[RFC PATCH 14/16] arm64: mm: Guard page table writes with kpkeys

Kevin Brodsky
2024-12-06T10:13:58Z

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 under the
kpkeys_hardened_pgtables guard, which sets POR_EL1 appropriately to
allow such writes.

Use this guard wherever page tables genuinely need to be written,
keeping its scope as small as possible (so that POR_EL1 is reset as
fast as possible). 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 | 19 +++++++++++++++++--
arch/arm64/mm/fault.c | 2 ++
2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 6986345b537a..5f9d748f08ee 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -39,6 +39,7 @@
#include <linux/mm_types.h>
#include <linux/sched.h>
#include <linux/page_table_check.h>
+#include <linux/kpkeys.h>

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
#define __HAVE_ARCH_FLUSH_PMD_TLB_RANGE
@@ -314,6 +315,7 @@ 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);
}

@@ -758,6 +760,7 @@ 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)) {
@@ -825,6 +828,7 @@ static inline void set_pud(pud_t *pudp, pud_t pud)
return;
}

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

if (pud_valid(pud)) {
@@ -906,6 +910,7 @@ static inline void set_p4d(p4d_t *p4dp, p4d_t p4d)
return;
}

+ guard(kpkeys_hardened_pgtables)();
WRITE_ONCE(*p4dp, p4d);
dsb(ishst);
isb();
@@ -1033,6 +1038,7 @@ static inline void set_pgd(pgd_t *pgdp, pgd_t pgd)
return;
}

+ guard(kpkeys_hardened_pgtables)();
WRITE_ONCE(*pgdp, pgd);
dsb(ishst);
isb();
@@ -1233,6 +1239,7 @@ 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;
@@ -1279,7 +1286,10 @@ static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
static inline pte_t __ptep_get_and_clear(struct mm_struct *mm,
unsigned long address, pte_t *ptep)
{
- 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));

page_table_check_pte_clear(mm, pte);

@@ -1322,7 +1332,10 @@ static inline pte_t __get_and_clear_full_ptes(struct mm_struct *mm,
static inline pmd_t pmdp_huge_get_and_clear(struct mm_struct *mm,
unsigned long address, pmd_t *pmdp)
{
- pmd_t pmd = __pmd(xchg_relaxed(&pmd_val(*pmdp), 0));
+ pmd_t pmd;
+
+ scoped_guard(kpkeys_hardened_pgtables)
+ pmd = __pmd(xchg_relaxed(&pmd_val(*pmdp), 0));

page_table_check_pmd_clear(mm, pmd);

@@ -1336,6 +1349,7 @@ 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);
@@ -1416,6 +1430,7 @@ 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 ef63651099a9..ab45047155b9 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -220,6 +220,8 @@ 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