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

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Date:	Fri, 23 Mar 2012 10:55:18 -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 15:09 +0000, Al Viro wrote:
> On Thu, Mar 22, 2012 at 10:53:04AM -0400, Mimi Zohar wrote:
> > > 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.
> 
> Don't do it, then?  If you _must_ write to xattr on final fput, I'd suggest
> starting to figure out if xattr needs its protection to be ->i_mutex - it
> might make sense to introduce a separate mutex for xattr crap.  Or not - I'm
> not familiar enough with the guts of xattr handling in individual filesystems
> to tell if that would work (e.g. if it would need unpleasant changes to
> ->setattr() instances)...

After looking into this, the individual filesystems do their own xattr
locking.  The i_mutex, however, is currently required to access
inode->i_op->setxattr() (and the isec).  In addition, IMA-appraisal
requires the i_mutex in order to calculate the file hash.

Calling ima_file_free() after the mmap_sem is released, as opposed to
queueing the __fput(), won't work, as the file needs to be open in order
to calculate the file hash.

Calling ima_file_free() before taking the mmap_sem, could work, but at
that point we don't have access to the file handle. Do you see any other
options?

thanks,

Mimi

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