[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>

1 files changed, 1 insertions, 2 deletions

diff --git a/mm/memory.c b/mm/memory.c
index ef09f0acb1d8..0047d3a4e364 100644
--- a/mm/memory.c
+++ b/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;
                 }