s390/mm,pageattr: add more page table walk sanity checks

The current page table walk code in pageattr.c only checks for large pages
while walking the kernel page table, but happily assumes that everything
else is just fine.
Add more checks so we never access invalid memory regions.
Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/pageattr.c

@@ -8,25 +8,38 @@
 #include <asm/cacheflush.h>
 #include <asm/pgtable.h>
 
-static void change_page_attr(unsigned long addr, int numpages,
-			     pte_t (*set) (pte_t))
+static pte_t *walk_page_table(unsigned long addr)
 {
-	pte_t *ptep, pte;
-	pmd_t *pmdp;
-	pud_t *pudp;
 	pgd_t *pgdp;
-	int i;
+	pud_t *pudp;
+	pmd_t *pmdp;
+	pte_t *ptep;
 
-	for (i = 0; i < numpages; i++) {
-		pgdp = pgd_offset(&init_mm, addr);
+	pgdp = pgd_offset_k(addr);
+	if (pgd_none(*pgdp))
+		return NULL;
 	pudp = pud_offset(pgdp, addr);
+	if (pud_none(*pudp))
+		return NULL;
 	pmdp = pmd_offset(pudp, addr);
-		if (pmd_large(*pmdp)) {
-			WARN_ON_ONCE(1);
-			continue;
-		}
+	if (pmd_none(*pmdp) || pmd_large(*pmdp))
+		return NULL;
 	ptep = pte_offset_kernel(pmdp, addr);
+	if (pte_none(*ptep))
+		return NULL;
+	return ptep;
+}
 
+static void change_page_attr(unsigned long addr, int numpages,
+			     pte_t (*set) (pte_t))
+{
+	pte_t *ptep, pte;
+	int i;
+
+	for (i = 0; i < numpages; i++) {
+		ptep = walk_page_table(addr);
+		if (WARN_ON_ONCE(!ptep))
+			break;
 		pte = *ptep;
 		pte = set(pte);
 		__ptep_ipte(addr, ptep);