[v6,1/3] dm: Add verity helpers for LoadPin

Commit Message

Matthias Kaehlcke May 23, 2022, 9:13 p.m. UTC

LoadPin limits loading of kernel modules, firmware and certain
other files to a 'pinned' file system (typically a read-only
rootfs). To provide more flexibility LoadPin is being extended
to also allow loading these files from trusted dm-verity
devices. For that purpose LoadPin can be provided with a list
of verity root digests that it should consider as trusted.

Add a bunch of helpers to allow LoadPin to check whether a DM
device is a trusted verity device. The new functions broadly
fall in two categories: those that need access to verity
internals (like the root digest), and the 'glue' between
LoadPin and verity. The new file dm-verity-loadpin.c contains
the glue functions.

Signed-off-by: Matthias Kaehlcke <[e-mail]>
Acked-by: Kees Cook <[e-mail]>
---

Changes in v6:
- s/loadpin_trusted_verity_root_digests/dm_verity_loadpin_trusted_root_digests/
- s/trusted_root_digest/dm_verity_loadpin_trusted_root_digest/
- removed unnecessary symbol exports

Changes in v5:
- changed dm_verity_loadpin_is_sb_trusted() to
  dm_verity_loadpin_is_bdev_trusted()
- bumped version number to 1.8.1
- deleted bad semicolon in declaration of stub for
  dm_verity_loadpin_is_bdev_trusted()
- added 'Acked-by' tag from Kees

Changes in v4:
- a trusted verity device must have a single target of
  type 'verity'
- share list of verity digests with loadpin, deleted
  dm_verity_loadpin_set_trusted_root_digests()
- dm_verity_loadpin_is_md_trusted() is now dm_verity_loadpin_is_sb_trusted(),
  it receives a super_block instead of mapped_device. Updated kernel doc.
- changed struct trusted_root_digest to have an unsized
  u8 array instead of a pointer
- extend 'dm-verity-objs' instead of 'dm-mod-objs'

Changes in v3:
- none

Changes in v2:
- none

 drivers/md/Makefile               |  6 +++
 drivers/md/dm-verity-loadpin.c    | 74 +++++++++++++++++++++++++++++++
 drivers/md/dm-verity-target.c     | 33 +++++++++++++-
 drivers/md/dm-verity.h            |  4 ++
 include/linux/dm-verity-loadpin.h | 27 +++++++++++
 5 files changed, 143 insertions(+), 1 deletion(-)
 create mode 100644 drivers/md/dm-verity-loadpin.c
 create mode 100644 include/linux/dm-verity-loadpin.h

Comments

Matthias Kaehlcke June 13, 2022, 5:15 p.m. UTC | #1

Mike, does this look good to you or are any further changes needed
to get this landed?

Thanks

Matthias

On Mon, May 23, 2022 at 02:13:58PM -0700, Matthias Kaehlcke wrote:
> LoadPin limits loading of kernel modules, firmware and certain
> other files to a 'pinned' file system (typically a read-only
> rootfs). To provide more flexibility LoadPin is being extended
> to also allow loading these files from trusted dm-verity
> devices. For that purpose LoadPin can be provided with a list
> of verity root digests that it should consider as trusted.
> 
> Add a bunch of helpers to allow LoadPin to check whether a DM
> device is a trusted verity device. The new functions broadly
> fall in two categories: those that need access to verity
> internals (like the root digest), and the 'glue' between
> LoadPin and verity. The new file dm-verity-loadpin.c contains
> the glue functions.
> 
> Signed-off-by: Matthias Kaehlcke <[e-mail]>
> Acked-by: Kees Cook <[e-mail]>
> ---
> 
> Changes in v6:
> - s/loadpin_trusted_verity_root_digests/dm_verity_loadpin_trusted_root_digests/
> - s/trusted_root_digest/dm_verity_loadpin_trusted_root_digest/
> - removed unnecessary symbol exports
> 
> Changes in v5:
> - changed dm_verity_loadpin_is_sb_trusted() to
>   dm_verity_loadpin_is_bdev_trusted()
> - bumped version number to 1.8.1
> - deleted bad semicolon in declaration of stub for
>   dm_verity_loadpin_is_bdev_trusted()
> - added 'Acked-by' tag from Kees
> 
> Changes in v4:
> - a trusted verity device must have a single target of
>   type 'verity'
> - share list of verity digests with loadpin, deleted
>   dm_verity_loadpin_set_trusted_root_digests()
> - dm_verity_loadpin_is_md_trusted() is now dm_verity_loadpin_is_sb_trusted(),
>   it receives a super_block instead of mapped_device. Updated kernel doc.
> - changed struct trusted_root_digest to have an unsized
>   u8 array instead of a pointer
> - extend 'dm-verity-objs' instead of 'dm-mod-objs'
> 
> Changes in v3:
> - none
> 
> Changes in v2:
> - none
> 
>  drivers/md/Makefile               |  6 +++
>  drivers/md/dm-verity-loadpin.c    | 74 +++++++++++++++++++++++++++++++
>  drivers/md/dm-verity-target.c     | 33 +++++++++++++-
>  drivers/md/dm-verity.h            |  4 ++
>  include/linux/dm-verity-loadpin.h | 27 +++++++++++
>  5 files changed, 143 insertions(+), 1 deletion(-)
>  create mode 100644 drivers/md/dm-verity-loadpin.c
>  create mode 100644 include/linux/dm-verity-loadpin.h
> 
> diff --git a/drivers/md/Makefile b/drivers/md/Makefile
> index 0454b0885b01..71771901c823 100644
> --- a/drivers/md/Makefile
> +++ b/drivers/md/Makefile
> @@ -108,6 +108,12 @@ ifeq ($(CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG),y)
>  dm-verity-objs			+= dm-verity-verify-sig.o
>  endif
>  
> +ifeq ($(CONFIG_DM_VERITY),y)
> +ifeq ($(CONFIG_SECURITY_LOADPIN),y)
> +dm-verity-objs			+= dm-verity-loadpin.o
> +endif
> +endif
> +
>  ifeq ($(CONFIG_DM_AUDIT),y)
>  dm-mod-objs			+= dm-audit.o
>  endif
> diff --git a/drivers/md/dm-verity-loadpin.c b/drivers/md/dm-verity-loadpin.c
> new file mode 100644
> index 000000000000..10c18bc1652c
> --- /dev/null
> +++ b/drivers/md/dm-verity-loadpin.c
> @@ -0,0 +1,74 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +#include <linux/list.h>
> +#include <linux/kernel.h>
> +#include <linux/dm-verity-loadpin.h>
> +
> +#include "dm.h"
> +#include "dm-verity.h"
> +
> +#define DM_MSG_PREFIX	"verity-loadpin"
> +
> +LIST_HEAD(dm_verity_loadpin_trusted_root_digests);
> +
> +static bool is_trusted_verity_target(struct dm_target *ti)
> +{
> +	u8 *root_digest;
> +	unsigned int digest_size;
> +	struct dm_verity_loadpin_trusted_root_digest *trd;
> +	bool trusted = false;
> +
> +	if (!dm_is_verity_target(ti))
> +		return false;
> +
> +	if (dm_verity_get_root_digest(ti, &root_digest, &digest_size))
> +		return false;
> +
> +	list_for_each_entry(trd, &dm_verity_loadpin_trusted_root_digests, node) {
> +		if ((trd->len == digest_size) &&
> +		    !memcmp(trd->data, root_digest, digest_size)) {
> +			trusted = true;
> +			break;
> +		}
> +	}
> +
> +	kfree(root_digest);
> +
> +	return trusted;
> +}
> +
> +/*
> + * Determines whether the file system of a superblock is located on
> + * a verity device that is trusted by LoadPin.
> + */
> +bool dm_verity_loadpin_is_bdev_trusted(struct block_device *bdev)
> +{
> +	struct mapped_device *md;
> +	struct dm_table *table;
> +	struct dm_target *ti;
> +	int srcu_idx;
> +	bool trusted = false;
> +
> +	if (list_empty(&dm_verity_loadpin_trusted_root_digests))
> +		return false;
> +
> +	md = dm_get_md(bdev->bd_dev);
> +	if (!md)
> +		return false;
> +
> +	table = dm_get_live_table(md, &srcu_idx);
> +
> +	if (dm_table_get_num_targets(table) != 1)
> +		goto out;
> +
> +	ti = dm_table_get_target(table, 0);
> +
> +	if (is_trusted_verity_target(ti))
> +		trusted = true;
> +
> +out:
> +	dm_put_live_table(md, srcu_idx);
> +	dm_put(md);
> +
> +	return trusted;
> +}
> diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> index 80133aae0db3..c5395e93525a 100644
> --- a/drivers/md/dm-verity-target.c
> +++ b/drivers/md/dm-verity-target.c
> @@ -19,6 +19,7 @@
>  #include <linux/module.h>
>  #include <linux/reboot.h>
>  #include <linux/scatterlist.h>
> +#include <linux/string.h>
>  
>  #define DM_MSG_PREFIX			"verity"
>  
> @@ -1310,9 +1311,39 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
>  	return r;
>  }
>  
> +/*
> + * Check whether a DM target is a verity target.
> + */
> +bool dm_is_verity_target(struct dm_target *ti)
> +{
> +	return ti->type->module == THIS_MODULE;
> +}
> +
> +/*
> + * Get the root digest of a verity target.
> + *
> + * Returns a copy of the root digest, the caller is responsible for
> + * freeing the memory of the digest.
> + */
> +int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest, unsigned int *digest_size)
> +{
> +	struct dm_verity *v = ti->private;
> +
> +	if (!dm_is_verity_target(ti))
> +		return -EINVAL;
> +
> +	*root_digest = kmemdup(v->root_digest, v->digest_size, GFP_KERNEL);
> +	if (*root_digest == NULL)
> +		return -ENOMEM;
> +
> +	*digest_size = v->digest_size;
> +
> +	return 0;
> +}
> +
>  static struct target_type verity_target = {
>  	.name		= "verity",
> -	.version	= {1, 8, 0},
> +	.version	= {1, 8, 1},
>  	.module		= THIS_MODULE,
>  	.ctr		= verity_ctr,
>  	.dtr		= verity_dtr,
> diff --git a/drivers/md/dm-verity.h b/drivers/md/dm-verity.h
> index 4e769d13473a..c832cc3e3d24 100644
> --- a/drivers/md/dm-verity.h
> +++ b/drivers/md/dm-verity.h
> @@ -129,4 +129,8 @@ extern int verity_hash(struct dm_verity *v, struct ahash_request *req,
>  extern int verity_hash_for_block(struct dm_verity *v, struct dm_verity_io *io,
>  				 sector_t block, u8 *digest, bool *is_zero);
>  
> +extern bool dm_is_verity_target(struct dm_target *ti);
> +extern int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest,
> +				     unsigned int *digest_size);
> +
>  #endif /* DM_VERITY_H */
> diff --git a/include/linux/dm-verity-loadpin.h b/include/linux/dm-verity-loadpin.h
> new file mode 100644
> index 000000000000..fb695ecaa5d5
> --- /dev/null
> +++ b/include/linux/dm-verity-loadpin.h
> @@ -0,0 +1,27 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef __LINUX_DM_VERITY_LOADPIN_H
> +#define __LINUX_DM_VERITY_LOADPIN_H
> +
> +#include <linux/list.h>
> +
> +struct block_device;
> +
> +extern struct list_head dm_verity_loadpin_trusted_root_digests;
> +
> +struct dm_verity_loadpin_trusted_root_digest {
> +	struct list_head node;
> +	unsigned int len;
> +	u8 data[];
> +};
> +
> +#if IS_ENABLED(CONFIG_SECURITY_LOADPIN) && IS_BUILTIN(CONFIG_DM_VERITY)
> +bool dm_verity_loadpin_is_bdev_trusted(struct block_device *bdev);
> +#else
> +static inline bool dm_verity_loadpin_is_bdev_trusted(struct block_device *bdev)
> +{
> +	return false;
> +}
> +#endif
> +
> +#endif /* __LINUX_DM_VERITY_LOADPIN_H */
> -- 
> 2.36.1.124.g0e6072fb45-goog
> 

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Patch

diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index 0454b0885b01..71771901c823 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -108,6 +108,12 @@  ifeq ($(CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG),y)
 dm-verity-objs			+= dm-verity-verify-sig.o
 endif
 
+ifeq ($(CONFIG_DM_VERITY),y)
+ifeq ($(CONFIG_SECURITY_LOADPIN),y)
+dm-verity-objs			+= dm-verity-loadpin.o
+endif
+endif
+
 ifeq ($(CONFIG_DM_AUDIT),y)
 dm-mod-objs			+= dm-audit.o
 endif
diff --git a/drivers/md/dm-verity-loadpin.c b/drivers/md/dm-verity-loadpin.c
new file mode 100644
index 000000000000..10c18bc1652c
--- /dev/null
+++ b/drivers/md/dm-verity-loadpin.c
@@ -0,0 +1,74 @@ 
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <linux/list.h>
+#include <linux/kernel.h>
+#include <linux/dm-verity-loadpin.h>
+
+#include "dm.h"
+#include "dm-verity.h"
+
+#define DM_MSG_PREFIX	"verity-loadpin"
+
+LIST_HEAD(dm_verity_loadpin_trusted_root_digests);
+
+static bool is_trusted_verity_target(struct dm_target *ti)
+{
+	u8 *root_digest;
+	unsigned int digest_size;
+	struct dm_verity_loadpin_trusted_root_digest *trd;
+	bool trusted = false;
+
+	if (!dm_is_verity_target(ti))
+		return false;
+
+	if (dm_verity_get_root_digest(ti, &root_digest, &digest_size))
+		return false;
+
+	list_for_each_entry(trd, &dm_verity_loadpin_trusted_root_digests, node) {
+		if ((trd->len == digest_size) &&
+		    !memcmp(trd->data, root_digest, digest_size)) {
+			trusted = true;
+			break;
+		}
+	}
+
+	kfree(root_digest);
+
+	return trusted;
+}
+
+/*
+ * Determines whether the file system of a superblock is located on
+ * a verity device that is trusted by LoadPin.
+ */
+bool dm_verity_loadpin_is_bdev_trusted(struct block_device *bdev)
+{
+	struct mapped_device *md;
+	struct dm_table *table;
+	struct dm_target *ti;
+	int srcu_idx;
+	bool trusted = false;
+
+	if (list_empty(&dm_verity_loadpin_trusted_root_digests))
+		return false;
+
+	md = dm_get_md(bdev->bd_dev);
+	if (!md)
+		return false;
+
+	table = dm_get_live_table(md, &srcu_idx);
+
+	if (dm_table_get_num_targets(table) != 1)
+		goto out;
+
+	ti = dm_table_get_target(table, 0);
+
+	if (is_trusted_verity_target(ti))
+		trusted = true;
+
+out:
+	dm_put_live_table(md, srcu_idx);
+	dm_put(md);
+
+	return trusted;
+}
diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index 80133aae0db3..c5395e93525a 100644
--- a/drivers/md/dm-verity-target.c
+++ b/drivers/md/dm-verity-target.c
@@ -19,6 +19,7 @@ 
 #include <linux/module.h>
 #include <linux/reboot.h>
 #include <linux/scatterlist.h>
+#include <linux/string.h>
 
 #define DM_MSG_PREFIX			"verity"
 
@@ -1310,9 +1311,39 @@  static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
 	return r;
 }
 
+/*
+ * Check whether a DM target is a verity target.
+ */
+bool dm_is_verity_target(struct dm_target *ti)
+{
+	return ti->type->module == THIS_MODULE;
+}
+
+/*
+ * Get the root digest of a verity target.
+ *
+ * Returns a copy of the root digest, the caller is responsible for
+ * freeing the memory of the digest.
+ */
+int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest, unsigned int *digest_size)
+{
+	struct dm_verity *v = ti->private;
+
+	if (!dm_is_verity_target(ti))
+		return -EINVAL;
+
+	*root_digest = kmemdup(v->root_digest, v->digest_size, GFP_KERNEL);
+	if (*root_digest == NULL)
+		return -ENOMEM;
+
+	*digest_size = v->digest_size;
+
+	return 0;
+}
+
 static struct target_type verity_target = {
 	.name		= "verity",
-	.version	= {1, 8, 0},
+	.version	= {1, 8, 1},
 	.module		= THIS_MODULE,
 	.ctr		= verity_ctr,
 	.dtr		= verity_dtr,
diff --git a/drivers/md/dm-verity.h b/drivers/md/dm-verity.h
index 4e769d13473a..c832cc3e3d24 100644
--- a/drivers/md/dm-verity.h
+++ b/drivers/md/dm-verity.h
@@ -129,4 +129,8 @@  extern int verity_hash(struct dm_verity *v, struct ahash_request *req,
 extern int verity_hash_for_block(struct dm_verity *v, struct dm_verity_io *io,
 				 sector_t block, u8 *digest, bool *is_zero);
 
+extern bool dm_is_verity_target(struct dm_target *ti);
+extern int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest,
+				     unsigned int *digest_size);
+
 #endif /* DM_VERITY_H */
diff --git a/include/linux/dm-verity-loadpin.h b/include/linux/dm-verity-loadpin.h
new file mode 100644
index 000000000000..fb695ecaa5d5
--- /dev/null
+++ b/include/linux/dm-verity-loadpin.h
@@ -0,0 +1,27 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef __LINUX_DM_VERITY_LOADPIN_H
+#define __LINUX_DM_VERITY_LOADPIN_H
+
+#include <linux/list.h>
+
+struct block_device;
+
+extern struct list_head dm_verity_loadpin_trusted_root_digests;
+
+struct dm_verity_loadpin_trusted_root_digest {
+	struct list_head node;
+	unsigned int len;
+	u8 data[];
+};
+
+#if IS_ENABLED(CONFIG_SECURITY_LOADPIN) && IS_BUILTIN(CONFIG_DM_VERITY)
+bool dm_verity_loadpin_is_bdev_trusted(struct block_device *bdev);
+#else
+static inline bool dm_verity_loadpin_is_bdev_trusted(struct block_device *bdev)
+{
+	return false;
+}
+#endif
+
+#endif /* __LINUX_DM_VERITY_LOADPIN_H */