Commit 00442ad0 authored Oct 08, 2012 by Mel Gorman Committed by Linus Torvalds Oct 09, 2012

mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma()



Commit 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>
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>

parent 63f74ca2

mm/mempolicy.c

+11 −1

Original line number	Diff line number	Diff line
		@@ -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;

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