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

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Message-ID: <CALLzPKaa8eOBA7VxiGijUzoUzk-05cwD8e4A89yyQtCL0=g7qg@mail.gmail.com>
Date:	Thu, 17 Jan 2013 18:27:38 +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 5:18 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Thu, Jan 17, 2013 at 04:58:02PM +0200, Kasatkin, Dmitry 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...
>
> Hm..., encrypted swap is interesting. It might have overhead too. It is
> one way of doing thing, but I think it would be better if we don't rely
> on user has setup encrypted swap and provide a way to lock down memory of
> signed executables.
>
>>
>> But certain locking extensions like you implemented can be added to IMA as well.
>
> Cool. That would help. I am very new to IMA. If you have some ideas on
> how to go about it, I can start looking into implementing it.
>
>>
>> It was said about ptrace already.
>
> Sorry, did not get what was said about ptrace already? We need to disable
> ptrace for signed executables. So can IMA set a process flag upon
> signature verification and exec() code can give additional capability. And
> as Eric mentioned that should automatically take care of ptrace issue.
>
>>
>> > - 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?
>
> If you have loaded all the pages from disk and locked them in memory and
> verified the signature, then even if somebody modifies a block on disk
> it does not matter. We will not read pages from disk anymore for this
> exec(). We verified the signature of executable loaded in memory and
> in-memory copy is intact.
>
> So next time somebody tries to execute same binary it will fail (because
> of modified block).
>

Ok. This is about the same as with swap.

It should be a 10 lines patch to IMA to lock down the memory.
It may be controlled by the policy.
IMA might also set new capability if needed.

- Dmitry


> Thanks
> Vivek
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