linux-kernel - Re: [RFC v2 7/7] modsig: build rules and scripts to generate keys and sign modules

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Date:	Fri, 17 Aug 2012 00:04:05 +0300
From:	"Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>
To:	Josh Boyer <jwboyer@...il.com>
Cc:	zohar@...ux.vnet.ibm.com, jmorris@...ei.org, rusty@...tcorp.com.au,
	dhowells@...hat.com, linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 7/7] modsig: build rules and scripts to generate keys and
 sign modules

On Thu, Aug 16, 2012 at 11:31 PM, Josh Boyer <jwboyer@...il.com> wrote:
> On Thu, Aug 16, 2012 at 4:12 PM, Kasatkin, Dmitry
> <dmitry.kasatkin@...el.com> wrote:
>>>> 1. signed_modules_install
>>>> This target creates an ephemeral key pair, signs the kernel modules with
>>>> the private key, destroys the private key, and embeds the public key in
>>>> the kernel. (Thanks to Dave Hansen for the target name.)
>>>
>>> This requires CONFIG_INTEGRITY_MODULES to be enabled to actually do
>>> anything useful with the signed modules, correct?
>>>
>>
>> Correct. It does not make sense to sign module if module support is disabled.
>> But there scripts/genkey.sh and ksign.sh which works without Makefiles.
>> So possible to generate keys and sign a module...
>
> Right, but it won't actually do anything if the config option isn't set.
> Which means someone calling 'make signed_modules_install' won't actually
> get signed modules.  That's confusing.
>

Yes.. It had to be behind "ifeq CONFIG_INTEGRITY_MODULES"

>>>> 2. modules_install
>>>> When CONFIG_INTEGRITY_MODULES is enabled, this target uses an existing
>>>> private key to sign kernel modules.
>>>
>>> If the answer to the above question is yes, then why can't we stick
>>> with a single modules_install command for signing?  It would seem to me
>>> that calling signed_modules_install could use an existing key or
>>> generate an ephemeral key in the absence of one and install the signed
>>> modules, and modules_install would simply install unsigned modules.
>>>
>>> Or, alternatively, just make modules_install sign or not sign depending
>>> on whether CONFIG_INTEGRITY_MODULES is enabled.
>>
>> This is what "make modules_install" does. It uses existing private key
>> and does not remove it after install.
>
> Right.  I should have been more clear.  I was suggesting that "make
> modules_install" generate a key as well if one does not already exist.
> Essentially removing the necessity for sign_modules_install.
>
>>>  I don't see why you
>>> would overload a target or create two different ones when both depend
>>> on that option.
>>>
>>> Could you explain the reasoning behind that a bit more?
>>
>> The reason for "signed_modules_install" is to limit existence of private key.
>> Private key is generate just before install, modules installed and
>> signed, then key is destroyed.
>> So existence of private key is limited to "time make
>> signed_modules_install" execution time.
>>
>> We had a debate about it, and strong message was that we might want to
>> do it like that...
>
> I guess I personally don't see the need to destroy they key so quickly.
> Is the concern that an intruder might grab the key and use it to sign a
> module that the developer would then later on somehow load?  Or
> similarly someone would grab a temporary key from a distro build
> machine?  That limits the attack surface, sure, but I'm not sure it's
> really reasonable.
>
> For a developer that isn't distributing kernels to others, it's just
> adding more time to the compile (which I know can be disabled, but
> still).  For a distribution, most of them are either using a private
> key already or they have a buildsystem that destroys a buildroot after
> a build completes.  The key is already going to be destroyed in that
> scenario.
>
> josh

Well... Will not argue here. I had similar opinion as well.

Mimi strongly wanted really to "reduce" the existence time of the key...

- Dmitry
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