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

[KOSAKI Motohiro]

(6/11/12 9:33 AM), Ben Hutchings wrote:
> On Mon, 2012-06-11 at 05:17 -0400, kosaki.motohiro@xxxxxxxxx wrote:
> > From: KOSAKI Motohiro<kosaki.motohiro@xxxxxxxxxxxxxx>
> >
> > 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 alloc_pages_vma() goes 'goto retry_cpuset' path, a policy
> > refcount will be decreased too much and therefore it will make a memory corruption.
> [...]
> > --- 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;
>
> = 0
>
> And this should really be a bool.

Good catch. Thanks.



From 6a635a77e7b192413525855e19df7a724c81ae5b Mon Sep 17 00:00:00 2001
From: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Date: Tue, 29 May 2012 22:23:46 -0400
Subject: [PATCH 5/6] mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma()

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 alloc_pages_vma() goes 'goto retry_cpuset' path, a policy
refcount will be decreased too much and therefore it will make a memory corruption.

This patch fixes it.

Cc: Dave Jones <davej@xxxxxxxxxx>,
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>,
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Cc: Miao Xie <miaox@xxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
---
 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..ddde834 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;
+	bool got_ref = 0;
 
 	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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