[PATCH 3/3] powerpc/mm: Mark __init memory no-execute when STRICT_KERNEL_RWX=y

[Michael Ellerman]

Christophe LEROY <christophe.leroy at c-s.fr> writes:
> Le 14/07/2017 à 08:51, Michael Ellerman a écrit :
>> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> index c0737c86a362..3d562b210c65 100644
>> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
>> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> @@ -1192,5 +1192,12 @@ static inline const int pud_pfn(pud_t pud)
>>   	BUILD_BUG();
>>   	return 0;
>>   }
>> +
>> +#ifdef CONFIG_STRICT_KERNEL_RWX
>> +void mark_initmem_nx(void);
>> +#else
>> +static inline void mark_initmem_nx(void) { }
>> +#endif
>> +
>
> Why do we want to limit that to CONFIG_STRICT_KERNEL_RWX ?
> Only the kernel text is marked X, even without CONFIG_STRICT_KERNEL_RWX 
> (at least on PPC32), so I believe we should clear X on init text in any 
> case, shouldn't we ?

You're right, but ..

On 64-bit when STRICT_KERNEL_RWX=n we make no effort to ensure the
start/end of the init text is on a page boundary.

eg. on 64-bit hash we will typically use a 16M page to map the whole
kernel, text/data/init_text/etc.

So yes we *should* always mark it no-execute but in practice we can't
because it's not page aligned.

But if that's different on (some?) 32-bit then we could introduce a new
CONFIG symbol that is enabled in the right cases.

cheers