linux-kernel - Re: [PATCH v2 02/10] arm64: Move fixmap page tables to end of kernel image

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Message-ID: <823c6406-5b70-490d-a318-25ac48950ec6@arm.com>
Date: Tue, 27 Jan 2026 09:42:09 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org
Cc: linux-arm-kernel@...ts.infradead.org, will@...nel.org,
 catalin.marinas@....com, mark.rutland@....com,
 Ard Biesheuvel <ardb@...nel.org>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Liz Prucka <lizprucka@...gle.com>, Seth Jenkins <sethjenkins@...gle.com>,
 Kees Cook <kees@...nel.org>, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 02/10] arm64: Move fixmap page tables to end of kernel
 image

On 26/01/2026 09:26, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
> 
> Move the fixmap page tables out of the BSS section, and place them at
> the end of the image, right before the init_pg_dir section where some of
> the other statically allocated page tables live.
> 
> These page tables are currently the only data objects in vmlinux that
> are meant to be accessed via the kernel image's linear alias, and so
> placing them together allows the remainder of the data/bss section to be
> remapped read-only or unmapped entirely.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>

Reviewed-by: Ryan Roberts <ryan.roberts@....com>

> ---
>  arch/arm64/kernel/vmlinux.lds.S | 5 +++++
>  arch/arm64/mm/fixmap.c          | 7 ++++---
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index b2a093f5b3fc..c089b83308a5 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -335,6 +335,11 @@ SECTIONS
>  	__pi___bss_start = __bss_start;
>  
>  	. = ALIGN(PAGE_SIZE);
> +	.pgdir : {
> +		__pgdir_start = .;
> +		*(.fixmap_bss)
> +	}

Observing that this still gets zero-initialized:

	/* Clear BSS and the initial page tables */
	memset(__bss_start, 0, (char *)init_pg_end - (char *)__bss_start);

Thanks,
Ryan

> +
>  	__pi_init_pg_dir = .;
>  	. += INIT_DIR_SIZE;
>  	__pi_init_pg_end = .;
> diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
> index c5c5425791da..b649ea1a46e4 100644
> --- a/arch/arm64/mm/fixmap.c
> +++ b/arch/arm64/mm/fixmap.c
> @@ -31,9 +31,10 @@ static_assert(NR_BM_PMD_TABLES == 1);
>  
>  #define BM_PTE_TABLE_IDX(addr)	__BM_TABLE_IDX(addr, PMD_SHIFT)
>  
> -static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __page_aligned_bss;
> -static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
> -static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
> +#define __fixmap_bss	__section(".fixmap_bss") __aligned(PAGE_SIZE)
> +static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __fixmap_bss;
> +static pmd_t bm_pmd[PTRS_PER_PMD] __fixmap_bss __maybe_unused;
> +static pud_t bm_pud[PTRS_PER_PUD] __fixmap_bss __maybe_unused;
>  
>  static inline pte_t *fixmap_pte(unsigned long addr)
>  {