linux-kernel - Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively

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Date:   Mon, 2 Oct 2017 09:34:02 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-integrity@...r.kernel.org
Subject: Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been
 taken exclusively

On Sun, Oct 01, 2017 at 11:41:48AM -0700, Linus Torvalds wrote:
> On Sun, Oct 1, 2017 at 5:08 AM, Mimi Zohar <zohar@...ux.vnet.ibm.com> wrote:
> >
> > Right, re-introducing the iint->mutex and a new i_generation field in
> > the iint struct with a separate set of locks should work.  It will be
> > reset if the file metadata changes (eg. setxattr, chown, chmod).
> 
> Note that the "inner lock" could possibly be omitted if the
> invalidation can be just a single atomic instruction.
> 
> So particularly if invalidation could be just an atomic_inc() on the
> generation count, there might not need to be any inner lock at all.
> 
> You'd have to serialize the actual measurement with the "read
> generation count", but that should be as simple as just doing a
> smp_rmb() between the "read generation count" and "do measurement on
> file contents".

We already have a change counter on the inode, which is modified on
any data or metadata write (i_version) under filesystem locks.  The
i_version counter has well defined semantics - it's required by
NFSv4 to increment on any metadata or data change - so we should be
able to rely on it's behaviour to implement IMA as well. Filesystems
that support i_version are marked with [SB|MS]_I_VERSION in the
superblock (IS_I_VERSION(inode)) so it should be easy to tell if IMA
can be supported on a specific filesystem (btrfs, ext4, fuse and xfs
ATM).

The IMA code should be able to sample that at measurement time and
either fail or be retried if i_version changes during measurement.
We can then simply make the IMA xattr write conditional on the
i_version value being unchanged from the sample the IMA code passes
into the filesystem once the filesystem holds all the locks it needs
to write the xattr...

I note that IMA already grabs the i_version in
ima_collect_measurement(), so this shouldn't be too hard to do.
Perhaps we don't need any new locks or counters at all, maybe just
the ability to feed a version cookie to the set_xattr method?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com