From: "Kirill A. Shutemov" kirill.shutemov@linux.intel.com Subject: 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.... 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 ---

mm/hugetlb.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)

diff -puN mm/hugetlb.c~mm-hugetlb-fix-null-pointer-dereference-on-5-level-paging-machine mm/hugetlb.c --- a/mm/hugetlb.c~mm-hugetlb-fix-null-pointer-dereference-on-5-level-paging-machine +++ a/mm/hugetlb.c @@ -4635,7 +4635,9 @@ pte_t *huge_pte_alloc(struct mm_struct * 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) { _