x86/mm: Prevent kernel Oops in PTDUMP code with HIGHPTE=y

The walk_pte_level() function just uses __va to get the virtual address of
the PTE page, but that breaks when the PTE page is not in the direct
mapping with HIGHPTE=y.

The result is an unhandled kernel paging request at some random address
when accessing the current_kernel or current_user file.

Use the correct API to access PTE pages.

Fixes: fe770bf0310d ('x86: clean up the page table dumper and add 32-bit support')
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Cc: jgross@suse.com
Cc: JBeulich@suse.com
Cc: hpa@zytor.com
Cc: aryabinin@virtuozzo.com
Cc: kirill.shutemov@linux.intel.com
Link: https://lkml.kernel.org/r/1523971636-4137-1-git-send-email-joro@8bytes.org

arch/x86/mm/dump_pagetables.c

@@ -18,6 +18,7 @@
 #include <linux/init.h>
 #include <linux/sched.h>
 #include <linux/seq_file.h>
+#include <linux/highmem.h>
 
 #include <asm/pgtable.h>
 
@@ -334,16 +335,16 @@ static void walk_pte_level(struct seq_file *m, struct pg_state *st, pmd_t addr,
 			   pgprotval_t eff_in, unsigned long P)
 {
 	int i;
-	pte_t *start;
+	pte_t *pte;
 	pgprotval_t prot, eff;
 
-	start = (pte_t *)pmd_page_vaddr(addr);
 	for (i = 0; i < PTRS_PER_PTE; i++) {
-		prot = pte_flags(*start);
-		eff = effective_prot(eff_in, prot);
 		st->current_address = normalize_addr(P + i * PTE_LEVEL_MULT);
+		pte = pte_offset_map(&addr, st->current_address);
+		prot = pte_flags(*pte);
+		eff = effective_prot(eff_in, prot);
 		note_page(m, st, __pgprot(prot), eff, 5);
-		start++;
+		pte_unmap(pte);
 	}
 }
 #ifdef CONFIG_KASAN