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

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Date:	Thu, 22 Mar 2012 10:53:04 -0400
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	Al Viro <viro@...IV.linux.org.uk>
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 Thu, 2012-03-22 at 14:22 +0000, Al Viro wrote:
> 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.

Al, thanks for replying!  I realize you are very prejudiced against IMA,
but have you taken a look at IMA-appraisal?  It probably would have been
better if we used another name in order to avoid this prejudice, but
unfortunately we didn't.  :(  IMA-appraisal is about enforcing file
integrity.

> 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.

I think I got that loud and clear; otherwise we wouldn't have come up
with deferring the __fput().  We have a very real problem here - writing
extended attributes requires taking the i_mutex.  I'm open to any
suggestions.

thanks,

Mimi

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