Re: [PATCH v2 0/7] Introduce bus firewall controller framework

[Benjamin GAIGNARD <benjamin.gaignard@st.com>]

On 1/28/20 11:06 PM, Robin Murphy wrote:
> On 2020-01-28 8:06 pm, Benjamin GAIGNARD wrote:
>>
>> On 1/28/20 6:17 PM, Sudeep Holla wrote:
>>> On Tue, Jan 28, 2020 at 04:46:41PM +0000, Benjamin GAIGNARD wrote:
>>>> On 1/28/20 5:36 PM, Sudeep Holla wrote:
>>>>> On Tue, Jan 28, 2020 at 04:37:59PM +0100, Benjamin Gaignard wrote:
>>>>>> Bus firewall framework aims to provide a kernel API to set the 
>>>>>> configuration
>>>>>> of the harware blocks in charge of busses access control.
>>>>>>
>>>>>> Framework architecture is inspirated by pinctrl framework:
>>>>>> - a default configuration could be applied before bind the driver.
>>>>>>      If a configuration could not be applied the driver is not bind
>>>>>>      to avoid doing accesses on prohibited regions.
>>>>>> - configurations could be apllied dynamically by drivers.
>>>>>> - device node provides the bus firewall configurations.
>>>>>>
>>>>>> An example of bus firewall controller is STM32 ETZPC hardware block
>>>>>> which got 3 possible configurations:
>>>>>> - trust: hardware blocks are only accessible by software running 
>>>>>> on trust
>>>>>>      zone (i.e op-tee firmware).
>>>>>> - non-secure: hardware blocks are accessible by non-secure 
>>>>>> software (i.e.
>>>>>>      linux kernel).
>>>>>> - coprocessor: hardware blocks are only accessible by the 
>>>>>> coprocessor.
>>>>>> Up to 94 hardware blocks of the soc could be managed by ETZPC.
>>>>>>
>>>>> /me confused. Is ETZPC accessible from the non-secure kernel space to
>>>>> begin with ? If so, is it allowed to configure hardware blocks as 
>>>>> secure
>>>>> or trusted ? I am failing to understand the overall design of a 
>>>>> system
>>>>> with ETZPC controller.
>>>> Non-secure kernel could read the values set in ETZPC, if it doesn't 
>>>> match
>>>> with what is required by the device node the driver won't be probed.
>>>>
>>> OK, but I was under the impression that it was made clear that Linux is
>>> not firmware validation suite. The firmware need to ensure all the 
>>> devices
>>> that are not accessible in the Linux kernel are marked as disabled and
>>> this needs to happen before entering the kernel. So if this is what 
>>> this
>>> patch series achieves, then there is no need for it. Please stop 
>>> pursuing
>>> this any further or provide any other reasons(if any) to have it. Until
>>> you have other reasons, NACK for this series.
>>
>> No it doesn't disable the nodes.
>>
>> When the firmware disable a node before the kernel that means it change
>>
>> the DTB and that is a problem when you want to sign it. With my proposal
>>
>> the DTB remains the same.
>
> ???
>
> :/
>
> The DTB is used to pass the kernel command line, memory reservations, 
> random seeds, and all manner of other things dynamically generated by 
> firmware at boot-time. Apologies for being blunt but if "changing the 
> DTB" is considered a problem then I can't help but think you're doing 
> it wrong.

Yes but I would like to limit the number of cases where a firmware has 
to change the DTB.

With this proposal nodes remain the same and embedded the firewall 
configuration(s).

Until now firewall configuration is "static", the firmware disable (or 
remove) the nodes not accessible from Linux.

If Linux can rely on node's firewall information it could allow switch 
dynamically an hardware block from Linux to a coprocessor.

For example Linux could manage the display pipe configuration and when 
going to suspend handover the display hardware block to a coprocessor in 
charge a refreshing only some pixels.

Benjamin

>
> Robin.
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