[PATCH] Fix hugepage crash on failing mmap()

This patch fixes a crash in the hugepage code. unmap_hugepage_area() was
assuming that (due to prefault) PTEs must exist for all the area in
question. However, this may not be the case, if mmap() encounters an error
before the prefault and calls unmap_region() to clean up any partial
mapping.

Depending on the hugepage configuration, this crash can be triggered by an
unpriveleged user.

Signed-off-by: David Gibson <[e-mail]>
Cc: William Lee Irwin III <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/hugetlb.c

···
 {
 	struct mm_struct *mm = vma->vm_mm;
 	unsigned long address;
+	pte_t *ptep;
 	pte_t pte;
 	struct page *page;
 
···
 	BUG_ON(end & ~HPAGE_MASK);
 
 	for (address = start; address < end; address += HPAGE_SIZE) {
-		pte = huge_ptep_get_and_clear(mm, address, huge_pte_offset(mm, address));
+		ptep = huge_pte_offset(mm, address);
+		if (! ptep)
+			/* This can happen on truncate, or if an
+			 * mmap() is aborted due to an error before
+			 * the prefault */
+			continue;
+
+		pte = huge_ptep_get_and_clear(mm, address, ptep);
 		if (pte_none(pte))
 			continue;
+
 		page = pte_page(pte);
 		put_page(page);
 	}