linux-kernel - Re: IMA & truncate

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Date:	Tue, 6 May 2014 15:47:40 +0200
From:	Javier González <javigon.napster@...il.com>
To:	Dmitry Kasatkin <d.kasatkin@...sung.com>
Cc:	linux-security-module <linux-security-module@...r.kernel.org>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>, viro@...IV.linux.org.uk,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: IMA & truncate

Hi,

> I have discovered one IMA related issue.
> 
> IMA file hash is re-calculate if needed on file close.
> 
> It works with ftruncate(fd, length) syscall, because it operates on
> "opened" file.
> Recalculation is happening on file close.
> 
> truncate(path, length) syscall works with path and no file open/close
> takes place.
> Recalculation does not happen.
> IMA denies file access later.
> 
> It looks like vfs_truncate() should possibly call IMA to recalculate the
> hash.

I have had the same issue in an integrity module I am working on to move the hash (and generally encryption) operations to a secure processor (e.g., TrustZone).
After a discussion with Al Viro I was convinced that file_close is not the right place to associate a hook in order to recalculate the hash.

I am now using the LSM hook associated with vfs_truncate to recalculate the hash and it seems to work good. I am still running some performance tests to calculate the overhead of attacking an operation here, so I don’t have data yet. 

Best,

-------------------------------------------------/--
Javier González

IT University of Copenhagen
Rued Langgaards Vej 7, 2300
København S, Danmark 
Office: 4D23

http://javigon.com/
Skype: javigon.napster
(+45)31376927

-------------------------------------------------/--

On 6 May 2014, at 15:32, Dmitry Kasatkin <d.kasatkin@...sung.com> wrote:

> Hi,
> 
> I have discovered one IMA related issue.
> 
> IMA file hash is re-calculate if needed on file close.
> 
> It works with ftruncate(fd, length) syscall, because it operates on
> "opened" file.
> Recalculation is happening on file close.
> 
> truncate(path, length) syscall works with path and no file open/close
> takes place.
> Recalculation does not happen.
> IMA denies file access later.
> 
> It looks like vfs_truncate() should possibly call IMA to recalculate the
> hash.
> 
> - Dmitry
> 
> --
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