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

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Date:   Fri, 13 May 2022 12:29:29 -0400
From:   Mike Snitzer <snitzer@...hat.com>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        Song Liu <song@...nel.org>,
        Douglas Anderson <dianders@...omium.org>,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v3 1/3] dm: Add verity helpers for LoadPin

On Thu, May 12 2022 at  4:44P -0400,
Matthias Kaehlcke <mka@...omium.org> wrote:

> On Thu, May 12, 2022 at 01:19:12PM -0400, Mike Snitzer wrote:
> > On Wed, May 11 2022 at  4:54P -0400,
> > Matthias Kaehlcke <mka@...omium.org> wrote:
> > 
> > > 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.
> > 
> > I'll be reviewing your patchset now. Comments inlined below.
> > 
> > > 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@...omium.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
> > > > +
> > 
> > Why are you extending dm-mod-objs?  Why not dm-verity-objs?
> 
> Sorry, I missed to address this comment in my earlier reply.
> 
> I don't recall why I chose dm-mod-objs initially, agreed that
> dm-verity-objs seems a better fit.

Yes, should be fixed even though the 3rd patch removes this change.

BTW, looking at the 2nd patch's loadpin_is_fs_trusted().  Seems to me
you'd do well to pass a 'struct block_device *' to a DM helper rather
than force security/loadpin/loadpin.c to mess around with DM device
refcounting, etc.