From 8c4ebc7023de80451c30f72d85588ee48455afeb Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Fri, 7 Sep 2012 10:23:13 +1000 Subject: [PATCH] mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma() 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 Signed-off-by: Mel Gorman Reviewed-by: Christoph Lameter Signed-off-by: Andrew Morton --- mm/mempolicy.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-) diff --git a/mm/mempolicy.c b/mm/mempolicy.c index 1763418f1ada..6434033f962d 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; -- 2.39.5