linux-kernel - Re: [PATCH v5 05/21] ima: move ima_file

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Message-ID: <20110520152552.GB31061@mail.hallyn.com>
Date:	Fri, 20 May 2011 10:25:52 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	James Morris <jmorris@...ei.org>,
	David Safford <safford@...son.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <greg@...ah.com>,
	Dmitry Kasatkin <dmitry.kasatkin@...ia.com>,
	Mimi Zohar <zohar@...ibm.com>
Subject: Re: [PATCH v5 05/21] ima: move ima_file_free before releasing the
 file

Quoting Mimi Zohar (zohar@...ux.vnet.ibm.com):
> On Fri, 2011-05-20 at 08:40 -0500, Serge E. Hallyn wrote:
> > Quoting Mimi Zohar (zohar@...ux.vnet.ibm.com):
> > > On Thu, 2011-05-19 at 17:06 -0500, Serge E. Hallyn wrote:
> > > > Quoting Mimi Zohar (zohar@...ux.vnet.ibm.com):
> > > > > Integrity appraisal measures files on file_free and stores the file
> > > > > measurement as an xattr.  Measure the file before releasing it.
> > > > 
> > > > Can you put a bit more in the commit msg about why?  What's magic
> > > > about the fs-specific release function?
> > > 
> > > ima_file_free(), called on __fput(), currently flags files that have
> > > changed, so that the file is re-measured.  However, for appraising a
> > > files's integrity, flagging the file is not enough.  The file's
> > > security.ima xattr must be updated to reflect any changes.  This patch
> > > moves releasing the file to after calculating the new file hash.
> > 
> > Sorry if I'm being dense, but I still don't understand (even though
> > apparently I used to :)  why the fs release is magic here.  The
> > dropping of the writelock comes later, so no file will be able to
> > open the file for execute or write until that point, meaning that
> > won't be happening without a re-measure with or without this patch.
> > 
> > So you must be thinking something about general opens(), but I
> > don't believe that file_operations->release makes a difference to
> > another tasks's ability to open the file, nor to the writing out
> > of changed contents.
> > 
> > security_file_free() doesn't appear to be hooked by ima or evm,
> > and if a security module changes its security.X xattr you'll
> > end up re-measuring the xattrs anyway.
> > 
> > So I'm still missing something, sorry  :)
> > 
> > -serge
> 
> No, my mistake. IMA-appraisal, not EVM, needs to be able to read the
> file in order to re-calculate the hash and update the 'security.ima'
> extended attribute.  Will move this patch to the IMA patchset.

Gotcha, thanks.  Please do keep my ack on it :)

-serge
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