mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma()

Commit cc9a6c877661 ("cpuset: mm: reduce large amounts of memory barrier
related damage v3") introduced a potential memory corruption.
shmem_alloc_page() uses a pseudo vma and it has one significant unique
combination, vma->vm_ops=NULL and vma->policy->flags & MPOL_F_SHARED.

get_vma_policy() does NOT increase a policy ref when vma->vm_ops=NULL
and mpol_cond_put() DOES decrease a policy ref when a policy has
MPOL_F_SHARED.  Therefore, when a cpuset update race occurs,
alloc_pages_vma() falls in 'goto retry_cpuset' path, decrements the
reference count and frees the policy prematurely.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Christoph Lameter <cl@linux.com>
Cc: Josh Boyer <jwboyer@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 1 deletions

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 1763418f1ada..3d64b369180d 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1552,8 +1552,18 @@ struct mempolicy *get_vma_policy(struct task_struct *task,
                                                                         addr);
                         if (vpol)
                                 pol = vpol;
-                } else if (vma->vm_policy)
+                } else if (vma->vm_policy) {
                         pol = vma->vm_policy;
+
+                        /*
+                         * shmem_alloc_page() passes MPOL_F_SHARED policy with
+                         * a pseudo vma whose vma->vm_ops=NULL. Take a reference
+                         * count on these policies which will be dropped by
+                         * mpol_cond_put() later
+                         */
+                        if (mpol_needs_cond_ref(pol))
+                                mpol_get(pol);
+                }
         }
         if (!pol)
                 pol = &default_policy;