Date: Sun, 17 Sep 2017 08:17:57 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>,
        LSM List <linux-security-module@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>, linux-ima-devel@lists.sourceforge.net,
        Christoph Hellwig <hch@infradead.org>,
        James Morris <jmorris@namei.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Matthew Garrett <mjg59@srcf.ucam.org>, Jan Kara <jack@suse.com>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jaegeuk Kim <jaegeuk@kernel.org>, Chao Yu <yuchao0@huawei.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Bob Peterson <rpeterso@redhat.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Dave Kleikamp <shaggy@kernel.org>,
        Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>,
        Mark Fasheh <mfasheh@versity.com>, Joel Becker <jlbec@evilplan.org>,
        Richard Weinberger <richard@nod.at>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Hugh Dickins <hughd@google.com>, Chris Mason <clm@fb.com>
Subject: Re: [PATCH 3/3] ima: use fs method to read integrity data (updated
 patch description)
Message-ID: <20170917151757.GA14262@infradead.org>
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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.