[PATCH v25 11/30] x86/mm: Update pte_modify for _PAGE_COW

[Yu-cheng Yu]

The read-only and Dirty PTE has been used to indicate copy-on-write pages.
However, newer x86 processors also regard a read-only and Dirty PTE as a
shadow stack page.  In order to separate the two, the software-defined
_PAGE_COW is created to replace _PAGE_DIRTY for the copy-on-write case, and
pte_*() are updated.

Pte_modify() changes a PTE to 'newprot', but it doesn't use the pte_*().
Introduce fixup_dirty_pte(), which sets a dirty PTE, based on _PAGE_RW,
to either _PAGE_DIRTY or _PAGE_COW.

Apply the same changes to pmd_modify().

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/pgtable.h | 37 ++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 9c056d5815de..e1739f590ca6 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -799,6 +799,23 @@ static inline pmd_t pmd_mkinvalid(pmd_t pmd)
 
 static inline u64 flip_protnone_guard(u64 oldval, u64 val, u64 mask);
 
+static inline pteval_t fixup_dirty_pte(pteval_t pteval)
+{
+	pte_t pte = __pte(pteval);
+
+	/*
+	 * Fix up potential shadow stack page flags because the RO, Dirty
+	 * PTE is special.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+		if (pte_dirty(pte)) {
+			pte = pte_mkclean(pte);
+			pte = pte_mkdirty(pte);
+		}
+	}
+	return pte_val(pte);
+}
+
 static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
 {
 	pteval_t val = pte_val(pte), oldval = val;
@@ -809,16 +826,36 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
 	 */
 	val &= _PAGE_CHG_MASK;
 	val |= check_pgprot(newprot) & ~_PAGE_CHG_MASK;
+	val = fixup_dirty_pte(val);
 	val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);
 	return __pte(val);
 }
 
+static inline int pmd_write(pmd_t pmd);
+static inline pmdval_t fixup_dirty_pmd(pmdval_t pmdval)
+{
+	pmd_t pmd = __pmd(pmdval);
+
+	/*
+	 * Fix up potential shadow stack page flags because the RO, Dirty
+	 * PMD is special.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+		if (pmd_dirty(pmd)) {
+			pmd = pmd_mkclean(pmd);
+			pmd = pmd_mkdirty(pmd);
+		}
+	}
+	return pmd_val(pmd);
+}
+
 static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
 {
 	pmdval_t val = pmd_val(pmd), oldval = val;
 
 	val &= _HPAGE_CHG_MASK;
 	val |= check_pgprot(newprot) & ~_HPAGE_CHG_MASK;
+	val = fixup_dirty_pmd(val);
 	val = flip_protnone_guard(oldval, val, PHYSICAL_PMD_PAGE_MASK);
 	return __pmd(val);
 }
-- 
2.21.0