[PATCH] fs: ubifs: Add i_version support

[Sascha Hauer]

On Tue, Sep 12, 2017 at 02:38:02PM +0200, Richard Weinberger wrote:
> Sascha,
> 
> Am Dienstag, 12. September 2017, 12:39:00 CEST schrieb Sascha Hauer:
> > This adds i_version support to UBIFS. The inodes i_version is used by
> > IMA to detect changes to an inode and thus necessary to support IMA on
> > UBIFS. The i_version is stored in the previously unused space in the
> > UBIFS inode struct. Unlike in ext4 i_version support is unconditionally
> > enabled in UBIFS as I saw no reason to make it optional.
> 
> But we need a new UBIFS feature flag to indicate that this filesystem has 
> valid i_version fields.

I assume you mean a new UBIFS_FLG_*, right?

Who should set this flag? The Kernel once the filesystem has been
mounted with iversion support enabled? This would mean we indeed need a
iversion mount flag to give the user a chance to continue without
iversion support and keep the filesystem compatible with older kernels.

> 
> > Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>
> > ---
> >  fs/ubifs/dir.c         | 30 +++++++++++++++++++-----------
> >  fs/ubifs/file.c        |  5 +++++
> >  fs/ubifs/journal.c     |  3 ++-
> >  fs/ubifs/super.c       |  2 ++
> >  fs/ubifs/ubifs-media.h |  3 ++-
> >  5 files changed, 30 insertions(+), 13 deletions(-)
> > 
> > I did this patch exclusively to support IMA on UBIFS. IMA uses the inode's
> > i_version field to detect changes on inodes. A proper i_version support
> > needs to make the i_version persistent on disk, although IMA itself doesn't
> > need a persistent i_version. Last time an earlier version of this patch
> > 
> > was sent by Oleksij Rempel Richard said:
> > > What about making i_version persistent?
> > > We still have some empty fields in UBIFS' inode data structure.
> > > But first we have to be very sure that we need it.
> > 
> > This patch exactly implements this suggestion, leaving the question if we
> > really need it. I added the IMA maintainers to Cc in the hope that Mimi or
> > Dmitry can give a good reason why there's no alternative to i_version for
> > IMA.
> 
> Yes, it would be good to know more about the user, IMA. Does IMA store the 
> version somewhere?

No. IMA solely uses i_version to detect if an inode has been changed since
the last time it has seen this inode.
IMA measures all files it hasn't seen before initially and stores the i_version
in a struct integrity_iint_cache *. When an inode is written to next time IMA
checks if the cached i_version still matches the inode's i_version and if it
doesn't, it re-measures the inode. All this is purely runtime.

> Are there requirements on ordering? i.e. What if UBIFS faces a power-cut
> and the UBIFS i_version is behind IMA's version.

Since IMA doesn't store the i_version anywhere this won't happen.

> Maybe we have to teach UBIFS to update an inode less lazy that it currently 
> does...

No, I don't think so.

Sascha

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