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

[Matthias Kaehlcke <mka@chromium.org>]

On Wed, May 18, 2022 at 03:52:21PM -0400, Mike Snitzer wrote:
> On Tue, May 17 2022 at  7:34P -0400,
> Matthias Kaehlcke <mka@chromium.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@chromium.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()

Agreed, passing a block_device is a better choice here, I'll
change it as suggested.