Re: [PATCH v2 7/9] i.MX8M: HABv4: add an option to allow key revocation

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Marco,

On 7/4/24 10:15, Marco Felsch wrote:
> Hi Ahmad,
> 
> On 24-07-03, Ahmad Fatoum wrote:
>> Hello Marco,
>>
>> On 03.07.24 19:20, Marco Felsch wrote:
>>> The HAB code needs an special [Unlock] instruction to keep the
>>> SRK_REVOKE fuse bank unlocked. This is required if a key needs to be
>>> revoked.
>>>
>>> Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
>>> ---
>>>  arch/arm/mach-imx/Kconfig            | 8 ++++++++
>>>  include/mach/imx/habv4-imx8-gencsf.h | 6 ++++++
>>>  2 files changed, 14 insertions(+)
>>>
>>> diff --git a/arch/arm/mach-imx/Kconfig b/arch/arm/mach-imx/Kconfig
>>> index 61258137736f..68f55971506b 100644
>>> --- a/arch/arm/mach-imx/Kconfig
>>> +++ b/arch/arm/mach-imx/Kconfig
>>> @@ -835,6 +835,14 @@ config HABV4_QSPI
>>>  	help
>>>  	  Enable this option to build signed QSPI/FlexSPI images.
>>>  
>>> +config HABV4_CSF_UNLOCK_SRK_REVOKE
>>> +	depends on HABV4
>>> +	bool "Unlock SRK revocation"
>>> +	help
>>> +	  Enable this option to instruct the HAB code to not lock
>>> +	  the SRK_REVOKE_LOCK sticky bit. This is required for key
>>> +	  revocation. Don't enable this if you are unsure.
>>
>> I think for added safety we should have an extra option that prompts
>> for the key to be revoked and an initcall that is activated depending
>> on it, e.g.:
>>
>> config HABV4_CSF_SRK_REVOKE_INDEX
>>         int "SRK to revoke"
>>         range 0 3
>>         default 0
>>         depends on HABV4_CSF_SRK_REVOKE_UNLOCK
>>         help
>>           Which of the first three SRKs to revoke. The SRK indices are
>>           1-based. Saying 0 here will just print the SRK Revocation
>>           register without modification. SRK #4 is immutable.
>>                                                                        
>>           Proceed with caution, revoking a SRK is irreversible and
>>           manual manipulation of this code can brick the board!
>>                                                                        
>> if HABV4_CSF_SRK_REVOKE_INDEX = HABV4_SRK_INDEX
>> comment "Can't revoke same SRK used for signing"
>> comment "Attempts to build a signed barebox image will fail"
>> endif
>>
>> and then some code that checks the same above condition during final
>> assembly of the signed image.
>>
>> What do you think?
> 
> That's an good idea to make it more user-friendly for most users :)
> Regarding this patchset I do see it more as an addition since for my
> project the revocation is checked on every startup and we do allow the
> revocation of multiple SRK slots at the same time.

Ok. I just have a single nit then, can you rename
HABV4_CSF_UNLOCK_SRK_REVOKE to HABV4_CSF_SRK_REVOKE_UNLOCK, so we can
add HABV4_CSF_SRK_REVOKE_INDEX in future with the same prefix?

Thanks,
Ahmad

> 
> Regards,
>   Marco
> 
>>>  config HAB_CERTS_ENV
>>>  	depends on HAB
>>>  	bool "Specify certificates in environment"
>>> diff --git a/include/mach/imx/habv4-imx8-gencsf.h b/include/mach/imx/habv4-imx8-gencsf.h
>>> index 5f92ceceab00..56d9ef2de92f 100644
>>> --- a/include/mach/imx/habv4-imx8-gencsf.h
>>> +++ b/include/mach/imx/habv4-imx8-gencsf.h
>>> @@ -36,6 +36,12 @@ hab [Unlock]
>>>  hab Engine = CAAM
>>>  hab Features = RNG, MID
>>>  
>>> +#if defined(CONFIG_HABV4_CSF_UNLOCK_SRK_REVOKE)
>>> +hab [Unlock]
>>> +hab Engine = OCOTP
>>> +hab Features = SRK REVOKE
>>> +#endif
>>> +
>>>  hab [Install Key]
>>>  /* verification key index in key store (0, 2...4) */
>>>  hab Verification index = 0
>>>
>>
>> -- 
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>>
>>
>