linux-kernel - Re: [PATCH 2/3] binfmt_elf: Verify signature of signed elf binary

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Message-ID: <1358364290.4593.178.camel@falcor1>
Date:	Wed, 16 Jan 2013 14:24:50 -0500
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, pjones@...hat.com, hpa@...or.com,
	dhowells@...hat.com, jwboyer@...hat.com,
	Dmitry Kasatkin <dmitry.kasatkin@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-security-module@...r.kernel.org
Subject: Re: [PATCH 2/3] binfmt_elf: Verify signature of signed elf binary

On Wed, 2013-01-16 at 13:28 -0500, Vivek Goyal wrote:
> On Wed, Jan 16, 2013 at 01:08:35PM -0500, Mimi Zohar wrote:
> > On Wed, 2013-01-16 at 11:34 -0500, Vivek Goyal wrote:
> > 
> > > I read the comment in ima_bprm_check() being called from security_bprm_check(). 
> > > It says that files already open for write can't executed and files already
> > > open for exec can't be open for writes. That's fine. 
> > > 
> > > I was worried about anonymous pages being modified on swap and then
> > > faulted back in. It is not necessarily signature verification but making
> > > sure signed processes memory is not modified later by any unsigned process
> > > in anyway. And that includes disabling ptrace too.
> > > 
> > > So IMA stuff does not do anything to protect against process memory being
> > > modified using ptrace or by playing tricks with swap.
> > 
> > > I am not sure what will happen if I can bypass the file system and directly
> > > write on a disk block and modify executable. (Assuming one can get block
> > > information somehow). Does anything protect such modification? Will IMA
> > > detect it?
> > 
> > Sorry, this is out of scope for IMA.  Dmitry has looked into this, but
> > I'm not sure where it stands at the moment.
> 
> Ok, so that's one reason that why I wrote these patcehs. IMA currently
> is not doing following things to make sure address space of signed images
> is not modified by others.
> 
> - Protecting against modifications to pages on swap.
> - Protecting against modifications by ptrace.
> - Protecting against modifications which bypassed filesystem and directly
>   wrote to the block.
> 
> Locking down all the pages of signed binaries in memory hopefully should
> solve above problems.

Signing and verifying ELF executables goes back a long time ~2003/4,
from a number of esteemed kernel developers, including Greg-KH and Serge
Hallyn.

IMA-appraisal isn't limited to appraising a single type of file, but is
a generic mechanism for appraising all files.  If there are issues that
aren't being addressed, then by all means, please help by addressing
them.  Duplicating a large portion of the code is not productive.

thanks,

Mimi


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