linux-kernel - Re: [PATCH 3/4] integrity: IMA as an integrity service provider

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Date:	Thu, 20 Nov 2008 13:22:21 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...x-foundation.org>,
	James Morris <jmorris@...ei.org>,
	Christoph Hellwig <hch@...radead.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	David Safford <safford@...son.ibm.com>,
	Serge Hallyn <serue@...ux.vnet.ibm.com>,
	Mimi Zohar <zohar@...ibm.com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 3/4] integrity: IMA as an integrity service provider

On Thu, 2008-11-20 at 11:43 -0500, Mimi Zohar wrote:
> 
> +       /* Invalidate PCR, if a measured file is already open for read
> */
> +       if ((mask == MAY_WRITE) || (mask == MAY_APPEND)) {
> +               int mask_sav = data->mask;
> +               int rc;
> +
> +               data->mask = MAY_READ;
> +               rc = ima_must_measure(&idata);
> +               if (!rc) {
> +                       if (atomic_read(&(data->dentry->d_count)) - 1 >
> +                           atomic_read(&(inode->i_writecount)))
> +                               ima_add_violation(inode, data->filename,
> +                                                 "invalid_pcr", "ToMToU");
> +               }
> +               data->mask = mask_sav;
> +               goto out;
> +       }

Following up on Christoph's comment...

I'm worried that this calculation isn't very precise.  The calculation
that you're trying to come up with here is the number of opens (d_count)
vs. the number of writers (i_writecount).  When they don't match, you
know that the new open is the first write, and you must 'invalidate the
PCR'?

There are a number of things that elevate d_count, and it is a lot more
than just an open() that can do it.  Is that OK?

-- Dave

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