[RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW

[Yu-cheng Yu]

When Shadow Stack is enabled, the read-only and PAGE_DIRTY_HW PTE
setting is reserved only for the Shadow Stack.  To track dirty of
non-Shadow Stack read-only PTEs, we use PAGE_DIRTY_SW.

Update ptep_set_wrprotect() and pmdp_set_wrprotect().

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
---
 arch/x86/include/asm/pgtable.h | 42 ++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 4d50de77ea96..556ef258eeff 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1203,7 +1203,28 @@ 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)
 {
+	pte_t pte;
+
 	clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
+	pte = *ptep;
+
+	/*
+	 * Some processors can start a write, but ending up seeing
+	 * a read-only PTE by the time they get to the Dirty bit.
+	 * In this case, they will set the Dirty bit, leaving a
+	 * read-only, Dirty PTE which looks like a Shadow Stack PTE.
+	 *
+	 * However, this behavior has been improved and will not occur
+	 * on processors supporting Shadow Stacks.  Without this
+	 * guarantee, a transition to a non-present PTE and flush the
+	 * TLB would be needed.
+	 *
+	 * When change a writable PTE to read-only and if the PTE has
+	 * _PAGE_DIRTY_HW set, we move that bit to _PAGE_DIRTY_SW so
+	 * that the PTE is not a valid Shadow Stack PTE.
+	 */
+	pte = pte_move_flags(pte, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW);
+	set_pte_at(mm, addr, ptep, pte);
 }
 
 #define flush_tlb_fix_spurious_fault(vma, address) do { } while (0)
@@ -1266,7 +1287,28 @@ static inline pud_t pudp_huge_get_and_clear(struct mm_struct *mm,
 static inline void pmdp_set_wrprotect(struct mm_struct *mm,
 				      unsigned long addr, pmd_t *pmdp)
 {
+	pmd_t pmd;
+
 	clear_bit(_PAGE_BIT_RW, (unsigned long *)pmdp);
+	pmd = *pmdp;
+
+	/*
+	 * Some processors can start a write, but ending up seeing
+	 * a read-only PTE by the time they get to the Dirty bit.
+	 * In this case, they will set the Dirty bit, leaving a
+	 * read-only, Dirty PTE which looks like a Shadow Stack PTE.
+	 *
+	 * However, this behavior has been improved and will not occur
+	 * on processors supporting Shadow Stacks.  Without this
+	 * guarantee, a transition to a non-present PTE and flush the
+	 * TLB would be needed.
+	 *
+	 * When change a writable PTE to read-only and if the PTE has
+	 * _PAGE_DIRTY_HW set, we move that bit to _PAGE_DIRTY_SW so
+	 * that the PTE is not a valid Shadow Stack PTE.
+	 */
+	pmd = pmd_move_flags(pmd, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW);
+	set_pmd_at(mm, addr, pmdp, pmd);
 }
 
 #define pud_write pud_write
-- 
2.17.1