lkml.org 
[lkml]   [2012]   [Jun]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH 5/6] mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma()
    From
    Date
    On Mon, 2012-06-11 at 05:17 -0400, kosaki.motohiro@gmail.com wrote:
    > From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    >
    > commit cc9a6c8776 (cpuset: mm: reduce large amounts of memory barrier related
    > damage v3) introduced a memory corruption.
    >
    > shmem_alloc_page() passes pseudo vma and it has one significant unique
    > combination, vma->vm_ops=NULL and (vma->policy->flags & MPOL_F_SHARED).
    >
    > Now, 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 alloc_pages_vma() goes 'goto retry_cpuset' path, a policy
    > refcount will be decreased too much and therefore it will make a memory corruption.
    [...]
    > --- a/mm/mempolicy.c
    > +++ b/mm/mempolicy.c
    > @@ -1544,18 +1544,29 @@ 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;

    = 0

    And this should really be a bool.

    > 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;
    > + got_ref = 1;
    > + }
    > } else if (vma->vm_policy)
    > pol = vma->vm_policy;
    > }
    > if (!pol)
    > pol = &default_policy;
    > +
    > + /*
    > + * shmem_alloc_page() passes MPOL_F_SHARED policy with vma->vm_ops=NULL.
    > + * Thus, we need to take additional ref for avoiding refcount imbalance.
    > + */
    > + if (!got_ref && mpol_needs_cond_ref(pol))
    > + mpol_get(pol);
    > +
    > return pol;
    > }
    >
    [...]

    --
    Ben Hutchings
    Computers are not intelligent. They only think they are.
    [unhandled content-type:application/pgp-signature]
    \
     
     \ /
      Last update: 2012-06-11 16:44    [from the cache]
    ©2003-2014 Jasper Spaans. hosted at Digital Ocean