[ 78/84] mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma()

Greg Kroah-Hartman
2012-10-11T02:16:25Z

From: Mel Gorman <mgorman@suse.de>

commit 00442ad04a5eac08a98255697c510e708f6082e2 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/mempolicy.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1511,8 +1511,18 @@ struct mempolicy *get_vma_policy(struct
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;