3.4-stable review patch.  If anyone has any objections, please let me know.

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

From: Sasha Levin <redacted>

commit 08fa29d916c6e271ad13978cd993e7238c68db97 upstream.

A missing validation of the value returned by find_vma() could cause a
NULL ptr dereference when walking the pagetable.

This is triggerable from usermode by a simple user by trying to read a
page info out of /proc/pid/pagemap which doesn't exist.

Introduced by commit 025c5b2451e4 ("thp: optimize away unnecessary page
table locking").

Signed-off-by: Sasha Levin <redacted>
Reviewed-by: Naoya Horiguchi <redacted>
Cc: David Rientjes <rientjes@google.com>
Cc: Andi Kleen <redacted>
Cc: Andrea Arcangeli <redacted>
Cc: KOSAKI Motohiro <redacted>
Cc: KAMEZAWA Hiroyuki <redacted>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/proc/task_mmu.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c

@@ -784,7 +784,7 @@ static int pagemap_pte_range(pmd_t *pmd,
 
 	/* find the first VMA at or above 'addr' */
 	vma = find_vma(walk->mm, addr);
-	if (pmd_trans_huge_lock(pmd, vma) == 1) {
+	if (vma && pmd_trans_huge_lock(pmd, vma) == 1) {
 		for (; addr != end; addr += PAGE_SIZE) {
 			unsigned long offset;