mm/cdm: new_pol can be NULL when mbind mode arg was MPOL_DEFAULT

hulk inclusion
category: bugfix
bugzilla: 16080
CVE: NA
-------------------

the pointer new_pol can be NULL when mbind() mode arg was MPOL_DEFAULT,
so, before using new_pol, make sure it was not NULL.

Fixes: a72a680ff761 ("mm: Tag VMA with VM_CDM flag explicitly during mbind(MPOL_BIND) and page fault")
Signed-off-by: NLijun Fang <fanglijun3@huawei.com>
Reviewed-by: NYang Yingliang <yangyingliang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

mm/mempolicy.c

@@ -763,8 +763,8 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
 		vmstart = max(start, vma->vm_start);
 		vmend   = min(end, vma->vm_end);
 
-		if ((new_pol->mode == MPOL_BIND)
-			&& nodemask_has_cdm(new_pol->v.nodes))
+		if (new_pol && (new_pol->mode == MPOL_BIND) &&
+		    nodemask_has_cdm(new_pol->v.nodes))
 			set_vm_cdm(vma);
 
 		if (mpol_equal(vma_policy(vma), new_pol))