From: Mel Gorman <mgorman@suse.de>
Date: Fri, 28 Sep 2012 00:19:11 +0000 (+1000)
Subject: mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma()
X-Git-Tag: next-20120928~1^2~344
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=05987ff43535e7679d08b7a1ad2a178502081086;p=karo-tx-linux.git

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

cc9a6c87 ("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>
---

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index a31d6b519f6f..3c23d89fe06d 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1588,15 +1588,28 @@ struct mempolicy *get_vma_policy(struct task_struct *task,
 		struct vm_area_struct *vma, unsigned long addr)
 {
 	struct mempolicy *pol = task->mempolicy;
+	int got_ref;
 
 	if (vma) {
 		if (vma->vm_ops && vma->vm_ops->get_policy) {
 			struct mempolicy *vpol = vma->vm_ops->get_policy(vma,
 									addr);
-			if (vpol)
+			if (vpol) {
 				pol = vpol;
-		} else if (vma->vm_policy)
+				got_ref = 1;
+			}
+		} 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;