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

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Date:	Thu, 17 Jan 2013 17:06:09 +0200
From:	"Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, pjones@...hat.com, hpa@...or.com,
	dhowells@...hat.com, jwboyer@...hat.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 Thu, Jan 17, 2013 at 4:58 PM, Kasatkin, Dmitry
<dmitry.kasatkin@...el.com> wrote:
> On Wed, Jan 16, 2013 at 11:53 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
>> On Wed, Jan 16, 2013 at 02:24:50PM -0500, Mimi Zohar wrote:
>> [..]
>>> > > 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.
>>
>> So do you have ideas on how to address above mentioned issues. Do they
>> fit into the realm of IMA/EVM or I just need to write separate code (which
>> I have already done).
>>
>> With above issues, IMA stuff for executable files sounds incomplete.
>>
>
> swap is a last resort. healthy system uses swap minimally.
> It is very easy to add /etc/crypttab which allows to have encrypted swap
>
> # <target name> <source device>         <key file>      <options>
> swap   /dev/sda6       /dev/urandom    swap
>
> And it will eliminate plenty of possible attacks.
> Processes have also RW data, modification of those will create huge
> risk for the system...
>
> But certain locking extensions like you implemented can be added to IMA as well.
>
> It was said about ptrace already.
>
>
>> - Protecting against modifications which bypassed filesystem and directly  wrote to the block.
>
> Can you please tell a bit more how this patch protect against direct
> writing to the blocks?
>
> Thanks,
> Dmitry
>
>> Thanks
>> Vivek

One important thing to mention.
Protecting ELF-only does not help too much in protecting the system.
There are plenty of init, upstart and systemd scripts which must be
verified as well. IMA does it.

- Dmitry
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