linux-kernel - Re: [PULL REQUEST] : ima-appraisal patches

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Date:	Wed, 18 Apr 2012 16:56:05 -0400
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	James Morris <jmorris@...ei.org>,
	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>
Subject: Re: [PULL REQUEST] : ima-appraisal patches

On Wed, 2012-04-18 at 19:39 +0100, Al Viro wrote:
> On Wed, Apr 18, 2012 at 02:07:52PM -0400, Mimi Zohar wrote:
> 
> > >From the 'ima: defer calling __fput()' patch description:
> > 
> > ima_file_free(), which is called on __fput(), updates the file data
> > hash stored as an extended attribute to reflect file changes.  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 the __fput() being called until after the mmap_sem is released.
> > 
> > The number of __fput() calls needing to be deferred is minimal.  Only
> > those files in policy, that were closed prior to the munmap and were
> > mmapped write, need to defer the __fput().
> > 
> > With this patch, on a clean F16 install, from boot to login, only
> > 5 out of ~100,000 mmap_sem held fput() calls were deferred.
> 
> Assuming that it's commit 3cee52ffe8ca925bb1e96f804daa87f7e2e34e46
> Author: Mimi Zohar <zohar@...ux.vnet.ibm.com>
> Date:   Fri Feb 24 06:23:12 2012 -0500
> 
>     ima: defer calling __fput()
> in your tree, the NAK still stands.  For starters, but you are creating a
> different locking rules for IMA-enabled builds and for everything else.
> Moreover, this deferral is done only for files opened for write; the
> rules are convoluted as hell *and* inviting abuses.  

Yes, that is the updated version.  For performance, we limited deferring
the __fput() to only those files that could possibly change - open for
write, were closed before being munmapped, and that IMA-appraisal
maintains a file data hash as an xattr.  If the main concern is
different locking rules when IMA is enabled, then we could remove the
IMA criteria and rename ima_defer_fput() to something more generic.

As for "*and* inviting abuses", I'm not sure what you mean.

> NAKed at least until you come up with formal proof that there's no other
> lock where fput() would be possible and ->i_mutex was not allowed.  This
> is not a way to go; that kind of kludges leads to locking code that is
> impossible to reason about.

On __fput(), we need to update the security.ima xattr with a hash of the
file data.  The original thread discussion suggested changing the xattr
locking.  The filesystems seem to do their own xattr locking, but in
fs/xattr.c the i_mutex is taken before accessing the inode
setxattr/removexattr ops. 

hm, lockdep isn't complaining about anything else.  Not sure if that
qualifies as formal proof. 

> PS: BTW, what the hell is "fput already scheduled" codepath about?
> Why is it pr_info() and not an outright BUG_ON()?

I'll fix this.

thanks,

Mimi

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