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

From: Sasha Levin
Date: Wed Jun 05 2019 - 12:46:29 EST

On Mon, May 20, 2019 at 02:54:22PM -0700, Jaskaran Khurana wrote:
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.

The signature being provided for verification must verify the root hash and
must be trusted by the builtin keyring for verification to succeed.

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@xxxxxxxxxxxxxxxxxxx>
---
drivers/md/Kconfig | 23 ++++++
drivers/md/Makefile | 2 +-
drivers/md/dm-verity-target.c | 44 ++++++++--
drivers/md/dm-verity-verify-sig.c | 129 ++++++++++++++++++++++++++++++
drivers/md/dm-verity-verify-sig.h | 32 ++++++++
5 files changed, 222 insertions(+), 8 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

Can we avoid building dm-verity-verify-sig.o when
DM_VERITY_VERIFY_ROOTHASH_SIG is not set?

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..53aebfa8bc38 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 +34,11 @@
#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)

It would be nice if DM_VERITY_OPTS_MAX would remain the same size as it
is now if DM_VERITY_VERIFY_ROOTHASH_SIG is not enabled.

+#define DM_VERITY_MANDATORY_ARGS 10

This cleanup should be in a separate patch.

+

static unsigned dm_verity_prefetch_cluster = DM_VERITY_DEFAULT_PREFETCH_SIZE;

@@ -855,7 +859,8 @@ 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 +909,15 @@ 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);

Hm, I don't see empty verity_verify_is_sig_opt_arg() or
verity_verify_sig_parse_opt_args() for when this config option is
disabled.

+ if (r) {
+ ti->error = "Could not parse the sig args";
+ return r;
+ }
+ continue;
}

ti->error = "Unrecognized verity feature request";
@@ -930,6 +944,7 @@ 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 +952,7 @@ 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 = NULL;

v = kzalloc(sizeof(struct dm_verity), GFP_KERNEL);
if (!v) {
@@ -956,7 +972,7 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
goto bad;
}

- if (argc < 10) {
+ if (argc < DM_VERITY_MANDATORY_ARGS) {
ti->error = "Not enough arguments";
r = -EINVAL;
goto bad;
@@ -1070,6 +1086,7 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
r = -EINVAL;
goto bad;
}
+ root_hash_digest_to_validate = argv[8];

Can we avoid magic numbers?


if (strcmp(argv[9], "-")) {
v->salt_size = strlen(argv[9]) / 2;
@@ -1087,19 +1104,28 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
}
}

- argv += 10;
- argc -= 10;
+ argv += DM_VERITY_MANDATORY_ARGS;
+ argc -= DM_VERITY_MANDATORY_ARGS;

/* Optional parameters */
if (argc) {
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*/
an
+ 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 +1191,13 @@ 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..491c84eb58ef
--- /dev/null
+++ b/drivers/md/dm-verity-verify-sig.c
@@ -0,0 +1,129 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2019 Microsoft Corporation.
+ *
+ * Author: Jaskaran Singh Khurana <jaskarankhurana@xxxxxxxxxxxxxxxxxxx>
+ *
+ * This file is released under the GPLv2.

We don't need to state this, that's why we have SPDX.

+ */
+#include <linux/device-mapper.h>
+#include <linux/verification.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));
+}
+EXPORT_SYMBOL_GPL(verity_verify_is_sig_opt_arg);

For whom are all these symbols exported for?

+
+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)
+{
+ const char *sig_size;
+ const char *sig_buf;
+ char dummy;
+ struct dm_target *ti = v->ti;
+ int r = 0;
+
+ if (*argc < DM_VERITY_ROOT_HASH_VERIFICATION_OPTS - 1) {
+ ti->error = DM_VERITY_VERIFY_ERR("sig values not specified");
+ return -EINVAL;
+ }
+
+ sig_size = dm_shift_arg(as);
+ (*argc)--;
+
+ if (strcasecmp(arg_name, DM_VERITY_ROOT_HASH_VERIFICATION_OPT_SIG) ||
+ sscanf(sig_size, "%u%c",
+ &sig_opts->sig_size, &dummy) != 1) {
+ ti->error = DM_VERITY_VERIFY_ERR("invalid signature size");
+ return -EINVAL;
+ }
+
+ sig_buf = dm_shift_arg(as);
+ (*argc)--;
+
+ if (strlen(sig_buf) != sig_opts->sig_size * 2) {
+ ti->error = DM_VERITY_VERIFY_ERR("sig buffer, size: mismatch");
+ return -EINVAL;
+ }
+
+ sig_opts->sig = kmalloc(sig_opts->sig_size, GFP_KERNEL);
+ if (!sig_opts->sig) {
+ r = -ENOMEM;
+ goto end;

Why not return directly here?

+ }
+
+ r = hex2bin(sig_opts->sig, sig_buf, sig_opts->sig_size);
+
+ if (r < 0) {
+ ti->error = DM_VERITY_VERIFY_ERR("invalid roothash sig buf");
+ r = -EINVAL;
+ goto end;

We'll get to 'end' anyway.

+ }
+
+end:
+ if (r < 0)
+ verity_verify_sig_opts_cleanup(sig_opts);
+ return r;
+}
+EXPORT_SYMBOL_GPL(verity_verify_sig_parse_opt_args);
+
+#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 r;
+
+ 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 -EINVAL;

I'm not sure -EINVAL is the right failure here.

+ else
+ return 0;
+ }
+
+ r = verify_pkcs7_signature(root_hash, root_hash_len, sig_data, sig_len,
+ NULL, VERIFYING_UNSPECIFIED_SIGNATURE, NULL,
+ NULL);
+ if (r < 0)
+ goto end;

This statement makes no sense :)

+
+end:
+ return r;
+}
+#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
+EXPORT_SYMBOL_GPL(verity_verify_root_hash);
+void verity_verify_sig_opts_cleanup(struct dm_verity_sig_opts *sig_opts)

Why doesn't this live somewhere inside verity_dtr()?

--
Thanks,
Sasha