Re: [PATCH v4 8/8] powerpc/mm: Disable set_memory() routines when strict RWX isn't enabled

[Christophe Leroy <christophe.leroy@c-s.fr>]

Le 26/02/2020 à 07:24, Russell Currey a écrit :
> There are a couple of reasons that the set_memory() functions are
> problematic when STRICT_KERNEL_RWX isn't enabled:
> 
>   - The linear mapping is a different size and apply_to_page_range()
> 	may modify a giant section, breaking everything

I don't understand.

>   - patch_instruction() doesn't know to work around a page being marked
>   	RO, and will subsequently crash

Is patch_instruction() involved at all ?

> 
> The latter can be replicated by building a kernel with the set_memory()
> patches but with STRICT_KERNEL_RWX off and running ftracetest.
> 
> Reported-by: Jordan Niethe <jniethe5@gmail.com>
> Signed-off-by: Russell Currey <ruscur@russell.cc>
> ---
> v4: new
> 
>   arch/powerpc/mm/pageattr.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/mm/pageattr.c b/arch/powerpc/mm/pageattr.c
> index ee6b5e3b7604..ff111930cf5e 100644
> --- a/arch/powerpc/mm/pageattr.c
> +++ b/arch/powerpc/mm/pageattr.c
> @@ -96,12 +96,17 @@ static int set_page_attr(pte_t *ptep, unsigned long addr, void *data)
>   
>   int set_memory_attr(unsigned long addr, int numpages, pgprot_t prot)

Isn't it change_memory_attr() that is a problem for you ?

>   {
> -	unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
> -	unsigned long sz = numpages * PAGE_SIZE;
> +	unsigned long start, size;
> +
> +	if (!IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
> +		return 0;


Doing this you break patch 7:
mark_initmem_nx() is called regardless of CONFIG_STRICT_KERNEL_RWX
__kernel_map_pages() depends on CONFIG_DEBUG_PAGEALLOC which doesn't 
depend on CONFIG_STRICT_KERNEL_RWX


>   
>   	if (!numpages)
>   		return 0;
>   
> -	return apply_to_page_range(&init_mm, start, sz, set_page_attr,
> +	start = ALIGN_DOWN(addr, PAGE_SIZE);
> +	size = numpages * PAGE_SIZE;
> +
> +	return apply_to_page_range(&init_mm, start, size, set_page_attr,

You don't need to move start and size calculation and change the above.

>   				   (void *)pgprot_val(prot));
>   }
> 

Christophe