[PATCH v14 3/9] powerpc/modules: Make module_alloc() Strict Module RWX aware

[Michael Ellerman]

Christophe Leroy <christophe.leroy at csgroup.eu> writes:
> Le 17/05/2021 à 13:01, Michael Ellerman a écrit :
>> Jordan Niethe <jniethe5 at gmail.com> writes:
>>> On Mon, May 17, 2021 at 4:37 PM Christophe Leroy
>>> <christophe.leroy at csgroup.eu> wrote:
>>>> Le 17/05/2021 à 05:28, Jordan Niethe a écrit :
>>>>> Make module_alloc() use PAGE_KERNEL protections instead of
>>>>> PAGE_KERNEL_EXEX if Strict Module RWX is enabled.
>>>>>
>>>>> Signed-off-by: Jordan Niethe <jniethe5 at gmail.com>
>>>>> ---
>>>>> v14: - Split out from powerpc: Set ARCH_HAS_STRICT_MODULE_RWX
>>>>>        - Add and use strict_module_rwx_enabled() helper
>>>>> ---
>>>>>    arch/powerpc/include/asm/mmu.h | 5 +++++
>>>>>    arch/powerpc/kernel/module.c   | 4 +++-
>>>>>    2 files changed, 8 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/arch/powerpc/include/asm/mmu.h b/arch/powerpc/include/asm/mmu.h
>>>>> index 607168b1aef4..7710bf0cbf8a 100644
>>>>> --- a/arch/powerpc/include/asm/mmu.h
>>>>> +++ b/arch/powerpc/include/asm/mmu.h
>>>>> @@ -357,6 +357,11 @@ static inline bool strict_kernel_rwx_enabled(void)
>>>>>        return false;
>>>>>    }
>>>>>    #endif
>>>>> +
>>>>> +static inline bool strict_module_rwx_enabled(void)
>>>>> +{
>>>>> +     return IS_ENABLED(CONFIG_STRICT_MODULE_RWX) && strict_kernel_rwx_enabled();
>>>>> +}
>>>>
>>>> Looking at arch/Kconfig, I have the feeling that it is possible to select CONFIG_STRICT_MODULE_RWX
>>>> without selecting CONFIG_STRICT_KERNEL_RWX.
>>>>
>>>> In that case, strict_kernel_rwx_enabled() will return false.
>> 
>>> Ok, if someone did that currently it would break things, e.g. code
>>> patching. I think it should it be made impossible to
>>> CONFIG_STRICT_MODULE_RWX without CONFIG_STRICT_KERNEL_RWX?
>> 
>> Yeah I don't see any reason to support that combination.
>> 
>> We should be moving to a world where both are on by default, or in fact
>> are always enabled.
>
> Would it work if we add the following in arch/powerpc/Kconfig ? :
>
> 	select STRICT_KERNEL_RWX if STRICT_MODULE_RWX
>
> There should be no dependency issue as powerpc only selects ARCH_HAS_STRICT_MODULE_RWX when 
> ARCH_HAS_STRICT_KERNEL_RWX is also selected.

I think it will work. It's slightly rude to select things like that, but
I think it's OK for something like this.

Medium term we can possibly just have the generic STRICT_MODULE_RWX
depend on STRICT_KERNEL_RWX.

cheers