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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 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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Mel Gorman 2012-10-08 16:29:20 -07:00 committed by Linus Torvalds
parent 63f74ca21f
commit 00442ad04a
1 changed files with 11 additions and 1 deletions
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12
mm/mempolicy.c
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@ -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;