[RFC PATCH v3 0/1] Add dm verity root hash pkcs7 sig validation.

[RFC PATCH v3 0/1] Add dm verity root hash pkcs7 sig validation.

[Jaskaran Khurana]

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?

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:

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
Jaskaran Khurana (1):
  Adds in-kernel pkcs7 sig checking the roothash of the dm-verity hash
    tree

 drivers/md/Kconfig                |  23 ++++++
 drivers/md/Makefile               |   2 +-
 drivers/md/dm-verity-target.c     |  34 +++++++-
 drivers/md/dm-verity-verify-sig.c | 132 ++++++++++++++++++++++++++++++
 drivers/md/dm-verity-verify-sig.h |  30 +++++++
 5 files changed, 216 insertions(+), 5 deletions(-)
 create mode 100644 drivers/md/dm-verity-verify-sig.c
 create mode 100644 drivers/md/dm-verity-verify-sig.h

-- 
2.17.1

[RFC PATCH v3 1/1] Add dm verity root hash pkcs7 sig validation

[Jaskaran Khurana]

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 <jaskarankhurana@linux.microsoft.com>
---
 drivers/md/Kconfig                |  23 ++++++
 drivers/md/Makefile               |   2 +-
 drivers/md/dm-verity-target.c     |  34 +++++++-
 drivers/md/dm-verity-verify-sig.c | 132 ++++++++++++++++++++++++++++++
 drivers/md/dm-verity-verify-sig.h |  30 +++++++
 5 files changed, 216 insertions(+), 5 deletions(-)
 create mode 100644 drivers/md/dm-verity-verify-sig.c
 create mode 100644 drivers/md/dm-verity-verify-sig.h

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..5a2f84d5bd8a 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;
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 <jaskarankhurana@linux.microsoft.com>
+ *
+ */
+#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 <jaskarankhurana@linux.microsoft.com>
+ *
+ */
+#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

Re: [RFC PATCH v3 0/1] Add dm verity root hash pkcs7 sig validation.

[Milan Broz]

On 08/06/2019 00:31, Jaskaran Khurana wrote:
> 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.

...
>  drivers/md/Kconfig                |  23 ++++++
>  drivers/md/Makefile               |   2 +-
>  drivers/md/dm-verity-target.c     |  34 +++++++-
>  drivers/md/dm-verity-verify-sig.c | 132 ++++++++++++++++++++++++++++++
>  drivers/md/dm-verity-verify-sig.h |  30 +++++++

Please could you also modify Documentation/device-mapper/verity.txt and
describe the new table parameter?

It would be also nice to have a reference example how to configure it,
including how to create the signature file.

Milan

Re: [RFC PATCH v3 1/1] Add dm verity root hash pkcs7 sig validation

[Milan Broz]

On 08/06/2019 00:31, Jaskaran Khurana wrote:
> 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.

AFAIK there are tools that use dm-verity internally (some container
functions in systemd can recognize and check dm-verity partitions) and with
this option we will just kill possibility to use it without signature.

Anyway, this is up to Mike and Mikulas, I guess generic distros will not
set this option.

Some minor details below:

> 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

Why is this different from existing FEC extension? 
FEC uses ifdefs in header to blind functions if config is not set.

ifeq ($(CONFIG_DM_VERITY_FEC),y)
dm-verity-objs                  += dm-verity-fec.o
endif

...

> 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

...

> +	key = request_key(&key_type_user,
> +			key_desc, NULL);
> +	if (IS_ERR(key))
> +		return PTR_ERR(key);

You will need dependence on keyring here (kernel can be configured without it),
try to compile it without CONFIG_KEYS selected.

I think it is ok that  DM_VERITY_VERIFY_ROOTHASH_SIG can directly require CONFIG_KEYS.
(Add depends on CONFIG_KEYS in KConfig)

Also please increase minor version of dm-verity target when adding functions, something like

@@ -1175,7 +1175,7 @@ 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},

Thanks,
Milan

Re: [RFC PATCH v3 1/1] Add dm verity root hash pkcs7 sig validation

[Jaskaran Singh Khurana]

On Sat, 8 Jun 2019, Milan Broz wrote:

> On 08/06/2019 00:31, Jaskaran Khurana wrote:
> Why is this different from existing FEC extension?
> FEC uses ifdefs in header to blind functions if config is not set.
>
> ifeq ($(CONFIG_DM_VERITY_FEC),y)
> dm-verity-objs                  += dm-verity-fec.o
> endif
>
> ...
>

The reasoning for doing it this way is that there might be scripts that 
create a device mapper device and then mount and use it, with the 
signature verification enabled in kernel the scripts would be passing the 
signature like:

veritysetup open params... --roothash-sig=<sig.p7>

If later due to some reason the DM_VERITY_VERIFY_ROOTHASH_SIG is disabled 
if we do not recognize the parameter then the scripts need to be changed 
or else they will fail with INVALID argument,
in current implementation the parameter for signature is always parsed but
enforced based on the config being set, so the scripts need not be 
changed.
Let me know if you still feel I should be changing this and I will be 
happy to make the change, just wanted to share my reasoning for this.

>
> Thanks,
> Milan
>
Regards,
Jaskaran

Re: [RFC PATCH v3 1/1] Add dm verity root hash pkcs7 sig validation

[Jaskaran Singh Khurana]

On Sat, 8 Jun 2019, Milan Broz wrote:

> On 08/06/2019 00:31, Jaskaran Khurana wrote:
>> The verification is to support cases where the roothash is not secured by
>
>> +	key = request_key(&key_type_user,
>> +			key_desc, NULL);
>> +	if (IS_ERR(key))
>> +		return PTR_ERR(key);
>
> You will need dependence on keyring here (kernel can be configured without it),
> try to compile it without CONFIG_KEYS selected.
>
> I think it is ok that  DM_VERITY_VERIFY_ROOTHASH_SIG can directly require CONFIG_KEYS.
> (Add depends on CONFIG_KEYS in KConfig)
>

DM_VERITY_VERIFY_ROOTHASH_SIG selects SYSTEM_DATA_VERIFICATION and 
SYSTEM_DATA_VERIFICATION selects KEYS so we should be OK here.

>
> Thanks,
> Milan
>
Thanks,
Jaskaran.

Re: [RFC PATCH v3 1/1] Add dm verity root hash pkcs7 sig validation

[James Morris]

On Sat, 8 Jun 2019, Milan Broz 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.
> 
> AFAIK there are tools that use dm-verity internally (some container
> functions in systemd can recognize and check dm-verity partitions) and with
> this option we will just kill possibility to use it without signature.
> 
> Anyway, this is up to Mike and Mikulas, I guess generic distros will not
> set this option.

Right, I think this option would not be for a general purpose distro, but 
for embedded systems and other cases where the user may want a more 
tightly locked-down system.

-- 
James Morris
<jmorris@namei.org>