Subject: [PATCH v4 14/15] arm64: mm: Generalize manipulation code of read-only descriptors

From: Ard Biesheuvel <[email-masked]>
Date: Mon, 27 Apr 2026 17:34:31 +0200

From: Ard Biesheuvel <[email-masked]>

Before moving the fixmap PUD/PMD tables into .rodata, update the
existing descriptor manipulation code so it will fallback to the fixmap
for any descriptor located in the .pgdir_rodata section.

This is slightly more costly, as it evaluates whether or not a
descriptor is in the kernel's rodata region at levels PMD and higher for
any configuration, rather than only when the level in question is the
root level.

Signed-off-by: Ard Biesheuvel <[email-masked]>
---
 arch/arm64/include/asm/pgtable.h | 27 ++++++++++----------
 arch/arm64/kernel/vmlinux.lds.S  |  8 ++++--
 arch/arm64/mm/mmu.c              | 24 ++++++++---------
 3 files changed, 31 insertions(+), 28 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index a1c5894332d9..94235dd428be 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -816,23 +816,22 @@ extern pgd_t swapper_pg_dir[];
 extern pgd_t idmap_pg_dir[];
 extern pgd_t tramp_pg_dir[];
 extern pgd_t reserved_pg_dir[];
+extern pgd_t __pgdir_rodata_start[], __pgdir_rodata_end[];
 
-extern void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd);
+extern void set_rodata_pte(pte_t *ptep, pte_t pte);
 
-static inline bool in_swapper_pgdir(void *addr)
+static inline bool in_pgdir_rodata(void *addr)
 {
-	return ((unsigned long)addr & PAGE_MASK) ==
-	        ((unsigned long)swapper_pg_dir & PAGE_MASK);
+	return addr >= (void *)__pgdir_rodata_start &&
+	       addr < (void *)__pgdir_rodata_end;
 }
 
 static inline void set_pmd(pmd_t *pmdp, pmd_t pmd)
 {
-#ifdef __PAGETABLE_PMD_FOLDED
-	if (in_swapper_pgdir(pmdp)) {
-		set_swapper_pgd((pgd_t *)pmdp, __pgd(pmd_val(pmd)));
+	if (in_pgdir_rodata(pmdp)) {
+		set_rodata_pte((pte_t *)pmdp, __pte(pmd_val(pmd)));
 		return;
 	}
-#endif /* __PAGETABLE_PMD_FOLDED */
 
 	WRITE_ONCE(*pmdp, pmd);
 
@@ -893,8 +892,8 @@ static inline bool pgtable_l4_enabled(void);
 
 static inline void set_pud(pud_t *pudp, pud_t pud)
 {
-	if (!pgtable_l4_enabled() && in_swapper_pgdir(pudp)) {
-		set_swapper_pgd((pgd_t *)pudp, __pgd(pud_val(pud)));
+	if (in_pgdir_rodata(pudp)) {
+		set_rodata_pte((pte_t *)pudp, __pte(pud_val(pud)));
 		return;
 	}
 
@@ -974,8 +973,8 @@ static inline bool mm_pud_folded(const struct mm_struct *mm)
 
 static inline void set_p4d(p4d_t *p4dp, p4d_t p4d)
 {
-	if (in_swapper_pgdir(p4dp)) {
-		set_swapper_pgd((pgd_t *)p4dp, __pgd(p4d_val(p4d)));
+	if (in_pgdir_rodata(p4dp)) {
+		set_rodata_pte((pte_t *)p4dp, __pte(p4d_val(p4d)));
 		return;
 	}
 
@@ -1102,8 +1101,8 @@ static inline bool mm_p4d_folded(const struct mm_struct *mm)
 
 static inline void set_pgd(pgd_t *pgdp, pgd_t pgd)
 {
-	if (in_swapper_pgdir(pgdp)) {
-		set_swapper_pgd(pgdp, __pgd(pgd_val(pgd)));
+	if (in_pgdir_rodata(pgdp)) {
+		set_rodata_pte((pte_t *)pgdp, __pte(pgd_val(pgd)));
 		return;
 	}
 
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 2dca18574619..e5e1d0fd7f27 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -243,8 +243,12 @@ SECTIONS
 	reserved_pg_dir = .;
 	. += PAGE_SIZE;
 
-	swapper_pg_dir = .;
-	. += PAGE_SIZE;
+	.pgdir_rodata : {
+		__pgdir_rodata_start = .;
+		swapper_pg_dir = .;
+		. += PAGE_SIZE;
+		__pgdir_rodata_end = .;
+	}
 
 	. = ALIGN(SEGMENT_ALIGN);
 	__init_begin = .;
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index a464f3d2d2df..84d81bae07a7 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -65,34 +65,34 @@ static bool rodata_is_rw __ro_after_init = true;
  */
 long __section(".mmuoff.data.write") __early_cpu_boot_status;
 
-static DEFINE_SPINLOCK(swapper_pgdir_lock);
+static DEFINE_SPINLOCK(rodata_pgdir_lock);
 static DEFINE_MUTEX(fixmap_lock);
 
-void noinstr set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
+void noinstr set_rodata_pte(pte_t *ptep, pte_t pte)
 {
-	pgd_t *fixmap_pgdp;
+	pte_t *fixmap_ptep;
 
 	/*
-	 * Don't bother with the fixmap if swapper_pg_dir is still mapped
-	 * writable in the kernel mapping.
+	 * Don't bother with the fixmap if rodata is still mapped
+	 * writable in the kernel and linear mappings.
 	 */
 	if (rodata_is_rw) {
-		WRITE_ONCE(*pgdp, pgd);
+		WRITE_ONCE(*ptep, pte);
 		dsb(ishst);
 		isb();
 		return;
 	}
 
-	spin_lock(&swapper_pgdir_lock);
-	fixmap_pgdp = pgd_set_fixmap(__pa_symbol(pgdp));
-	WRITE_ONCE(*fixmap_pgdp, pgd);
+	spin_lock(&rodata_pgdir_lock);
+	fixmap_ptep = pte_set_fixmap(__pa_nodebug(ptep));
+	WRITE_ONCE(*fixmap_ptep, pte);
 	/*
 	 * We need dsb(ishst) here to ensure the page-table-walker sees
 	 * our new entry before set_p?d() returns. The fixmap's
 	 * flush_tlb_kernel_range() via clear_fixmap() does this for us.
 	 */
-	pgd_clear_fixmap();
-	spin_unlock(&swapper_pgdir_lock);
+	pte_clear_fixmap();
+	spin_unlock(&rodata_pgdir_lock);
 }
 
 pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
@@ -1071,6 +1071,7 @@ void __init mark_linear_text_alias_ro(void)
 	/*
 	 * Remove the write permissions from the linear alias of .text/.rodata
 	 */
+	WRITE_ONCE(rodata_is_rw, false);
 	update_mapping_prot(__pa_symbol(_text), (unsigned long)lm_alias(_text),
 			    (unsigned long)__init_begin - (unsigned long)_text,
 			    pgprot_tagged(PAGE_KERNEL_RO));
@@ -1221,7 +1222,6 @@ void mark_rodata_ro(void)
 	 * to cover NOTES and EXCEPTION_TABLE.
 	 */
 	section_size = (unsigned long)__init_begin - (unsigned long)__start_rodata;
-	WRITE_ONCE(rodata_is_rw, false);
 	update_mapping_prot(__pa_symbol(__start_rodata), (unsigned long)__start_rodata,
 			    section_size, PAGE_KERNEL_RO);
 	/* mark the range between _text and _stext as read only. */
-- 
2.54.0.rc2.544.gc7ae2d5bb8-goog