linux-kernel - Re: [PATCH v3 10/12] ima: defer calling _

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Message-ID: <20120322142212.GV6589@ZenIV.linux.org.uk>
Date:	Thu, 22 Mar 2012 14:22:12 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	David Safford <safford@...ux.vnet.ibm.com>,
	Dmitry Kasatkin <dmitry.kasatkin@...el.com>,
	Matt Helsley <matt.helsley@...il.com>,
	Mimi Zohar <zohar@...ibm.com>
Subject: Re: [PATCH v3 10/12] ima: defer calling __fput()

On Wed, Mar 21, 2012 at 02:54:15PM -0400, Mimi Zohar wrote:
> If a file is closed before it is munmapped, __fput() is called with
> the mmap_sem taken.  With IMA-appraisal enabled, this results in an
> mmap_sem/i_mutex lockdep.  ima_defer_fput() increments the f_count
> to defer __fput() being called until after the mmap_sem is released.

NAK.  It's far too heavy-weight for what's a very common path.  Worse,
it causes different locking conditions on IMA and non-IMA kernels and makes
bugs harder to spot.  Rejected with extreme prejudice; please, fix your
locking instead.

BTW, you've missed several other places in mm/* doing fput(), so it wouldn't
be enough to paper over your problem anyway.

Final fput() *can* happen under mmap_sem.  Period.
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