[PATCH 3.16 165/366] x86/mm: Prevent kernel Oops in PTDUMP code with HIGHPTE=y

[Ben Hutchings]

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Joerg Roedel <jroedel@xxxxxxx>

commit d6ef1f194b7569af8b8397876dc9ab07649d63cb upstream.

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@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: jgross@xxxxxxxx
Cc: JBeulich@xxxxxxxx
Cc: hpa@xxxxxxxxx
Cc: aryabinin@xxxxxxxxxxxxx
Cc: kirill.shutemov@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/1523971636-4137-1-git-send-email-joro@xxxxxxxxxx
[bwh: Backported to 3.16:
 - Keep using pte_pgprot() to get protection flags
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -16,6 +16,7 @@
 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/seq_file.h>
+#include <linux/highmem.h>
 
 #include <asm/pgtable.h>
 
@@ -263,15 +264,16 @@ static void walk_pte_level(struct seq_fi
 							unsigned long P)
 {
 	int i;
-	pte_t *start;
+	pte_t *pte;
 
-	start = (pte_t *) pmd_page_vaddr(addr);
 	for (i = 0; i < PTRS_PER_PTE; i++) {
-		pgprot_t prot = pte_pgprot(*start);
+		pgprot_t prot;
 
 		st->current_address = normalize_addr(P + i * PTE_LEVEL_MULT);
+		pte = pte_offset_map(&addr, st->current_address);
+		prot = pte_pgprot(*pte);
 		note_page(m, st, prot, 4);
-		start++;
+		pte_unmap(pte);
 	}
 }