[PATCH 5/6] mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma()

[kosaki.motohiro@gmail.com]

From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

commit cc9a6c8776 (cpuset: mm: reduce large amounts of memory barrier related
damage v3) introduced a memory corruption.

shmem_alloc_page() passes pseudo vma and it has one significant unique
combination, vma->vm_ops=NULL and (vma->policy->flags & MPOL_F_SHARED).

Now, 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 cpuset race is happen and alloc_pages_vma() fall in
'goto retry_cpuset' path, a policy refcount will be decreased too much and
therefore it will make memory corruption.

This patch fixes it.

Cc: Dave Jones <davej@redhat.com>,
Cc: Mel Gorman <mgorman@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>,
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Cc: Miao Xie <miaox@cn.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 mm/mempolicy.c |   13 ++++++++++++-
 mm/shmem.c     |    9 +++++----
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 7fb7d51..0da0969 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1544,18 +1544,29 @@ 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;
+				got_ref = 1;
+			}
 		} else if (vma->vm_policy)
 			pol = vma->vm_policy;
 	}
 	if (!pol)
 		pol = &default_policy;
+
+	/*
+	 * shmem_alloc_page() passes MPOL_F_SHARED policy with vma->vm_ops=NULL.
+	 * Thus, we need to take additional ref for avoiding refcount imbalance.
+	 */
+	if (!got_ref && mpol_needs_cond_ref(pol))
+		mpol_get(pol);
+
 	return pol;
 }
 
diff --git a/mm/shmem.c b/mm/shmem.c
index d576b84..eb5f1eb 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -919,6 +919,7 @@ static struct page *shmem_alloc_page(gfp_t gfp,
 			struct shmem_inode_info *info, pgoff_t index)
 {
 	struct vm_area_struct pvma;
+	struct page *page;
 
 	/* Create a pseudo vma that just contains the policy */
 	pvma.vm_start = 0;
@@ -926,10 +927,10 @@ static struct page *shmem_alloc_page(gfp_t gfp,
 	pvma.vm_ops = NULL;
 	pvma.vm_policy = mpol_shared_policy_lookup(&info->policy, index);
 
-	/*
-	 * alloc_page_vma() will drop the shared policy reference
-	 */
-	return alloc_page_vma(gfp, &pvma, 0);
+	page = alloc_page_vma(gfp, &pvma, 0);
+
+	mpol_put(pvma.vm_policy);
+	return page;
 }
 #else /* !CONFIG_NUMA */
 #ifdef CONFIG_TMPFS
-- 
1.7.1

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