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

[Feng Tang]

On Mon, Apr 27, 2026 at 05:34:19PM +0200, 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.

Reviewed-by: Feng Tang <feng.tang at linux.alibaba.com>

We did hit the issue that zero page got corrupted due to non-kernel
reason earlier this year, and it took us weeks to track it down to
zero page corruption and the final root cause, as it randomly happened
to different user space tasks. Thanks for the patch!

> 
> 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;
> -- 
> 2.54.0.rc2.544.gc7ae2d5bb8-goog
>