Re: [PATCH v9 11/42] x86/mm: Update ptep/pmdp_set_wrprotect() for _PAGE_SAVED_DIRTY

+++ b/arch/x86/include/asm/pgtable.h

@@ -1189,7 +1189,17 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
 static inline void ptep_set_wrprotect(struct mm_struct *mm,
                                      unsigned long addr, pte_t *ptep)
 {
-       clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
+       /*
+        * Avoid accidentally creating shadow stack PTEs
+        * (Write=0,Dirty=1).  Use cmpxchg() to prevent races with
+        * the hardware setting Dirty=1.
+        */
+       pte_t old_pte, new_pte;
+
+       old_pte = READ_ONCE(*ptep);
+       do {
+               new_pte = pte_wrprotect(old_pte);
+       } while (!try_cmpxchg((long *)&ptep->pte, (long *)&old_pte, *(long *)&new_pte));
 }