mm, thp: Do not make page table dirty unconditionally in touch_p[mu]d()

Currently, we unconditionally make page table dirty in touch_pmd().
It may result in false-positive can_follow_write_pmd().

We may avoid the situation, if we would only make the page table entry
dirty if caller asks for write access -- FOLL_WRITE.

The patch also changes touch_pud() in the same way.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(backported from commit a8f97366)
[cascardo: dropped touch_pud parts]
[cascardo: fix in-place of where touch_pmd would be called]
CVE-2017-1000405
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

mm/huge_memory.c

@@ -1304,17 +1304,11 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 	VM_BUG_ON_PAGE(!PageHead(page), page);
 	if (flags & FOLL_TOUCH) {
 		pmd_t _pmd;
-		/*
-		 * We should set the dirty bit only for FOLL_WRITE but
-		 * for now the dirty bit in the pmd is meaningless.
-		 * And if the dirty bit will become meaningful and
-		 * we'll only set it with FOLL_WRITE, an atomic
-		 * set_bit will be required on the pmd to set the
-		 * young bit, instead of the current set_pmd_at.
-		 */
-		_pmd = pmd_mkyoung(pmd_mkdirty(*pmd));
+		_pmd = pmd_mkyoung(*pmd);
+		if (flags & FOLL_WRITE)
+			_pmd = pmd_mkdirty(_pmd);
 		if (pmdp_set_access_flags(vma, addr & HPAGE_PMD_MASK,
-					  pmd, _pmd,  1))
+					  pmd, _pmd, flags & FOLL_WRITE))
 			update_mmu_cache_pmd(vma, addr, pmd);
 	}
 	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {