Subject: [PATCH v4 15/15] arm64: mm: Remap linear aliases of the fixmap page tables read-only

From: Ard Biesheuvel <[email-masked]>
Date: Mon, 27 Apr 2026 17:34:32 +0200

From: Ard Biesheuvel <[email-masked]>

The fixmap page tables are statically allocated, and are currently
mapped read-write both in the kernel mapping as well as its linear
alias. Due to lack of randomization of the linear map, these tables will
appear at a priori known offsets in the virtual address space when
booting without physical randomization, which means that a single kernel
write primitive is sufficient for an attacker to map memory of their own
choosing with any permissions at a known virtual address in the kernel's
address space.

To harden against this, move the fixmap PUD and PMD tables to
.pgdir_rodata, so that both their kernel mappings as well as their
linear aliases are mapped read-only during ordinary execution.
The PTE table needs to remain read-write accessible via the kernel
mapping, but its linear alias can be remapped read-only as well.

Signed-off-by: Ard Biesheuvel <[email-masked]>
---
 arch/arm64/include/asm/pgtable.h | 6 ++++--
 arch/arm64/kernel/vmlinux.lds.S  | 1 +
 arch/arm64/mm/fixmap.c           | 5 +++--
 arch/arm64/mm/mmu.c              | 5 +++++
 4 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 94235dd428be..21afe923cd71 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -822,8 +822,10 @@ extern void set_rodata_pte(pte_t *ptep, pte_t pte);
 
 static inline bool in_pgdir_rodata(void *addr)
 {
-	return addr >= (void *)__pgdir_rodata_start &&
-	       addr < (void *)__pgdir_rodata_end;
+	phys_addr_t pa = __pa_nodebug(addr);
+
+	return pa >= __pa_symbol_nodebug(__pgdir_rodata_start) &&
+	       pa < __pa_symbol_nodebug(__pgdir_rodata_end);
 }
 
 static inline void set_pmd(pmd_t *pmdp, pmd_t pmd)
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index e5e1d0fd7f27..9b346dd24d1c 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -247,6 +247,7 @@ SECTIONS
 		__pgdir_rodata_start = .;
 		swapper_pg_dir = .;
 		. += PAGE_SIZE;
+		*(.fixmap_rodata)
 		__pgdir_rodata_end = .;
 	}
 
diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
index b649ea1a46e4..ad6d46e5c23e 100644
--- a/arch/arm64/mm/fixmap.c
+++ b/arch/arm64/mm/fixmap.c
@@ -32,9 +32,10 @@ static_assert(NR_BM_PMD_TABLES == 1);
 #define BM_PTE_TABLE_IDX(addr)	__BM_TABLE_IDX(addr, PMD_SHIFT)
 
 #define __fixmap_bss	__section(".fixmap_bss") __aligned(PAGE_SIZE)
+#define __fixmap_rodata	__section(".fixmap_rodata") __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 pmd_t bm_pmd[PTRS_PER_PMD] __fixmap_rodata __maybe_unused;
+static pud_t bm_pud[PTRS_PER_PUD] __fixmap_rodata __maybe_unused;
 
 static inline pte_t *fixmap_pte(unsigned long addr)
 {
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 84d81bae07a7..e76fe5b0c5fe 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1076,6 +1076,11 @@ void __init mark_linear_text_alias_ro(void)
 			    (unsigned long)__init_begin - (unsigned long)_text,
 			    pgprot_tagged(PAGE_KERNEL_RO));
 
+	/* Map the fixmap PTE table at __fixmap_pgdir_start R/O in linear map too */
+	update_mapping_prot(__pa_symbol(__fixmap_pgdir_start),
+			    (unsigned long)lm_alias(__fixmap_pgdir_start),
+			    PAGE_SIZE, pgprot_tagged(PAGE_KERNEL_RO));
+
 	remap_linear_data_alias(true);
 
 	if (IS_ENABLED(CONFIG_HIBERNATION)) {
-- 
2.54.0.rc2.544.gc7ae2d5bb8-goog