Re: [PATCH v3 1/4] firmware: qcom_scm: Export SCM call functions

[Maximilian Luz <luzmaximilian@gmail.com>]

On 3/8/23 13:53, Srinivas Kandagatla wrote:
> 
> 
> On 07/03/2023 15:23, Dmitry Baryshkov wrote:
>>
>>> Make qcom_scm_call, qcom_scm_call_atomic and associated types accessible
>>> to other modules.
>>
>> Generally all the qcom_scm calls are a part of qcom_scm.c. I think it is better to make qseecom_scm_call a part qcom_scm.c (as we were previously doing) rather than exporting the core function.
>>
> 
> Other big issue I see in exporting qcom_scm_call() is that there is danger of misuse of this api as this could lead to a path where new apis and its payloads can come directly from userspace via a rogue/hacking modules. This will bypass scm layer completely within kernel.

I'm not sure I follow your argument here. If you have the possibility to
load your own kernel modules, can you not always bypass the kernel and
just directly invoke the respective SCM calls manually? So this is
superficial security at best.

I guess keeping it in qcom_scm could make it easier to spot new
in-kernel users of that function and with that better prevent potential
misuse in the kernel itself. But then again I'd hope that our review
system is good enough to catch such issues regardless and thoroughly
question calls to that function (especially ones involving user-space
APIs).

Regards,
Max

> 
> --srini
> 
>> If you wish to limit the kernel bloat, you can split the qcom_scm into per-driver backend and add Kconfig symbols to limit the impact. However I think that these functions are pretty small to justify the effort.
>>
> 
> 
>>>
>>> Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>