mm/ksm: handle protnone saved writes when making page write protect

Without this KSM will consider the page write protected, but a numa
fault can later mark the page writable.  This can result in memory
corruption.

Link: http://lkml.kernel.org/r/1487498625-10891-3-git-send-email-aneesh.kumar@linux.vnet.ibm.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/pgtable.h

@@ -233,6 +233,10 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
 #define pte_mk_savedwrite pte_mkwrite
 #endif
 
+#ifndef pte_clear_savedwrite
+#define pte_clear_savedwrite pte_wrprotect
+#endif
+
 #ifndef pmd_savedwrite
 #define pmd_savedwrite pmd_write
 #endif
@@ -241,6 +245,10 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
 #define pmd_mk_savedwrite pmd_mkwrite
 #endif
 
+#ifndef pmd_clear_savedwrite
+#define pmd_clear_savedwrite pmd_wrprotect
+#endif
+
 #ifndef __HAVE_ARCH_PMDP_SET_WRPROTECT
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 static inline void pmdp_set_wrprotect(struct mm_struct *mm,

mm/ksm.c

@@ -880,7 +880,8 @@ static int write_protect_page(struct vm_area_struct *vma, struct page *page,
 	if (WARN_ONCE(!pvmw.pte, "Unexpected PMD mapping?"))
 		goto out_unlock;
 
-	if (pte_write(*pvmw.pte) || pte_dirty(*pvmw.pte)) {
+	if (pte_write(*pvmw.pte) || pte_dirty(*pvmw.pte) ||
+	    (pte_protnone(*pvmw.pte) && pte_savedwrite(*pvmw.pte))) {
 		pte_t entry;
 
 		swapped = PageSwapCache(page);
@@ -905,7 +906,11 @@ static int write_protect_page(struct vm_area_struct *vma, struct page *page,
 		}
 		if (pte_dirty(entry))
 			set_page_dirty(page);
-		entry = pte_mkclean(pte_wrprotect(entry));
+
+		if (pte_protnone(entry))
+			entry = pte_mkclean(pte_clear_savedwrite(entry));
+		else
+			entry = pte_mkclean(pte_wrprotect(entry));
 		set_pte_at_notify(mm, pvmw.address, pvmw.pte, entry);
 	}
 	*orig_pte = *pvmw.pte;