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

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Message-Id: <1506643967.5691.46.camel@linux.vnet.ibm.com>
Date:   Thu, 28 Sep 2017 20:12:47 -0400
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Chinner <david@...morbit.com>
Cc:     LSM List <linux-security-module@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-integrity@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, "Theodore Ts'o" <tytso@....edu>
Subject: Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been
 taken exclusively

On Thu, 2017-09-28 at 16:39 -0700, Linus Torvalds wrote:
> On Thu, Sep 28, 2017 at 3:02 PM, Dave Chinner <david@...morbit.com> wrote:
> > On Thu, Sep 28, 2017 at 08:39:33AM -0400, Mimi Zohar wrote:
> >> Don't attempt to take the i_rwsem, if it has already been taken
> >> exclusively.
> >>
> >> Signed-off-by:  Mimi Zohar <zohar@...ux.vnet.ibm.com>
> >
> > That's bloody awful.
> >
> > The locking in filesystem IO paths is already complex enough without
> > adding a new IO path semantic that says "caller has already locked
> > the i_rwsem in some order and some dependencies that we have no idea
> > about".
> 
> I do have to admit that I never got a satisfactory answer on why IMA
> doesn't just use its own private per-inode lock for this all.
> 
> It isn't using the i_rwsem for file consistency reasons anyway, so it
> seems to be purely about serializing the actual signature generation
> with the xattr writing, but since IMA does those both, why isn't IMA
> just using its own lock (not the filesystem lock) to do that?

Originally IMA did define it's own lock, prior to IMA-appraisal.  IMA-
appraisal introduced writing the file hash as an xattr, which required
taking the i_mutex.  process_measurement() and ima_file_free() took
the iint->mutex first and then the i_mutex, while setxattr, chmod and
chown took the locks in reverse order.  To resolve the potential
deadlock, the iint->mutex was eliminated.

Mimi