linux-kernel - [PATCH 5/5] mempolicy: fix a memory corruption by refcount imbalance in alloc_pages

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Message-Id: <1349801921-16598-6-git-send-email-mgorman@suse.de>
Date:	Tue,  9 Oct 2012 17:58:41 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Stable <stable@...r.kernel.org>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Dave Jones <davej@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	Hugh Dickins <hughd@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, Mel Gorman <mgorman@...e.de>
Subject: [PATCH 5/5] mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma()

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@...fujitsu.com>
Signed-off-by: Mel Gorman <mgorman@...e.de>
Reviewed-by: Christoph Lameter <cl@...ux.com>
Cc: Josh Boyer <jwboyer@...il.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@...e.de>
---
 mm/mempolicy.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 1763418..3d64b36 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1552,8 +1552,18 @@ struct mempolicy *get_vma_policy(struct task_struct *task,
 									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;
-- 
1.7.9.2

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