[patch 6/9] mm/mempolicy: fix mpol_new leak in shared_policy_replace

[Andrew Morton <akpm@linux-foundation.org>]

From: Miaohe Lin <linmiaohe@huawei.com>
Subject: mm/mempolicy: fix mpol_new leak in shared_policy_replace

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);
  }

Link: https://lkml.kernel.org/r/20220329111416.27954-1-linmiaohe@huawei.com
Fixes: 42288fe366c4 ("mm: mempolicy: Convert shared_policy mutex to spinlock")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: <stable@vger.kernel.org>	[3.8]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/mempolicy.c |    1 +
 1 file changed, 1 insertion(+)

--- a/mm/mempolicy.c~mm-mempolicy-fix-mpol_new-leak-in-shared_policy_replace
+++ a/mm/mempolicy.c
@@ -2733,6 +2733,7 @@ alloc_new:
 	mpol_new = kmem_cache_alloc(policy_cache, GFP_KERNEL);
 	if (!mpol_new)
 		goto err_out;
+	atomic_set(&mpol_new->refcnt, 1);
 	goto restart;
 }
 
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