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Date: Tue, 28 Apr 2026 14:27:23 +0200
From: Mike Rapoport <rppt@kernel.org>
To: Ard Biesheuvel <ardb+git@google.com>
Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
	will@kernel.org, catalin.marinas@arm.com, mark.rutland@arm.com,
	Ard Biesheuvel <ardb@kernel.org>,
	Ryan Roberts <ryan.roberts@arm.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Liz Prucka <lizprucka@google.com>,
	Seth Jenkins <sethjenkins@google.com>, Kees Cook <kees@kernel.org>,
	David Hildenbrand <david@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH v4 02/15] mm: Make empty_zero_page __ro_after_init
Message-ID: <afCnq17NWvM4T4vC@kernel.org>
References: <20260427153416.2103979-17-ardb+git@google.com>
 <20260427153416.2103979-19-ardb+git@google.com>
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In-Reply-To: <20260427153416.2103979-19-ardb+git@google.com>

On Mon, Apr 27, 2026 at 05:34:19PM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@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@kernel.org>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@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
> 

-- 
Sincerely yours,
Mike.