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Subject[PATCH 5/5] mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma()
Date
[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>
---
mm/mempolicy.c | 17 +++++++++++++++--
1 files changed, 15 insertions(+), 2 deletions(-)
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 45f9825..82e872f 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1545,15 +1545,28 @@ 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;
--
1.7.7


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