Re: [PATCH v3 1/3] dm: Add verity helpers for LoadPin

Matthias Kaehlcke
2022-05-11T20:55:11Z

Alasdar/Mike, I'd be interested in your take on adding these functions
to verity/DM, to get an idea whether this series has a path forward to
landing upstream.

Thanks

Matthias

On Wed, May 04, 2022 at 12:54:17PM -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 <mka@chromium.org>
> ---
>
> Changes in v3:
> - none
>
> Changes in v2:
> - none
>
> drivers/md/Makefile | 6 +++
> drivers/md/dm-verity-loadpin.c | 80 +++++++++++++++++++++++++++++++
> drivers/md/dm-verity-target.c | 33 +++++++++++++
> drivers/md/dm-verity.h | 4 ++
> include/linux/dm-verity-loadpin.h | 27 +++++++++++
> 5 files changed, 150 insertions(+)
> 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..e12cd004d375 100644
> --- a/drivers/md/Makefile
> +++ b/drivers/md/Makefile
> @@ -100,6 +100,12 @@ ifeq ($(CONFIG_IMA),y)
> dm-mod-objs += dm-ima.o
> endif
>
> +ifeq ($(CONFIG_DM_VERITY),y)
> +ifeq ($(CONFIG_SECURITY_LOADPIN),y)
> +dm-mod-objs += dm-verity-loadpin.o
> +endif
> +endif
> +
> ifeq ($(CONFIG_DM_VERITY_FEC),y)
> dm-verity-objs += dm-verity-fec.o
> endif
> diff --git a/drivers/md/dm-verity-loadpin.c b/drivers/md/dm-verity-loadpin.c
> new file mode 100644
> index 000000000000..972ca93a2231
> --- /dev/null
> +++ b/drivers/md/dm-verity-loadpin.c
> @@ -0,0 +1,80 @@
> +// 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"
> +
> +static struct list_head *trusted_root_digests;
> +
> +/*
> + * Sets the root digests of verity devices which LoadPin considers as trusted.
> + *
> + * This function must only be called once.
> + */
> +void dm_verity_loadpin_set_trusted_root_digests(struct list_head *digests)
> +{
> + if (!trusted_root_digests)
> + trusted_root_digests = digests;
> + else
> + pr_warn("verity root digests trusted by LoadPin are already set!!!\n");
> +}
> +
> +static bool is_trusted_verity_target(struct dm_target *ti)
> +{
> + u8 *root_digest;
> + unsigned int digest_size;
> + struct 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, 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 a mapped device is a verity device that is trusted
> + * by LoadPin.
> + */
> +bool dm_verity_loadpin_is_md_trusted(struct mapped_device *md)
> +{
> + int srcu_idx;
> + struct dm_table *table;
> + unsigned int num_targets;
> + bool trusted = false;
> + int i;
> +
> + if (!trusted_root_digests || list_empty(trusted_root_digests))
> + return false;
> +
> + table = dm_get_live_table(md, &srcu_idx);
> + num_targets = dm_table_get_num_targets(table);
> + for (i = 0; i < num_targets; i++) {
> + struct dm_target *ti = dm_table_get_target(table, i);
> +
> + if (is_trusted_verity_target(ti)) {
> + trusted = true;
> + break;
> + }
> + }
> +
> + dm_put_live_table(md, srcu_idx);
> +
> + return trusted;
> +}
> diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> index 80133aae0db3..6f07b849fcb2 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"
>
> @@ -1310,6 +1311,38 @@ 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;
> +}
> +EXPORT_SYMBOL_GPL(dm_is_verity_target);
> +
> +/*
> + * 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;
> +}
> +EXPORT_SYMBOL_GPL(dm_verity_get_root_digest);
> +
> static struct target_type verity_target = {
> .name = "verity",
> .version = {1, 8, 0},
> 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..12a86911d05a
> --- /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 mapped_device;
> +
> +struct trusted_root_digest {
> + u8 *data;
> + unsigned int len;
> + struct list_head node;
> +};
> +
> +#if IS_ENABLED(CONFIG_SECURITY_LOADPIN) && IS_BUILTIN(CONFIG_DM_VERITY)
> +void dm_verity_loadpin_set_trusted_root_digests(struct list_head *digests);
> +bool dm_verity_loadpin_is_md_trusted(struct mapped_device *md);
> +#else
> +static inline void dm_verity_loadpin_set_trusted_root_digests(struct list_head *digests) {}
> +static inline bool dm_verity_loadpin_is_md_trusted(struct mapped_device *md)
> +{
> + return false;
> +}
> +#endif
> +
> +#endif /* __LINUX_DM_LOADPIN_H */
> --
> 2.36.0.464.gb9c8b46e94-goog
>