[PATCH] page_mkwrite caller race fix

After do_wp_page has tested page_mkwrite, it must release old_page after
acquiring page table lock, not before: at some stage that ordering got
reversed, leaving a (very unlikely) window in which old_page might be
truncated, freed, and reused in the same position.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -1531,8 +1531,6 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			if (vma->vm_ops->page_mkwrite(vma, old_page) < 0)
 				goto unwritable_page;
 
-			page_cache_release(old_page);
-
 			/*
 			 * Since we dropped the lock we need to revalidate
 			 * the PTE as someone else may have changed it.  If
@@ -1541,6 +1539,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			 */
 			page_table = pte_offset_map_lock(mm, pmd, address,
 							 &ptl);
+			page_cache_release(old_page);
 			if (!pte_same(*page_table, orig_pte))
 				goto unlock;
 		}