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 08:28:40 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        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, 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.

> 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?

                 Linus