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

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Date:	Fri, 20 May 2011 10:34:11 -0400
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	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

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.

thanks,

Mimi

> > > > Signed-off-by: Mimi Zohar <zohar@...ibm.com>
> > > > Acked-by: Serge Hallyn <serge.hallyn@...ntu.com>
> > > > ---
> > > >  fs/file_table.c |    2 +-
> > > >  1 files changed, 1 insertions(+), 1 deletions(-)
> > > > 
> > > > diff --git a/fs/file_table.c b/fs/file_table.c
> > > > index 01e4c1e..33f54a1 100644
> > > > --- a/fs/file_table.c
> > > > +++ b/fs/file_table.c
> > > > @@ -243,10 +243,10 @@ static void __fput(struct file *file)
> > > >  		if (file->f_op && file->f_op->fasync)
> > > >  			file->f_op->fasync(-1, file, 0);
> > > >  	}
> > > > +	ima_file_free(file);
> > > >  	if (file->f_op && file->f_op->release)
> > > >  		file->f_op->release(inode, file);
> > > >  	security_file_free(file);
> > > > -	ima_file_free(file);
> > > >  	if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL &&
> > > >  		     !(file->f_mode & FMODE_PATH))) {
> > > >  		cdev_put(inode->i_cdev);
> > > > -- 
> > 
> --
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