linux-kernel - Re: [PATCH 3/3] ima: use fs method to read integrity data (updated patch description)

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Date:   Sun, 17 Sep 2017 12:15:35 -0400
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>
Cc:     LSM List <linux-security-module@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        linux-ima-devel@...ts.sourceforge.net,
        James Morris <jmorris@...ei.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Jan Kara <jack@...e.com>, "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Bob Peterson <rpeterso@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Dave Kleikamp <shaggy@...nel.org>,
        Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>,
        Mark Fasheh <mfasheh@...sity.com>,
        Joel Becker <jlbec@...lplan.org>,
        Richard Weinberger <richard@....at>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Hugh Dickins <hughd@...gle.com>, Chris Mason <clm@...com>
Subject: Re: [PATCH 3/3] ima: use fs method to read integrity data (updated
 patch description)

On Sun, 2017-09-17 at 08:28 -0700, Linus Torvalds wrote:
> On Sun, Sep 17, 2017 at 8:17 AM, Christoph Hellwig <hch@...radead.org> wrote:
> >
> > Only for direct I/O, and IMA and direct I/O don't work together.
> > From ima_collect_measurement:
> >
> >                 if (file->f_flags & O_DIRECT) {
> >                         audit_cause = "failed(directio)";
> >                         result = -EACCES;
> >                         goto out;
> >                 }
> 
> That's not the issue.
> 
> The issue is that somebody else can come in - using direct IO - at the
> same time as the first person is collecting measurements, and thus
> race with the collector.
> 
> So now the measurements are not trustworthy any more.

Unless I'm missing something, that would only be possible with an IMA
policy rule that permits direct IO (eg. permit_directio).  Otherwise
the direct IO is denied.

> > Well, that's exactly the point of the new ->integrity_read routine
> > I proposed and prototype.  The important thing is that it is called
> > with i_rwsem held because code mugh higher in the chain already
> > acquired it, but except for that it's entirely up to the file system.
> 
> .. and *my* point is that it's the wrong lock for actually checking
> integrity (it doesn't actually guarantee exclusion, even though in
> practice it's almost always the case), and so we're adding a nasty
> callback that in 99% of all cases is the same as the normal read, and
> we *could* have just added it with a RWF flag instead.
> 
> Is there some reason why integrity has to use that particular lock
> that is so inconvenient for the filesystems it wants to check?

Originally IMA had its own lock (iint->mutex), prior to IMA-appraisal
being upstreamed.  With a separate lock, the iint->mutex and i_rwsem
would be taken in reverse order in process_measurements() and in the
setxattr, chown, chmod syscalls.

I'm at the airport on my way back home.

Mimi