity hash tree.
The verification is to support cases where the roothash is not secured by
Trusted Boot, UEFI Secureboot or similar technologies.
One of the use cases for this is for dm-verity volumes mounted after boot,
the root hash provided during the creation of the dm-verity volume has to
be secure and thus in-kernel validation implemented here will be used
before we trust the root hash and allow the block device to be created.
Why we are doing validation in the Kernel?
The reason is to still be secure in cases where the attacker is able to
compromise the user mode application in which case the user mode validation
could not have been trusted.
The root hash signature validation in the kernel along with existing
dm-verity implementation gives a higher level of confidence in the
executable code or the protected data. Before allowing the creation of
the device mapper block device the kernel code will check that the detached
pkcs7 signature passed to it validates the roothash and the signature is
trusted by builtin keys set at kernel creation. The kernel should be
secured using Verified boot, UEFI Secure Boot or similar technologies so we
can trust it.
What about attacker mounting non dm-verity volumes to run executable
code?
This verification can be used to have a security architecture where a LSM
can enforce this verification for all the volumes and by doing this it can
ensure that all executable code runs from signed and trusted dm-verity
volumes.
Further patches will be posted that build on this and enforce this
verification based on policy for all the volumes on the system.
How are these changes tested?
To generate and sign the roothash just dump the roothash returned by veritysetup
format in a text file and then sign using the tool in the topic branch here:
(fsverity uses the tool for signing, I just added a parameter there for testing)
https://github.com/jaskarankhurana/fsverity-sign/tree/fs_verity_detached_pkcs7_for_dm_verity
fsverity sign-dm-verity <ROOTHASH_IN_A_FILE> <OUTSIG> --key=<KEYFILE>
--cert=<CERTFILE>
veritysetup part of cryptsetup library was modified to take a optional
root-hash-sig parameter.
Commandline used to test the changes:
veritysetup open <data_device> <name> <hash_device> <root_hash>
--root-hash-sig=<root_hash_pkcs7_detached_sig>
The changes for veritysetup are in a topic branch for now at:
https://github.com/jaskarankhurana/veritysetup/tree/veritysetup_add_sig
Changelog:
v4:
- Code review feedback given by Milan Broz.
- Add documentation about the root hash signature parameter.
- Bump up the dm-verity target version.
- Provided way to sign and test with veritysetup in cover letter.
v3:
- Code review feedback given by Sasha Levin.
- Removed EXPORT_SYMBOL_GPL since this was not required.
- Removed "This file is released under the GPLv2" since we have SPDX
identifier.
- Inside verity_verify_root_hash changed EINVAL to ENOKEY when the key
descriptor is not specified but due to force option being set it is
expected.
- Moved CONFIG check to inside verity_verify_get_sig_from_key.
(Did not move the sig_opts_cleanup to inside verity_dtr as the
sig_opts do not need to be allocated for the entire duration the block
device is active unlike the verity structure, note verity_dtr is called
only if verity_ctr fails or after the lifetime of the block device.)
v2:
- Code review feedback to pass the signature binary blob as a key that can be
looked up in the kernel and be used to verify the roothash.
[Suggested by Milan Broz]
- Made the code related change suggested in review of v1.
[Suggested by Balbir Singh]
v1:
- Add kconfigs to control dm-verity root has signature verification and
use the signature if specified to verify the root hash.
Jaskaran Khurana (1):
Adds in-kernel pkcs7 sig checking the roothash of the dm-verity hash
tree
Documentation/device-mapper/verity.txt | 7 ++
drivers/md/Kconfig | 23 +++++
drivers/md/Makefile | 2 +-
drivers/md/dm-verity-target.c | 36 ++++++-
drivers/md/dm-verity-verify-sig.c | 132 +++++++++++++++++++++++++
drivers/md/dm-verity-verify-sig.h | 30 ++++++
6 files changed, 224 insertions(+), 6 deletions(-)
create mode 100644 drivers/md/dm-verity-verify-sig.c
create mode 100644 drivers/md/dm-verity-verify-sig.h
--
2.17.1
The verification is to support cases where the roothash is not secured by
Trusted Boot, UEFI Secureboot or similar technologies.
One of the use cases for this is for dm-verity volumes mounted after boot,
the root hash provided during the creation of the dm-verity volume has to
be secure and thus in-kernel validation implemented here will be used
before we trust the root hash and allow the block device to be created.
The signature being provided for verification must verify the root hash and
must be trusted by the builtin keyring for verification to succeed.
The hash is added as a key of type "user" and the description is passed to
the kernel so it can look it up and use it for verification.
Adds DM_VERITY_VERIFY_ROOTHASH_SIG: roothash verification
against the roothash signature file *if* specified, if signature file is
specified verification must succeed prior to creation of device mapper
block device.
Adds DM_VERITY_VERIFY_ROOTHASH_SIG_FORCE: roothash signature *must* be
specified for all dm verity volumes and verification must succeed prior
to creation of device mapper block device.
Signed-off-by: Jaskaran Khurana <[email protected]>
---
Documentation/device-mapper/verity.txt | 7 ++
drivers/md/Kconfig | 23 +++++
drivers/md/Makefile | 2 +-
drivers/md/dm-verity-target.c | 36 ++++++-
drivers/md/dm-verity-verify-sig.c | 132 +++++++++++++++++++++++++
drivers/md/dm-verity-verify-sig.h | 30 ++++++
6 files changed, 224 insertions(+), 6 deletions(-)
create mode 100644 drivers/md/dm-verity-verify-sig.c
create mode 100644 drivers/md/dm-verity-verify-sig.h
diff --git a/Documentation/device-mapper/verity.txt b/Documentation/device-mapper/verity.txt
index b3d2e4a42255..df7ef1d553cc 100644
--- a/Documentation/device-mapper/verity.txt
+++ b/Documentation/device-mapper/verity.txt
@@ -121,6 +121,13 @@ check_at_most_once
blocks, and a hash block will not be verified any more after all the data
blocks it covers have been verified anyway.
+root_hash_sig_key_desc <key_description>
+ This is the description of the USER_KEY that the kernel will lookup to get
+ the pkcs7 signature of the roothash. The pkcs7 signature is used to validate
+ the root hash during the creation of the device mapper block device.
+ Verification of roothash depends on the config DM_VERITY_VERIFY_ROOTHASH_SIG
+ being set in the kernel.
+
Theory of operation
===================
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index db269a348b20..da4115753f25 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -489,6 +489,29 @@ config DM_VERITY
If unsure, say N.
+config DM_VERITY_VERIFY_ROOTHASH_SIG
+ def_bool n
+ bool "Verity data device root hash signature verification support"
+ depends on DM_VERITY
+ select SYSTEM_DATA_VERIFICATION
+ help
+ The device mapper target created by DM-VERITY can be validated if the
+ pre-generated tree of cryptographic checksums passed has a pkcs#7
+ signature file that can validate the roothash of the tree.
+
+ If unsure, say N.
+
+config DM_VERITY_VERIFY_ROOTHASH_SIG_FORCE
+ def_bool n
+ bool "Forces all dm verity data device root hash should be signed"
+ depends on DM_VERITY_VERIFY_ROOTHASH_SIG
+ help
+ The device mapper target created by DM-VERITY will succeed only if the
+ pre-generated tree of cryptographic checksums passed also has a pkcs#7
+ signature file that can validate the roothash of the tree.
+
+ If unsure, say N.
+
config DM_VERITY_FEC
bool "Verity forward error correction support"
depends on DM_VERITY
diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index be7a6eb92abc..8a8c142bcfe1 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -61,7 +61,7 @@ obj-$(CONFIG_DM_LOG_USERSPACE) += dm-log-userspace.o
obj-$(CONFIG_DM_ZERO) += dm-zero.o
obj-$(CONFIG_DM_RAID) += dm-raid.o
obj-$(CONFIG_DM_THIN_PROVISIONING) += dm-thin-pool.o
-obj-$(CONFIG_DM_VERITY) += dm-verity.o
+obj-$(CONFIG_DM_VERITY) += dm-verity.o dm-verity-verify-sig.o
obj-$(CONFIG_DM_CACHE) += dm-cache.o
obj-$(CONFIG_DM_CACHE_SMQ) += dm-cache-smq.o
obj-$(CONFIG_DM_ERA) += dm-era.o
diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index f4c31ffaa88e..adf7f376be7d 100644
--- a/drivers/md/dm-verity-target.c
+++ b/drivers/md/dm-verity-target.c
@@ -16,7 +16,7 @@
#include "dm-verity.h"
#include "dm-verity-fec.h"
-
+#include "dm-verity-verify-sig.h"
#include <linux/module.h>
#include <linux/reboot.h>
@@ -34,7 +34,8 @@
#define DM_VERITY_OPT_IGN_ZEROES "ignore_zero_blocks"
#define DM_VERITY_OPT_AT_MOST_ONCE "check_at_most_once"
-#define DM_VERITY_OPTS_MAX (2 + DM_VERITY_OPTS_FEC)
+#define DM_VERITY_OPTS_MAX (2 + DM_VERITY_OPTS_FEC + \
+ DM_VERITY_ROOT_HASH_VERIFICATION_OPTS)
static unsigned dm_verity_prefetch_cluster = DM_VERITY_DEFAULT_PREFETCH_SIZE;
@@ -855,7 +856,8 @@ static int verity_alloc_zero_digest(struct dm_verity *v)
return r;
}
-static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v)
+static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
+ struct dm_verity_sig_opts *verify_args)
{
int r;
unsigned argc;
@@ -904,6 +906,14 @@ static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v)
if (r)
return r;
continue;
+ } else if (verity_verify_is_sig_opt_arg(arg_name)) {
+ r = verity_verify_sig_parse_opt_args(as, v,
+ verify_args,
+ &argc, arg_name);
+ if (r)
+ return r;
+ continue;
+
}
ti->error = "Unrecognized verity feature request";
@@ -930,6 +940,7 @@ static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v)
static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
{
struct dm_verity *v;
+ struct dm_verity_sig_opts verify_args = {0};
struct dm_arg_set as;
unsigned int num;
unsigned long long num_ll;
@@ -937,6 +948,7 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
int i;
sector_t hash_position;
char dummy;
+ char *root_hash_digest_to_validate;
v = kzalloc(sizeof(struct dm_verity), GFP_KERNEL);
if (!v) {
@@ -1070,6 +1082,7 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
r = -EINVAL;
goto bad;
}
+ root_hash_digest_to_validate = argv[8];
if (strcmp(argv[9], "-")) {
v->salt_size = strlen(argv[9]) / 2;
@@ -1095,11 +1108,20 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
as.argc = argc;
as.argv = argv;
- r = verity_parse_opt_args(&as, v);
+ r = verity_parse_opt_args(&as, v, &verify_args);
if (r < 0)
goto bad;
}
+ /* Root hash signature is a optional parameter*/
+ r = verity_verify_root_hash(root_hash_digest_to_validate,
+ strlen(root_hash_digest_to_validate),
+ verify_args.sig,
+ verify_args.sig_size);
+ if (r < 0) {
+ ti->error = "Root hash verification failed";
+ goto bad;
+ }
v->hash_per_block_bits =
__fls((1 << v->hash_dev_block_bits) / v->digest_size);
@@ -1165,9 +1187,13 @@ 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));
+ verity_verify_sig_opts_cleanup(&verify_args);
+
return 0;
bad:
+
+ verity_verify_sig_opts_cleanup(&verify_args);
verity_dtr(ti);
return r;
@@ -1175,7 +1201,7 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
static struct target_type verity_target = {
.name = "verity",
- .version = {1, 4, 0},
+ .version = {1, 5, 0},
.module = THIS_MODULE,
.ctr = verity_ctr,
.dtr = verity_dtr,
diff --git a/drivers/md/dm-verity-verify-sig.c b/drivers/md/dm-verity-verify-sig.c
new file mode 100644
index 000000000000..1a889be76ede
--- /dev/null
+++ b/drivers/md/dm-verity-verify-sig.c
@@ -0,0 +1,132 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2019 Microsoft Corporation.
+ *
+ * Author: Jaskaran Singh Khurana <[email protected]>
+ *
+ */
+#include <linux/device-mapper.h>
+#include <linux/verification.h>
+#include <keys/user-type.h>
+#include "dm-verity.h"
+#include "dm-verity-verify-sig.h"
+
+#define DM_VERITY_VERIFY_ERR(s) DM_VERITY_ROOT_HASH_VERIFICATION " " s
+
+
+bool verity_verify_is_sig_opt_arg(const char *arg_name)
+{
+ return (!strcasecmp(arg_name,
+ DM_VERITY_ROOT_HASH_VERIFICATION_OPT_SIG_KEY));
+}
+
+static int verity_verify_get_sig_from_key(const char *key_desc,
+ struct dm_verity_sig_opts *sig_opts)
+{
+ struct key *key;
+ const struct user_key_payload *ukp;
+ int ret = 0;
+
+ if (!IS_ENABLED(CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG))
+ return 0;
+
+ key = request_key(&key_type_user,
+ key_desc, NULL);
+ if (IS_ERR(key))
+ return PTR_ERR(key);
+
+ down_read(&key->sem);
+
+ ukp = user_key_payload_locked(key);
+ if (!ukp) {
+ ret = -EKEYREVOKED;
+ goto end;
+ }
+
+ sig_opts->sig = kmalloc(ukp->datalen, GFP_KERNEL);
+ if (!sig_opts->sig) {
+ ret = -ENOMEM;
+ goto end;
+ }
+ sig_opts->sig_size = ukp->datalen;
+
+ memcpy(sig_opts->sig, ukp->data, sig_opts->sig_size);
+
+end:
+ up_read(&key->sem);
+ key_put(key);
+
+ return ret;
+}
+
+int verity_verify_sig_parse_opt_args(struct dm_arg_set *as,
+ struct dm_verity *v,
+ struct dm_verity_sig_opts *sig_opts,
+ unsigned int *argc,
+ const char *arg_name)
+{
+ struct dm_target *ti = v->ti;
+ int ret = 0;
+ const char *sig_key = NULL;
+
+ if (!*argc) {
+ ti->error = DM_VERITY_VERIFY_ERR("Signature key not specified");
+ return -EINVAL;
+ }
+
+ sig_key = dm_shift_arg(as);
+ (*argc)--;
+
+ ret = verity_verify_get_sig_from_key(sig_key, sig_opts);
+ if (ret < 0)
+ ti->error = DM_VERITY_VERIFY_ERR("Invalid key specified");
+
+ return ret;
+}
+
+#ifdef CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG
+/*
+ * verify_verify_roothash - Verify the root hash of the verity hash device
+ * using builtin trusted keys.
+ *
+ * @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 ret;
+
+ if (!root_hash || root_hash_len == 0)
+ return -EINVAL;
+
+ if (!sig_data || sig_len == 0) {
+ if (IS_ENABLED(CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG_FORCE))
+ return -ENOKEY;
+ else
+ return 0;
+ }
+
+ ret = verify_pkcs7_signature(root_hash, root_hash_len, sig_data,
+ sig_len, NULL, VERIFYING_UNSPECIFIED_SIGNATURE,
+ NULL, NULL);
+
+ return ret;
+}
+#else
+int verity_verify_root_hash(const void *root_hash, size_t root_hash_len,
+ const void *sig_data, size_t sig_len)
+{
+ return 0;
+}
+#endif
+
+void verity_verify_sig_opts_cleanup(struct dm_verity_sig_opts *sig_opts)
+{
+ kfree(sig_opts->sig);
+ sig_opts->sig = NULL;
+ sig_opts->sig_size = 0;
+}
diff --git a/drivers/md/dm-verity-verify-sig.h b/drivers/md/dm-verity-verify-sig.h
new file mode 100644
index 000000000000..339818e6b527
--- /dev/null
+++ b/drivers/md/dm-verity-verify-sig.h
@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2019 Microsoft Corporation.
+ *
+ * Author: Jaskaran Singh Khurana <[email protected]>
+ *
+ */
+#ifndef DM_VERITY_SIG_VERIFICATION_H
+#define DM_VERITY_SIG_VERIFICATION_H
+
+#define DM_VERITY_ROOT_HASH_VERIFICATION "DM Verity Sig Verification"
+#define DM_VERITY_ROOT_HASH_VERIFICATION_OPT_SIG_KEY "root_hash_sig_key_desc"
+#define DM_VERITY_ROOT_HASH_VERIFICATION_OPTS 2
+
+struct dm_verity_sig_opts {
+ unsigned int sig_size;
+ u8 *sig;
+};
+int verity_verify_root_hash(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,
+ struct dm_verity_sig_opts *sig_opts,
+ unsigned int *argc, const char *arg_name);
+
+void verity_verify_sig_opts_cleanup(struct dm_verity_sig_opts *sig_opts);
+
+#endif /* DM_VERITY_SIG_VERIFICATION_H */
--
2.17.1
On 13/06/2019 03:06, Jaskaran Khurana wrote:
...
> Adds DM_VERITY_VERIFY_ROOTHASH_SIG_FORCE: roothash signature *must* be
> specified for all dm verity volumes and verification must succeed prior
> to creation of device mapper block device.
I had a quick discussion about this and one suggestion was
to add dm-verity kernel module parameter instead of a new config option.
The idea is that if you can control kernel boot commandline, you can add it
there with the same effect (expecting that root device is on dm-verity as well).
Isn't this better option or it is not going to work for you?
Milan
On Mon, 17 Jun 2019, Milan Broz wrote:
> On 13/06/2019 03:06, Jaskaran Khurana wrote:
> ...
>
>> Adds DM_VERITY_VERIFY_ROOTHASH_SIG_FORCE: roothash signature *must* be
>> specified for all dm verity volumes and verification must succeed prior
>> to creation of device mapper block device.
>
> I had a quick discussion about this and one suggestion was
> to add dm-verity kernel module parameter instead of a new config option.
>
> The idea is that if you can control kernel boot commandline, you can add it
> there with the same effect (expecting that root device is on dm-verity as well).
>
> Isn't this better option or it is not going to work for you?
Seems like a better option to me, I will make the change and remove both
the configs.
>
> Milan
>
Regards,
Jaskaran