linux-kernel - Re: MODSIGN without RTC?

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Date:	Thu, 07 Feb 2013 11:54:32 +0100
From:	Alexander Holler <holler@...oftware.de>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
CC:	linux-kernel@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	David Miller <davem@...emloft.net>,
	Rusty Russell <rusty@...tcorp.com.au>,
	David Howells <dhowells@...hat.com>
Subject: Re: MODSIGN without RTC?

Am 07.02.2013 08:01, schrieb Alexander Holler:
> Am 07.02.2013 07:42, schrieb Geert Uytterhoeven:
>> On Thu, Feb 7, 2013 at 2:06 AM, Alexander Holler <holler@...oftware.de> wrote:
>>> Am 07.02.2013 00:42, schrieb Alexander Holler:
>>>> I wanted to try out MODSIGN with kernel 3.7.6 and I've just got hit by:
>>>>
>>>> [    1.346445] X.509: Cert 6a23533cec71c4c52a1618fb4d830e06aa90474e is
>>>> not yet valid
>>>>
>>>> The reason is likely that the (ARM) device in question doesn't have a
>>>> RTC (oh, that topic again ;) ) and gets it's time on boot through NTP.
>>>>
>>>> The used certificate was generated automatically. Having a look at it,
>>>> the following is shown:
>>>>
>>>>          Validity
>>>>               Not Before: Feb  6 02:56:46 2013 GMT
>>>>               Not After : Jan 13 02:56:46 2113 GMT
>>>>
>>>> Without having thought about possible security problems, my first idea
>>>> would be to let the validity start at 1970. As I never did such I never
>>>> had thought about possible implications when doing such (e.g. I don't
>>>> know if someone checks the start date for plausabilitiy)
>>>>
>>>> Another solution would be to retry loading of the certificate if the
>>>> time gets set (and e.g. differs more than a year).
>>>>
>>>> Has someone already thought about how to solve that problem? Or did
>>>> everyone use sane systems which have a (working) RTC?
>>>
>>>
>>> Another option would be to make a configure option to just ignore the date.
>>
>> Or an option to auto-advance the clock to the "Not Before" date if needed...
>>
>>> I'm not sure if I would like to use MODSIGN when I have to fear that the
>>> machine wouldn't start when the RTC fails or got set to a wrong date.
>>
>> Hmm, nice failure mode...
>
> And the dream of every vendor, finally a working expiration date. And a
> nice TV-B-Gone, just feed a wrong date once. ;)

I've digged a bit around about how to disable the date check, but then 
decided that I shouldn't try to implement that 
(CONFIG_MODSIGN_IGNORE_DATES) because of missing knowledge about the 
(usage of) crypto-api.

I think adding attributes to the key and the parsed key like bool 
ingore_dates and bool parsed_dates_invalid might be an option. Using 
such x509_key_preparse() could just set parsed_dates_invalid instead of 
returning with -EKEYREJECTED or -EKEYEXPIRED, if it encounters invalid 
dates.

Regards,

Alexander

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