[PATCH v2] 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 issue could be triggered easily with the below
code snippet if there're 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>
Cc: <stable@xxxxxxxxxxxxxxx> # 3.8
---
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..4cdd425b2752 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;
+	refcount_set(&mpol_new->refcnt, 1);
 	goto restart;
 }
 
-- 
2.23.0