From: Mel Gorman Date: Mon, 8 Oct 2012 23:29:20 +0000 (-0700) Subject: mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma() X-Git-Tag: v3.6.2~20 X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=f66a52d6f1561fb601b0ad4e54ae3363b2af2486;p=karo-tx-linux.git 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 Signed-off-by: Mel Gorman Reviewed-by: Christoph Lameter Cc: Josh Boyer Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- diff --git a/mm/mempolicy.c b/mm/mempolicy.c index 1763418f1ada..3d64b369180d 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;