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

[Florian Fainelli <f.fainelli@gmail.com>]

On 1/28/20 12:06 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.

Could you use an overlay then which is the result of the firewalling
results by your firewall block, which is smaller than the main SoC/board
DTB and can be easily audited not to accidentally enable blocks, but
only disable them by adding/changing the respective "status" property.
Worst case, your driver probes, has been firewalled and this is not
reflected in the DTB, you get a bus error, or a hang, or however it gets
implemented.

Like Robin and Sudeep here, I do not understand why the kernel should
have any business in this, let alone allowing blocks to change owners,
that sounds contrary to the purpose of a firewall being controlled under
an untrusted entity (Linux).
-- 
Florian

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