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 <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>

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 6e2cd87e031328e4a0446f507c4471db1c3af725..f14ae9adec2091671cee13c8c52e4327f83599fd 100644 (file)
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1604,15 +1604,28 @@ struct mempolicy *get_vma_policy(struct task_struct *task,
                struct vm_area_struct *vma, unsigned long addr)
 {
        struct mempolicy *pol = get_task_policy(task);
+       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;