linux-kernel - [PATCH 1/4] arm64: Move fixmap page tables to end of kernel image

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Message-ID: <20260119164747.1402434-7-ardb+git@google.com>
Date: Mon, 19 Jan 2026 17:47:49 +0100
From: Ard Biesheuvel <ardb+git@...gle.com>
To: 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>, Ryan Roberts <ryan.roberts@....com>, 
	Liz Prucka <lizprucka@...gle.com>, Seth Jenkins <sethjenkins@...gle.com>, 
	Kees Cook <kees@...nel.org>, linux-hardening@...r.kernel.org
Subject: [PATCH 1/4] arm64: Move fixmap page tables to end of kernel image

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>
---
 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 ad6133b89e7a..df530e6f3e53 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -334,6 +334,11 @@ SECTIONS
 	__pi___bss_start = __bss_start;
 
 	. = ALIGN(PAGE_SIZE);
+	.pgdir : {
+		__pgdir_start = .;
+		*(.fixmap_bss)
+	}
+
 	__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)
 {
-- 
2.52.0.457.g6b5491de43-goog