[PATCH v4 02/15] mm: Make empty_zero_page __ro_after_init

[David Hildenbrand (Arm)]

On 4/27/26 17:34, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb at kernel.org>
> 
> The empty zero page is used to back any kernel or user space mapping
> that is supposed to remain cleared, and so the page itself is never
> supposed to be modified.
> 
> So make it __ro_after_init rather than __page_aligned_bss: on most
> architectures, this ensures that both the kernel's mapping of it and any
> aliases that are accessible via the kernel direct (linear) map are
> mapped read-only, and cannot be used (inadvertently or maliciously) to
> corrupt the contents of the zero page.
> 
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  mm/mm_init.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index f9f8e1af921c..6ca01ed2a5a4 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -57,7 +57,7 @@ unsigned long zero_page_pfn __ro_after_init;
>  EXPORT_SYMBOL(zero_page_pfn);
>  
>  #ifndef __HAVE_COLOR_ZERO_PAGE
> -uint8_t empty_zero_page[PAGE_SIZE] __page_aligned_bss;
> +uint8_t empty_zero_page[PAGE_SIZE] __ro_after_init __aligned(PAGE_SIZE);
>  EXPORT_SYMBOL(empty_zero_page);
>  
>  struct page *__zero_page __ro_after_init;

I am no expert on BSS etc, but from what I understand, we'll still get zeroed
page-aligned memory. I don't know if there is any other impact on not having it
in bss.page_aligned. I assume no

Acked-by: David Hildenbrand (Arm) <david at kernel.org>

-- 
Cheers,

David