userfaultfd: hugetlbfs: fix userfaultfd_huge_must_wait() pte access

Use huge_ptep_get() to translate huge ptes to normal ptes so we can
check them with the huge_pte_* functions.  Otherwise some architectures
will check the wrong values and will not wait for userspace to bring in
the memory.

Link: http://lkml.kernel.org/r/20180626132421.78084-1-frankja@linux.ibm.com
Fixes: 369cd2121be4 ("userfaultfd: hugetlbfs: userfaultfd_huge_must_wait for hugepmd ranges")
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/userfaultfd.c

@@ -222,24 +222,26 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
 					 unsigned long reason)
 {
 	struct mm_struct *mm = ctx->mm;
-	pte_t *pte;
+	pte_t *ptep, pte;
 	bool ret = true;
 
 	VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
 
-	pte = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));
-	if (!pte)
+	ptep = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));
+
+	if (!ptep)
 		goto out;
 
 	ret = false;
+	pte = huge_ptep_get(ptep);
 
 	/*
 	 * Lockless access: we're in a wait_event so it's ok if it
 	 * changes under us.
 	 */
-	if (huge_pte_none(*pte))
+	if (huge_pte_none(pte))
 		ret = true;
-	if (!huge_pte_write(*pte) && (reason & VM_UFFD_WP))
+	if (!huge_pte_write(pte) && (reason & VM_UFFD_WP))
 		ret = true;
 out:
 	return ret;