mm/hugetlb: fix NULL-pointer dereference on 5-level paging machine

I made a mistake during converting hugetlb code to 5-level paging: in
huge_pte_alloc() we have to use p4d_alloc(), not p4d_offset().

Otherwise it leads to crash -- NULL-pointer dereference in pud_alloc()
if p4d table is not yet allocated.

It only can happen in 5-level paging mode.  In 4-level paging mode
p4d_offset() always returns pgd, so we are fine.

Link: http://lkml.kernel.org/r/20171122121921.64822-1-kirill.shutemov@linux.intel.com
Fixes: c2febafc6773 ("mm: convert generic code to 5-level paging")
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: <stable@vger.kernel.org>	[4.11+]

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 698e8fb34031..9a334f5fb730 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4635,7 +4635,9 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
         pte_t *pte = NULL;
 
         pgd = pgd_offset(mm, addr);
-        p4d = p4d_offset(pgd, addr);
+        p4d = p4d_alloc(mm, pgd, addr);
+        if (!p4d)
+                return NULL;
         pud = pud_alloc(mm, p4d, addr);
         if (pud) {
                 if (sz == PUD_SIZE) {