linux-kernel - Re: [PATCH] ima: prevent dead lock when a file is opened for direct io

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Date:	Tue, 26 Feb 2013 16:20:24 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:	linux-security-module <linux-security-module@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ima: prevent dead lock when a file is opened for direct
 io

On Wed, Feb 20, 2013 at 04:27:51PM -0500, Mimi Zohar wrote:
> Hi Al,
> 
> Are there any negative repercussions to temporarily removing the
> o_direct flag in order to calculate the file hash?
> 
> thanks,
> 
> Mimi
> -----
> 
> Files are measured or appraised based on the IMA policy.  When a file
> in policy is opened for read with the O_DIRECT flag set, a deadlock
> occurs due to do_blockdev_direct_IO() taking i_mutex before calling
> filemap_write_and_wait_range(). The i_mutex was previously taken in
> process_measurement().  This patch temporarily removes the O_DIRECT
> flag in order to calculate the hash and restores it once completed.

	Why does process_measurement() hold ->i_mutex across that?
It really sounds like "we kinda hope no ->read() will take ->i_mutex,
oops, at least one case does, umm... let's kludge around a bit and
hope no other case shows up".  

Locking rules should be documented and they should make sense.  You are
introducing a new one and it's really convoluted - "no ->read() instance
for a regular file shall take ->i_mutex unless it's an O_DIRECT open".
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