Re: [PATCH v2 0/5] imx8mp: add support for the IMX AIPSTZ bridge

[Laurentiu Mihalcea <laurentiumihalcea111@gmail.com>]

On 2/27/2025 1:28 PM, Marco Felsch wrote:
> Hi Laurentiu,
>
> On 25-02-26, Marco Felsch wrote:
>> Hi,
>>
>> On 25-02-26, Laurentiu Mihalcea wrote:
>>> From: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>
>>>
>>> The AIPSTZ bridge offers some security-related configurations which can
>>> be used to restrict master access to certain peripherals on the bridge.
>>>
>>> Normally, this could be done from a secure environment such as ATF before
>>> Linux boots but the configuration of AIPSTZ5 is lost each time the power
>>> domain is powered off and then powered on. Because of this, it has to be
>>> configured each time the power domain is turned on and before any master
>>> tries to access the peripherals (e.g: AP, CM7, DSP, on i.MX8MP).
>> My question still stands:
>>
>> Setting these bits requires very often that the core is running at EL3
>> (e.g. secure-monitor) which is not the case for Linux. Can you please
>> provide more information how Linux can set these bits?
> Sorry I didn't noticed your response:
>
> https://lore.kernel.org/all/a62ab860-5e0e-4ebc-af1f-6fb7ac621e2b@gmail.com/
>
> If EL1 is allowed to set the security access configuration of the IP
> cores doesn't this mean that a backdoor can be opened? E.g. your
> secure-boot system configures one I2C IP core to be accessible only from
> secure-world S-EL1 (OP-TEE) and after the power-domain was power-cycled
> it's accessible from EL1 again. This doesn't seem right. Why should a
> user be able to limit the access permissions to an IP core to only be
> accessible from secure-world if the IP core is accessible from
> normal-world after the power-domain was power-cycled.
>
> Regards,
>   Marco

I'm no security expert so please feel free to correct me if I get something wrong.

This isn't about S/NS world. The bridge AC doesn't offer any configurations for
denying access to peripherals based on S/NS world. AFAIK that's the job of the CSU
(central security unit), which is a different IP.

Perhaps I shouldn't have used the term "trusted" as it might have ended up creating
more confusion? If so, please do let me know so I can maybe add a comment about
it in one of the commit messages. In this context, "master X is trusted for read/writes"
means "master X is allowed to perform read/write transactions".

Even if the bridge is configured to allow read/write transactions from a master
(i.e: master is marked as trusted for read/writes) that wouldn't be very helpful.
You'd still have to bypass the CSU configuration which as far as I understand is also
used by the bridge to deny access to peripherals (e.g: if transaction is secure+privileged
then forward to peripheral, otherwise abort it). See the "4.7.6.1 Security Block"
and "4.7.4  Access Protections" chapters from the IMX8MP RM.

Given all of this, I think the purpose of this IP's AC is to add some extra,
light, security features on top of the CSU.