linux-kernel - Re: [PATCH v4 1/3] dm: Add verity helpers for LoadPin

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Message-ID: <YoVOdYHJd/LbYIaE@redhat.com>
Date:   Wed, 18 May 2022 15:52:21 -0400
From:   Mike Snitzer <snitzer@...nel.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Alasdair Kergon <agk@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>, dm-devel@...hat.com,
        Douglas Anderson <dianders@...omium.org>,
        linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        Song Liu <song@...nel.org>,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v4 1/3] dm: Add verity helpers for LoadPin

On Tue, May 17 2022 at  7:34P -0400,
Matthias Kaehlcke <mka@...omium.org> 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@...omium.org>
> ---
> 
> 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, 144 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..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..3226fbe4a1fe
> --- /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(loadpin_trusted_verity_root_digests);
> +
> +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, &loadpin_trusted_verity_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_sb_trusted(struct super_block *sb)
> +{
> +	struct mapped_device *md;
> +	struct dm_table *table;
> +	struct dm_target *ti;
> +	int srcu_idx;
> +	bool trusted = false;
> +
> +	if (list_empty(&loadpin_trusted_verity_root_digests))
> +		return false;
> +
> +	md = dm_get_md(sb->s_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;
> +}

Not seeing why passing a super_block a block layer interface was
chosen.

Please pass the super_block's block_device and rename to
dm_verity_loadpin_is_bdev_trusted()

Thanks,
Mike