rmap: anon_vma_prepare() can leak anon_vma_chain

If find_mergeable_anon_vma() succeeds but another thread installs
->anon_vma before we take ptl, then allocated == NULL but avc should be
freed.  Change the code to check avc != NULL to detect this case.

Also, a couple of whitespace changes to make the critical section more
visible.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Pete Zaitcev <zaitcev@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/rmap.c b/mm/rmap.c
index 526704e..07fc947 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -133,8 +133,8 @@ int anon_vma_prepare(struct vm_area_struct *vma)
                                goto out_enomem_free_avc;
                        allocated = anon_vma;
                }
-               spin_lock(&anon_vma->lock);
 
+               spin_lock(&anon_vma->lock);
                /* page_table_lock to protect against threads */
                spin_lock(&mm->page_table_lock);
                if (likely(!vma->anon_vma)) {
@@ -144,14 +144,15 @@ int anon_vma_prepare(struct vm_area_struct *vma)
                        list_add(&avc->same_vma, &vma->anon_vma_chain);
                        list_add(&avc->same_anon_vma, &anon_vma->head);
                        allocated = NULL;
+                       avc = NULL;
                }
                spin_unlock(&mm->page_table_lock);
-
                spin_unlock(&anon_vma->lock);
-               if (unlikely(allocated)) {
+
+               if (unlikely(allocated))
                        anon_vma_free(allocated);
+               if (unlikely(avc))
                        anon_vma_chain_free(avc);
-               }
        }
        return 0;