[PATCH v4] mm/mempolicy: fix mpol_new leak in shared_policy_replace

[Miaohe Lin <linmiaohe@xxxxxxxxxx>]

If mpol_new is allocated but not used in restart loop, mpol_new will be
freed via mpol_put before returning to the caller.  But refcnt is not
initialized yet, so mpol_put could not do the right things and might leak
the unused mpol_new. This would happen if mempolicy was updated on the
shared shmem file while the sp->lock has been dropped during the memory
allocation.

This issue could be triggered easily with the below code snippet if there
are many processes doing the below work at the same time:

  shmid = shmget((key_t)5566, 1024 * PAGE_SIZE, 0666|IPC_CREAT);
  shm = shmat(shmid, 0, 0);
  loop many times {
    mbind(shm, 1024 * PAGE_SIZE, MPOL_LOCAL, mask, maxnode, 0);
    mbind(shm + 128 * PAGE_SIZE, 128 * PAGE_SIZE, MPOL_DEFAULT, mask,
          maxnode, 0);
  }

Fixes: 42288fe366c4 ("mm: mempolicy: Convert shared_policy mutex to spinlock")
Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 3.8
---
v3->v4:
  rebase on mainline.
v2->v3:
  enhance commit log and collect Acked-by tag per Michal Hocko. Thanks!
v1->v2:
  Add reproducer snippet and Cc stable.
  Thanks Michal Hocko for review and comment!
---
 mm/mempolicy.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index a2516d31db6c..6b34c5d5e65a 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2733,6 +2733,7 @@ static int shared_policy_replace(struct shared_policy *sp, unsigned long start,
 	mpol_new = kmem_cache_alloc(policy_cache, GFP_KERNEL);
 	if (!mpol_new)
 		goto err_out;
+	atomic_set(&mpol_new->refcnt, 1);
 	goto restart;
 }
 
-- 
2.23.0