[RFC PATCH v5 15/18] arm64: mm: Guard page table writes with kpkeys

Kevin Brodsky
2025-08-15T08:56:42Z

When CONFIG_KPKEYS_HARDENED_PGTABLES is enabled, page tables (both
user and kernel) are mapped with a privileged pkey in the linear
mapping. As a result, they can only be written at an elevated kpkeys
level.

Introduce a kpkeys guard that sets POR_EL1 appropriately to allow
writing to page tables, and use this guard wherever necessary. The
scope is kept as small as possible, so that POR_EL1 is quickly reset
to its default value. Where atomics are involved, the guard's scope
encompasses the whole loop to avoid switching POR_EL1 unnecessarily.

This patch is a no-op if CONFIG_KPKEYS_HARDENED_PGTABLES is disabled
(default).

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
arch/arm64/include/asm/pgtable.h | 22 +++++++++++++++++++++-
arch/arm64/mm/fault.c | 2 ++
2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index abd2dee416b3..1694fb839854 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -39,6 +39,14 @@
#include <linux/mm_types.h>
#include <linux/sched.h>
#include <linux/page_table_check.h>
+#include <linux/kpkeys.h>
+
+#ifdef CONFIG_KPKEYS_HARDENED_PGTABLES
+KPKEYS_GUARD_COND(kpkeys_hardened_pgtables, KPKEYS_LVL_PGTABLES,
+ kpkeys_hardened_pgtables_enabled())
+#else
+KPKEYS_GUARD_NOOP(kpkeys_hardened_pgtables)
+#endif

static inline void emit_pte_barriers(void)
{
@@ -390,6 +398,7 @@ static inline pte_t pte_clear_uffd_wp(pte_t pte)

static inline void __set_pte_nosync(pte_t *ptep, pte_t pte)
{
+ guard(kpkeys_hardened_pgtables)();
WRITE_ONCE(*ptep, pte);
}

@@ -858,6 +867,7 @@ static inline void set_pmd(pmd_t *pmdp, pmd_t pmd)
}
#endif /* __PAGETABLE_PMD_FOLDED */

+ guard(kpkeys_hardened_pgtables)();
WRITE_ONCE(*pmdp, pmd);

if (pmd_valid(pmd))
@@ -918,6 +928,7 @@ static inline void set_pud(pud_t *pudp, pud_t pud)
return;
}

+ guard(kpkeys_hardened_pgtables)();
WRITE_ONCE(*pudp, pud);

if (pud_valid(pud))
@@ -999,6 +1010,7 @@ static inline void set_p4d(p4d_t *p4dp, p4d_t p4d)
return;
}

+ guard(kpkeys_hardened_pgtables)();
WRITE_ONCE(*p4dp, p4d);
queue_pte_barriers();
}
@@ -1127,6 +1139,7 @@ static inline void set_pgd(pgd_t *pgdp, pgd_t pgd)
return;
}

+ guard(kpkeys_hardened_pgtables)();
WRITE_ONCE(*pgdp, pgd);
queue_pte_barriers();
}
@@ -1316,6 +1329,7 @@ static inline int __ptep_test_and_clear_young(struct vm_area_struct *vma,
{
pte_t old_pte, pte;

+ guard(kpkeys_hardened_pgtables)();
pte = __ptep_get(ptep);
do {
old_pte = pte;
@@ -1363,7 +1377,10 @@ static inline pte_t __ptep_get_and_clear_anysz(struct mm_struct *mm,
pte_t *ptep,
unsigned long pgsize)
{
- pte_t pte = __pte(xchg_relaxed(&pte_val(*ptep), 0));
+ pte_t pte;
+
+ scoped_guard(kpkeys_hardened_pgtables)
+ pte = __pte(xchg_relaxed(&pte_val(*ptep), 0));

switch (pgsize) {
case PAGE_SIZE:
@@ -1436,6 +1453,7 @@ static inline void ___ptep_set_wrprotect(struct mm_struct *mm,
{
pte_t old_pte;

+ guard(kpkeys_hardened_pgtables)();
do {
old_pte = pte;
pte = pte_wrprotect(pte);
@@ -1469,6 +1487,7 @@ static inline void __clear_young_dirty_pte(struct vm_area_struct *vma,
{
pte_t old_pte;

+ guard(kpkeys_hardened_pgtables)();
do {
old_pte = pte;

@@ -1516,6 +1535,7 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
unsigned long address, pmd_t *pmdp, pmd_t pmd)
{
page_table_check_pmd_set(vma->vm_mm, pmdp, pmd);
+ guard(kpkeys_hardened_pgtables)();
return __pmd(xchg_relaxed(&pmd_val(*pmdp), pmd_val(pmd)));
}
#endif
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index d816ff44faff..c4ab361bba72 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -214,6 +214,8 @@ int __ptep_set_access_flags(struct vm_area_struct *vma,
if (pte_same(pte, entry))
return 0;

+ guard(kpkeys_hardened_pgtables)();
+
/* only preserve the access flags and write permission */
pte_val(entry) &= PTE_RDONLY | PTE_AF | PTE_WRITE | PTE_DIRTY;

--
2.47.0