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>

1 files changed, 5 insertions, 4 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 526704e8215d..07fc94758799 100644
--- 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;