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 08:17:57 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.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,
        Christoph Hellwig <hch@...radead.org>,
        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 Sat, Sep 16, 2017 at 11:20:47AM -0700, Linus Torvalds wrote:
> Sure, generic_file_write_iter() does take that lock exclusively, but
> not everybody uses generic_file_write_iter() at all for writing.
> 
> For example, xfs still uses that i_rwsem, but for block-aligned writes
> it will only get it shared. And I'm not convinced some other
> filesystem might not end up using some other lock entirely.

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;
		}

(and yes, it should be checking for IOCB_DIRECT to avoid racy
f_flags manipulations, but that's another issue)

> The filesystem can do its own locking, and I'm starting to think that
> it would be better to just pass this "this is an integrity read" down
> to the filesystem, and expect the filesystem to do the locking based
> on that.

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.